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MICHAEL SHORT: So
today, I wanted

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to give you some context for
why we're learning about all

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the neutron stuff and go
over all the reactor types

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that, until this year,
the first time you learned

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about the non-light
water reactors at MIT

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was once you left MIT.

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I remember that as
an undergrad as well.

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The only exposure we had
to non-light water reactors

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00:00:40,410 --> 00:00:42,903
is in our design course, because
we decided to design one.

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00:00:42,903 --> 00:00:45,570
So I wanted to show you guys all
the different types of reactors

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that are out there,
how they work,

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and start generating
and marinating

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in all the different
variables and nomenclature

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that we'll use to develop the
neutron transport and neutron

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00:00:55,350 --> 00:00:56,910
diffusion equations.

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00:00:56,910 --> 00:00:59,430
The nice part is
now, until quiz two,

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00:00:59,430 --> 00:01:01,800
you can pretty much forget
about the concept of charge.

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00:01:01,800 --> 00:01:04,050
So 8.02 can go
back on the shelf,

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00:01:04,050 --> 00:01:06,870
because every interaction
we do here is neutral,

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00:01:06,870 --> 00:01:08,250
charge neutral.

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00:01:08,250 --> 00:01:10,650
There'll be radioactive
decays that are not the case.

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00:01:10,650 --> 00:01:12,793
But everything
neutron is neutral.

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00:01:12,793 --> 00:01:14,460
It doesn't mean it's
going to be simple.

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00:01:14,460 --> 00:01:16,350
It's just going to be different.

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00:01:16,350 --> 00:01:18,420
But in the meantime,
today is not

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going to be
particularly intense,

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00:01:20,040 --> 00:01:22,620
but I do want to show
you where we're going.

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00:01:22,620 --> 00:01:25,260
And this goes with
the pedagogical switch

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00:01:25,260 --> 00:01:27,580
that we made in this
department starting this year.

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00:01:27,580 --> 00:01:30,270
And you guys are the
first trial of this.

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00:01:30,270 --> 00:01:33,180
We're switching to context
first and theory second.

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00:01:33,180 --> 00:01:35,190
I personally find it
much more interesting

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00:01:35,190 --> 00:01:37,830
to study the theory of
something for which I

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00:01:37,830 --> 00:01:39,415
know the application exists.

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00:01:39,415 --> 00:01:40,290
Who here would agree?

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Just about actually everybody.

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00:01:44,950 --> 00:01:45,520
OK.

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00:01:45,520 --> 00:01:45,760
Yeah.

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00:01:45,760 --> 00:01:46,960
That's what I thought too.

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00:01:46,960 --> 00:01:50,943
So in the end, we had arguments
amongst the faculty about,

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00:01:50,943 --> 00:01:52,360
well, you have to
learn the theory

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00:01:52,360 --> 00:01:54,960
to understand the application.

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00:01:54,960 --> 00:01:57,490
And that works really well when
you say it behind the closed

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office door by yourself.

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00:01:58,780 --> 00:02:01,120
But the fact is, I'm in it for--

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00:02:01,120 --> 00:02:02,160
yeah.

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00:02:02,160 --> 00:02:05,060
I'm in it for maximum
subject matter retention,

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00:02:05,060 --> 00:02:07,630
so in whatever order
that works the best.

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00:02:07,630 --> 00:02:10,030
And sounds like, for you
guys, this works the best.

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00:02:10,030 --> 00:02:12,697
That's what we're doing with the
whole undergrad curriculum, not

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00:02:12,697 --> 00:02:14,540
just this class.

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00:02:14,540 --> 00:02:17,890
So let's launch into all the
different methods of making

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nuclear power, both
fission and fusion,

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and to switch gears since
we're dealing with neutrons.

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I don't know what happened
with the-- oh, there we go.

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The idea here is that
neutrons hit things

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like uranium and
plutonium, the fissile

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isotopes that you
guys saw on the exam,

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00:02:32,830 --> 00:02:34,900
and caused the release
of other neutrons.

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And as we come up
with these variables,

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I'm going to start
laying them out here.

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It might take more than
a board to fill them all.

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And I'll warn you
ahead of time, this

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is the only time in
this course that we're

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going to have V and nu,
the Greek letter nu,

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00:02:48,520 --> 00:02:50,230
on the board at the same time.

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And I'm going to make it
really obvious which one is nu

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and which one is V.

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So this parameter that describes
how many neutrons come out

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from each fission reaction
we refer to as nu,

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or the average number you'll see
in the data tables as nu bar.

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And so as we come up with
these sorts of things,

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I will start going over them.

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And the idea here is that each
uranium-235, or plutonium,

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or whatever nucleus begets
two to three neutrons,

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the exact number for which
is still under a hot debate,

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and I don't think
it actually matters,

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will make a couple of fission
products that take away

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most of the heat of
the nuclear reaction.

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And I just want to stop there,
even though you know there's

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00:03:36,512 --> 00:03:37,720
going to be a chain reaction.

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00:03:37,720 --> 00:03:40,200
And that's what makes
nuclear power happen.

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And we can go over the timeline
of what actually happens

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00:03:42,790 --> 00:03:47,182
in fission and what kind of a
nuclear reaction it really is.

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00:03:47,182 --> 00:03:48,640
So in this case,
this is a reaction

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where a neutron is
heading towards,

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00:03:51,372 --> 00:03:53,080
this time we're actually
going to give it

95
00:03:53,080 --> 00:03:57,220
a label, a uranium-235 nucleus.

96
00:03:57,220 --> 00:03:59,770
And it very temporarily,
like I showed you yesterday,

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00:03:59,770 --> 00:04:03,280
forms a compound
nucleus, some sort

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00:04:03,280 --> 00:04:07,660
of large excited nucleus
that lasts for about 10

99
00:04:07,660 --> 00:04:10,660
to the minus 14 seconds.

100
00:04:10,660 --> 00:04:12,580
So it doesn't
instantly fizz apart.

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00:04:12,580 --> 00:04:14,950
There's actually a
neutron absorption event,

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some sort of nuclear
instability, at which point

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your two fission
products break off.

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Notice, you don't have-- let's
call them fission product one

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00:04:28,800 --> 00:04:30,320
and fission product two.

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00:04:30,320 --> 00:04:33,500
Notice, you don't quite
have any neutrons yet.

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00:04:33,500 --> 00:04:37,010
Neutron production is not
instantaneous for the following

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reason.

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00:04:39,290 --> 00:04:42,620
If you remember back to nuclear
stability, when we plotted,

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00:04:42,620 --> 00:04:47,090
let's say, I think that
was maybe Z and this was N.

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00:04:47,090 --> 00:04:49,130
And I think this was
a homework problem.

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00:04:49,130 --> 00:04:51,380
And you had to come up
with some sort of curve

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00:04:51,380 --> 00:04:56,960
of best fit for the most
stable combination of NZ

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00:04:56,960 --> 00:04:57,590
for a nucleus.

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00:04:57,590 --> 00:04:59,300
It was not a straight line.

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00:04:59,300 --> 00:05:03,670
It was something on the
order of like N equals--

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what is it?

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--1.0055Z plus some constant,
something with a rather small

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slope.

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00:05:12,240 --> 00:05:17,910
Well, if you have a heavy
nucleus, like uranium-235,

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00:05:17,910 --> 00:05:20,670
and you split it apart
evenly, let's just

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00:05:20,670 --> 00:05:23,310
pretend it splits
evenly for now,

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00:05:23,310 --> 00:05:25,830
you're kind of
splitting that nucleus

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00:05:25,830 --> 00:05:28,450
along a rather unstable line.

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00:05:28,450 --> 00:05:31,530
And, as you saw in the
semi-empirical mass formula,

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00:05:31,530 --> 00:05:34,260
a little bit of instability
goes a really long way

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00:05:34,260 --> 00:05:37,240
towards making the nucleus
extremely unstable.

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00:05:37,240 --> 00:05:41,280
So let's say you'd make a
couple of fission products

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00:05:41,280 --> 00:05:43,920
that just cleaved that nucleus
with the same proportion

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00:05:43,920 --> 00:05:45,900
of protons and neutrons.

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How would they decay?

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Or how can they decay?

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There's a couple different ways.

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What do you guys think?

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AUDIENCE: It can emit neutrons.

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MICHAEL SHORT: It
can emit neutrons

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00:05:55,650 --> 00:05:58,255
if it's really
unstable, at which point

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00:05:58,255 --> 00:05:59,880
it would just go down
a neutron number.

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Or how else could it decay?

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00:06:02,632 --> 00:06:03,980
AUDIENCE: Alpha decay.

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00:06:03,980 --> 00:06:05,280
MICHAEL SHORT: Alpha decay.

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00:06:05,280 --> 00:06:07,220
Let's see, yeah, a
lot of those will--

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00:06:07,220 --> 00:06:09,020
the heavier ones tend
to do alpha decay.

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00:06:09,020 --> 00:06:10,670
What would it do at alpha decay?

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00:06:10,670 --> 00:06:14,457
For alpha, I guess it will be
going that direction, right?

146
00:06:14,457 --> 00:06:15,040
You know what?

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00:06:15,040 --> 00:06:16,498
I'm not going to
rule that out yet.

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00:06:16,498 --> 00:06:17,860
So let's go with that.

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00:06:17,860 --> 00:06:19,147
How else could they decay?

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AUDIENCE: Through beta decay.

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MICHAEL SHORT:
Through beta decay,

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let's say in that direction.

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Pretty much all
these happen, just

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not necessarily in this order.

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When you have a really,
really asymmetric nucleus,

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a lot of these
fission products will

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00:06:36,450 --> 00:06:40,280
emit neutrons almost
instantaneously

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00:06:40,280 --> 00:06:43,610
in the realm of like 10
to the minus 17 seconds,

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00:06:43,610 --> 00:06:46,390
some incredibly short timeline.

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You will start to decay
downwards a little bit.

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00:06:50,188 --> 00:06:51,730
But you're not quite
at the stability

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00:06:51,730 --> 00:06:56,980
line, which is why a lot of the
fission products then go on.

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00:06:56,980 --> 00:06:59,020
And they deposit
their kinetic energy

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00:06:59,020 --> 00:07:01,480
by bouncing around the
different atoms in material

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00:07:01,480 --> 00:07:02,650
creating heat.

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00:07:02,650 --> 00:07:08,340
But a lot of them will also
send off betas or gammas.

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And it may take 10 to the
minus 13 seconds for them

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00:07:16,880 --> 00:07:20,150
to whatever the half-life of
that particular isotope is.

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00:07:20,150 --> 00:07:24,740
And after around, let's say,
10 to the minus 10 to 10

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00:07:24,740 --> 00:07:27,230
to the minus 6
seconds, depending

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on the isotope in the medium,
those two fission products

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00:07:31,610 --> 00:07:32,210
will stop.

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00:07:35,250 --> 00:07:37,866
And let's just say
that they stop there.

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So the whole process of
fission, it's actually

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00:07:40,030 --> 00:07:41,890
quite a compound process.

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00:07:41,890 --> 00:07:45,940
First, the neutron is absorbed,
forming a compound nucleus.

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Then it splits apart.

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00:07:47,290 --> 00:07:48,940
Then those individual
fission products

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00:07:48,940 --> 00:07:51,400
undergo whatever
decays suit them best.

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00:07:51,400 --> 00:07:54,250
And that's the source of
the neutrons in fission.

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00:07:54,250 --> 00:07:55,960
Sometimes one of
those fission products

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00:07:55,960 --> 00:07:58,000
might be particularly unstable.

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And it might send
off two neutrons.

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00:08:00,100 --> 00:08:01,930
In other cases, though
I don't know of one

185
00:08:01,930 --> 00:08:03,850
off the top my head,
it might be none.

186
00:08:03,850 --> 00:08:06,550
But this is the whole
timeline of events in fission

187
00:08:06,550 --> 00:08:09,460
and the justification for
why this happens straight

188
00:08:09,460 --> 00:08:12,046
from the first month of 22.01.

189
00:08:12,046 --> 00:08:14,410
And I wanted to pull up
some of the nuclear data

190
00:08:14,410 --> 00:08:17,670
so you can see what these values
tend to look like and also

191
00:08:17,670 --> 00:08:19,520
where to find them.

192
00:08:19,520 --> 00:08:22,430
I'm going to do that
screen cloning thing again.

193
00:08:26,910 --> 00:08:27,930
There we go.

194
00:08:27,930 --> 00:08:31,970
So I've already pre-pulled
up the JANIS library.

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00:08:31,970 --> 00:08:33,950
I've already clicked
on uranium-235.

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Thanks to you guys, I have
all the data now on my shirt

197
00:08:36,708 --> 00:08:38,000
so you can see a little better.

198
00:08:38,000 --> 00:08:39,955
I also have it on the screen.

199
00:08:39,955 --> 00:08:41,330
So let's look at
this value right

200
00:08:41,330 --> 00:08:45,172
here, nu bar total,
neutron production.

201
00:08:45,172 --> 00:08:48,150
And I'll make it bigger
so it's easier to see.

202
00:08:48,150 --> 00:08:50,190
Did I click on the right one?

203
00:08:50,190 --> 00:08:51,300
Yeah.

204
00:08:51,300 --> 00:08:52,330
So take a look at that.

205
00:08:52,330 --> 00:08:57,660
The total number of neutrons
produced during U-235,

206
00:08:57,660 --> 00:09:02,100
for most energies it's
hovering around the 2.4 or so.

207
00:09:02,100 --> 00:09:06,740
There's been arguments about
whether it's 2.43 or 2.44.

208
00:09:06,740 --> 00:09:08,810
And that's a linear scale.

209
00:09:08,810 --> 00:09:09,960
That's not very helpful.

210
00:09:09,960 --> 00:09:12,050
Let's go to a logarithmic scale.

211
00:09:12,050 --> 00:09:14,210
That's more like what
I'm used to seeing.

212
00:09:14,210 --> 00:09:18,860
Most of the fission happens for
U-235 in the thermal region,

213
00:09:18,860 --> 00:09:21,350
in the region where the neutrons
are at values, let's say,

214
00:09:21,350 --> 00:09:24,450
the cutoff is usually about
one electron volt or lower

215
00:09:24,450 --> 00:09:25,820
in average energy.

216
00:09:25,820 --> 00:09:30,620
And nu bar is fantastically
constant at that level.

217
00:09:30,620 --> 00:09:33,140
Then as you go up
and up in energy,

218
00:09:33,140 --> 00:09:35,030
you start to make more
and more neutrons.

219
00:09:35,030 --> 00:09:36,905
Why do you guys think
that would be the case?

220
00:09:42,640 --> 00:09:44,520
What are you doing to
that compound nucleus

221
00:09:44,520 --> 00:09:48,052
as you increase the
incoming neutron energy?

222
00:09:48,052 --> 00:09:49,760
AUDIENCE: It's going
to have more energy.

223
00:09:49,760 --> 00:09:51,690
MICHAEL SHORT: It's going
to have more energy itself.

224
00:09:51,690 --> 00:09:53,420
You might excite
other nuclear states

225
00:09:53,420 --> 00:09:55,460
that can then lead to
other sorts of decays

226
00:09:55,460 --> 00:09:56,960
or other neutron emission.

227
00:09:56,960 --> 00:10:00,740
So to me, that's the reason
why, once you hit about 1 MeV,

228
00:10:00,740 --> 00:10:05,120
you can start to see a lot
more neutrons being given off.

229
00:10:05,120 --> 00:10:07,850
The reason we usually
treat this as a constant,

230
00:10:07,850 --> 00:10:10,510
notice I haven't given
it an energy dependence,

231
00:10:10,510 --> 00:10:12,680
is because most of the
fission that happens

232
00:10:12,680 --> 00:10:14,480
is at thermal energies.

233
00:10:14,480 --> 00:10:18,896
For that, I want to show you
the fission cross section.

234
00:10:18,896 --> 00:10:22,125
There are a lot
of cross sections.

235
00:10:22,125 --> 00:10:24,250
And it's probably going to
be on a different graph,

236
00:10:24,250 --> 00:10:26,013
because it's in different units.

237
00:10:26,013 --> 00:10:27,430
And this gives you
a rough measure

238
00:10:27,430 --> 00:10:29,710
per atom, what's the
probability of fission

239
00:10:29,710 --> 00:10:33,010
happening as a function of
incoming neutron energy?

240
00:10:33,010 --> 00:10:37,060
At those high energies, you have
relatively low cross sections,

241
00:10:37,060 --> 00:10:39,520
or low probabilities,
of fission happening.

242
00:10:39,520 --> 00:10:41,560
Then there's this crazy
resonance region that

243
00:10:41,560 --> 00:10:43,780
looks like a sideways mustache.

244
00:10:43,780 --> 00:10:46,540
But then as you get down
to the lower energy levels,

245
00:10:46,540 --> 00:10:49,150
it gets much more, in
fact, exponentially more,

246
00:10:49,150 --> 00:10:50,800
likely that fission will happen.

247
00:10:50,800 --> 00:10:53,380
So almost all the fissioning
in a light water reactor,

248
00:10:53,380 --> 00:10:55,330
or any sort of other
thermal reactor,

249
00:10:55,330 --> 00:10:57,010
happens at thermal energies.

250
00:10:57,010 --> 00:10:59,870
And that's why we take
nu bar as a constant.

251
00:10:59,870 --> 00:11:02,110
You don't have to,
especially if you're

252
00:11:02,110 --> 00:11:04,255
analyzing what's
called a fast reactor

253
00:11:04,255 --> 00:11:07,780
or a reactor whose neutron
population remains fast

254
00:11:07,780 --> 00:11:09,093
on purpose.

255
00:11:09,093 --> 00:11:10,510
And so with that,
I want to launch

256
00:11:10,510 --> 00:11:14,976
into some of the different types
of reactors that you might see.

257
00:11:14,976 --> 00:11:17,420
And you guys already
did those calculations

258
00:11:17,420 --> 00:11:21,670
in problem set one, so I don't
have to repeat them for you.

259
00:11:21,670 --> 00:11:23,290
Let's get right
into the acronyms.

260
00:11:23,290 --> 00:11:25,440
So if you haven't
figured this out already,

261
00:11:25,440 --> 00:11:29,310
nuclear is a pretty
acronym dense field.

262
00:11:29,310 --> 00:11:34,125
Can anyone say they know all
the acronyms on this slide?

263
00:11:34,125 --> 00:11:37,230
You're going to know about 90%
of them in about 90 minutes.

264
00:11:37,230 --> 00:11:38,370
So it's OK.

265
00:11:38,370 --> 00:11:41,720
Or you'll have
seen them at least.

266
00:11:41,720 --> 00:11:44,772
Any look completely unfamiliar?

267
00:11:44,772 --> 00:11:45,730
AUDIENCE: Most of them.

268
00:11:45,730 --> 00:11:46,380
MICHAEL SHORT: Most of them?

269
00:11:46,380 --> 00:11:47,310
[LAUGHTER]

270
00:11:47,310 --> 00:11:48,895
Well, let's knock them off.

271
00:11:48,895 --> 00:11:50,520
So [INAUDIBLE],, last
Thursday, already

272
00:11:50,520 --> 00:11:53,670
showed you the basic layout
of a boiling water reactor,

273
00:11:53,670 --> 00:11:56,010
one of the types of
light water reactors.

274
00:11:56,010 --> 00:11:58,440
And the reason that this
is a thermal reactor

275
00:11:58,440 --> 00:12:00,180
is because it's full of water.

276
00:12:00,180 --> 00:12:03,480
Water, as we saw in our
old q equation argument,

277
00:12:03,480 --> 00:12:06,120
is very good at
stopping neutrons,

278
00:12:06,120 --> 00:12:08,130
because, if you
guys remember this,

279
00:12:08,130 --> 00:12:12,480
the maximum change in energy
that a neutron can get

280
00:12:12,480 --> 00:12:16,750
is related to alpha times
its incoming energy.

281
00:12:16,750 --> 00:12:24,250
Or this alpha is just A minus
1 over A plus 1 squared.

282
00:12:24,250 --> 00:12:29,050
And I think this would actually
be a 1 minus right there.

283
00:12:29,050 --> 00:12:31,580
A is that mass
number of whatever

284
00:12:31,580 --> 00:12:34,230
the neutrons are hitting.

285
00:12:34,230 --> 00:12:37,140
And that one comes directly
from the neutron mass number.

286
00:12:42,090 --> 00:12:45,270
If you remember, this was
the simplest reduction

287
00:12:45,270 --> 00:12:48,090
of the q equation,
the generalized q

288
00:12:48,090 --> 00:12:50,377
equation for kinematics
that we looked at.

289
00:12:50,377 --> 00:12:51,960
When I said let's
do the general form,

290
00:12:51,960 --> 00:12:53,850
then OK, let's take
the simplest form,

291
00:12:53,850 --> 00:12:55,560
neutron elastic scattering.

292
00:12:55,560 --> 00:12:57,480
Here's where it comes back.

293
00:12:57,480 --> 00:13:01,650
If a neutron hits water, which
is made mostly of hydrogen,

294
00:13:01,650 --> 00:13:07,230
and A is 1, then it can transfer
a maximum of all of its energy,

295
00:13:07,230 --> 00:13:09,477
let's say, to that
hydrogen atom, therefore,

296
00:13:09,477 --> 00:13:12,060
giving the neutron no energy and
thermalizing it or slowing it

297
00:13:12,060 --> 00:13:14,940
down very quickly.

298
00:13:14,940 --> 00:13:17,580
To show you what one of these
things actually looks like,

299
00:13:17,580 --> 00:13:19,818
that's the underside of a BWR.

300
00:13:19,818 --> 00:13:21,360
Did [INAUDIBLE] show
you this before?

301
00:13:21,360 --> 00:13:21,860
OK.

302
00:13:21,860 --> 00:13:25,022
So you've already seen what
this generally looks like.

303
00:13:25,022 --> 00:13:25,980
What about the turbine?

304
00:13:25,980 --> 00:13:29,640
Has anyone actually seen a
turbine this size close up,

305
00:13:29,640 --> 00:13:31,990
a gigawatt electric turbine?

306
00:13:31,990 --> 00:13:34,350
I'm trying to see which one
of those pixels is a person.

307
00:13:38,833 --> 00:13:40,250
I don't see anything
person-sized.

308
00:13:40,250 --> 00:13:43,910
There's a ladder that looks
to be about 6 feet tall,

309
00:13:43,910 --> 00:13:47,500
so to give you guys a sense of
scale of the sort of turbines

310
00:13:47,500 --> 00:13:50,530
that we say, oh, yeah, we
draw a turbine on our diagram.

311
00:13:50,530 --> 00:13:53,380
Well, it's not
actually that simple .

312
00:13:53,380 --> 00:13:55,210
These things take
up entire hallways,

313
00:13:55,210 --> 00:13:57,100
or kind of airport
hangar sized buildings.

314
00:13:57,100 --> 00:13:59,980
I've never seen one in the US,
but I've seen one in Japan.

315
00:13:59,980 --> 00:14:01,450
It was a lot cleaner than this.

316
00:14:01,450 --> 00:14:03,900
But, otherwise, it looked
pretty much the same.

317
00:14:03,900 --> 00:14:05,650
And the way this
actually works, for those

318
00:14:05,650 --> 00:14:08,650
who haven't taken any
thermo classes yet,

319
00:14:08,650 --> 00:14:12,850
is this turbine is full of
different sets of blades that

320
00:14:12,850 --> 00:14:15,520
are curved at an angle so
that when steam shoots in,

321
00:14:15,520 --> 00:14:19,030
it transfers some of its energy
to get the turbine rotating.

322
00:14:19,030 --> 00:14:21,640
And there's going to
be a generator, kind

323
00:14:21,640 --> 00:14:24,770
of like an alternator, to
generate the electricity there,

324
00:14:24,770 --> 00:14:27,940
which looks to be
roughly 100 feet away.

325
00:14:27,940 --> 00:14:30,900
Just to give you a sense
of scale for this stuff.

326
00:14:30,900 --> 00:14:33,207
As [INAUDIBLE] showed you,
a pressurized water reactor

327
00:14:33,207 --> 00:14:35,290
is another kind of light
water reactor with what's

328
00:14:35,290 --> 00:14:37,120
called an indirect cycle.

329
00:14:37,120 --> 00:14:39,190
So this water stays pressurized.

330
00:14:39,190 --> 00:14:42,700
It also stays liquid, which
is good for neutron moderation

331
00:14:42,700 --> 00:14:44,170
or slowing down.

332
00:14:44,170 --> 00:14:48,520
Because in addition to the
probability of any interaction,

333
00:14:48,520 --> 00:14:51,790
some probability sigma, if you
want to get the total reaction

334
00:14:51,790 --> 00:14:56,310
probability, you have to
multiply by its number density

335
00:14:56,310 --> 00:14:58,800
to get a macroscopic
cross section.

336
00:14:58,800 --> 00:15:00,630
This is why I introduce
this stuff way

337
00:15:00,630 --> 00:15:02,310
at the beginning
of class, so you'd

338
00:15:02,310 --> 00:15:05,640
have time to marinate in it and
then bring it back and remember

339
00:15:05,640 --> 00:15:07,420
what it was all about.

340
00:15:07,420 --> 00:15:09,270
And so every single
reaction that

341
00:15:09,270 --> 00:15:14,610
goes on in a nuclear reactor
has got its own cross section.

342
00:15:14,610 --> 00:15:19,400
We'll probably need half
the board for this one.

343
00:15:19,400 --> 00:15:25,740
You can say you have a total
microscopic cross section.

344
00:15:25,740 --> 00:15:29,070
These are all going to be as
a function of neutron energy.

345
00:15:29,070 --> 00:15:31,740
What's the probability of
anything happening at all?

346
00:15:31,740 --> 00:15:36,300
And these are actually tabulated
up on the JANIS website.

347
00:15:36,300 --> 00:15:39,930
So let's unclick that, get
rid of neutron production,

348
00:15:39,930 --> 00:15:44,260
and go all the way to
the top, n comma total.

349
00:15:44,260 --> 00:15:47,020
So all this stuff is written
in nuclear reaction parlance,

350
00:15:47,020 --> 00:15:50,500
where if you have, let's
say, n comma total, that

351
00:15:50,500 --> 00:15:55,320
means a neutron comes in, and
that's the reaction that you're

352
00:15:55,320 --> 00:15:57,530
looking at.

353
00:15:57,530 --> 00:16:01,160
So this data file here,
once I open it up,

354
00:16:01,160 --> 00:16:03,560
will give you the probability
that anything at all

355
00:16:03,560 --> 00:16:05,790
will happen.

356
00:16:05,790 --> 00:16:08,920
You can see as the neutron
energy gets higher,

357
00:16:08,920 --> 00:16:11,070
the probability of
anything happening at all

358
00:16:11,070 --> 00:16:12,900
gets less, and less, and less.

359
00:16:12,900 --> 00:16:15,750
And it follows the shape of most
of the other cross sections.

360
00:16:15,750 --> 00:16:17,910
And I'm going to leave
this up right there.

361
00:16:17,910 --> 00:16:21,150
You've also got a few
different kinds of reactions.

362
00:16:21,150 --> 00:16:24,710
You can have a scatter.

363
00:16:24,710 --> 00:16:29,060
Let's call that scatter,
which we've already said

364
00:16:29,060 --> 00:16:31,520
can either be
elastic or inelastic.

365
00:16:38,000 --> 00:16:39,830
It may not matter
to us from the point

366
00:16:39,830 --> 00:16:43,610
of view of neutron physics
whether the collision

367
00:16:43,610 --> 00:16:46,770
is elastic or inelastic.

368
00:16:46,770 --> 00:16:48,720
All that matters is
the neutron goes in,

369
00:16:48,720 --> 00:16:50,760
and a slower neutron comes out.

370
00:16:50,760 --> 00:16:53,460
Because what we're really
concerned with here

371
00:16:53,460 --> 00:16:57,330
is tracking the full
population of neutrons

372
00:16:57,330 --> 00:16:59,760
at any point in the reactor.

373
00:16:59,760 --> 00:17:01,470
So we'll give this
a position vector

374
00:17:01,470 --> 00:17:07,380
r, which has just
got x, y, and z in it

375
00:17:07,380 --> 00:17:10,710
or whatever other coordinate
system you might happen to use.

376
00:17:10,710 --> 00:17:13,710
I prefer Cartesian,
because it makes sense.

377
00:17:13,710 --> 00:17:17,800
At every energy going
in any direction,

378
00:17:17,800 --> 00:17:20,700
so we now have a
solid angled vector

379
00:17:20,700 --> 00:17:26,187
that's got both theta and
phi in it any given time.

380
00:17:26,187 --> 00:17:28,520
And the whole goal of what
we're going to be doing today

381
00:17:28,520 --> 00:17:30,410
and all of next
week is to find out,

382
00:17:30,410 --> 00:17:33,110
how do you solve
for and simplify

383
00:17:33,110 --> 00:17:34,610
this population of neutrons?

384
00:17:40,900 --> 00:17:42,970
Make sure to fill
that in as velocity.

385
00:17:51,430 --> 00:17:52,370
Let's see.

386
00:17:52,370 --> 00:17:55,300
Let me get back to the
cross sections and stuff.

387
00:17:55,300 --> 00:17:57,400
If we want to know
how many neutrons

388
00:17:57,400 --> 00:18:03,940
are in a certain little volume
element, in some d volume,

389
00:18:03,940 --> 00:18:07,330
in some certain little
increment of energy, dE,

390
00:18:07,330 --> 00:18:11,250
traveling in some very
small, solid angle,

391
00:18:11,250 --> 00:18:14,170
d omega, supposedly, if
you have this function,

392
00:18:14,170 --> 00:18:16,810
then you know the
direction, and location,

393
00:18:16,810 --> 00:18:18,280
and speed of every
single neutron

394
00:18:18,280 --> 00:18:20,110
everywhere in the reactor.

395
00:18:20,110 --> 00:18:22,460
And this is eventually what
the goal of things like Ben

396
00:18:22,460 --> 00:18:24,820
and Kord's group does, the
Computational Reactor Physics

397
00:18:24,820 --> 00:18:28,287
Group, is solve for this or
a simplified version of it,

398
00:18:28,287 --> 00:18:30,370
over, and over, and over
again for different sorts

399
00:18:30,370 --> 00:18:31,780
of geometries.

400
00:18:31,780 --> 00:18:36,100
And in order to do so, you need
to know the rates of reactions

401
00:18:36,100 --> 00:18:37,960
of every kind of
possible reaction

402
00:18:37,960 --> 00:18:40,995
that could take a neutron
out of its current position,

403
00:18:40,995 --> 00:18:43,120
like if it happens to be
moving, which most of them

404
00:18:43,120 --> 00:18:46,390
are, out of its
current energy group.

405
00:18:46,390 --> 00:18:49,420
Which pretty much any reaction
will cause the neutron

406
00:18:49,420 --> 00:18:50,920
to lose energy.

407
00:18:50,920 --> 00:18:52,570
What's the only
reaction we've talked

408
00:18:52,570 --> 00:18:55,210
about where the neutron
loses absolutely no energy?

409
00:19:00,930 --> 00:19:02,567
It's a type of scattering.

410
00:19:02,567 --> 00:19:04,030
AUDIENCE: Forward scattering?

411
00:19:04,030 --> 00:19:06,660
MICHAEL SHORT: Yep,
exactly, forward scattering.

412
00:19:06,660 --> 00:19:08,790
So for forward
scattering for that case

413
00:19:08,790 --> 00:19:11,730
where theta scattering equals 0.

414
00:19:11,730 --> 00:19:15,243
Again, you missed.

415
00:19:15,243 --> 00:19:17,410
The neutron didn't actually
change direction at all.

416
00:19:17,410 --> 00:19:19,940
And, therefore, it didn't
transfer any energy.

417
00:19:19,940 --> 00:19:23,350
But for everything else, for
every other possible reaction,

418
00:19:23,350 --> 00:19:25,990
there's going to be an energy
change associated with it

419
00:19:25,990 --> 00:19:28,960
and probably some
corresponding change in angle,

420
00:19:28,960 --> 00:19:30,970
because a neutron can't
just be moving, and hit

421
00:19:30,970 --> 00:19:34,210
something, and continue
moving more slowly.

422
00:19:34,210 --> 00:19:38,050
There's got to be some change
in momentum to balance along

423
00:19:38,050 --> 00:19:40,020
with that change of energy.

424
00:19:40,020 --> 00:19:42,840
And it might slightly move
in some different direction.

425
00:19:42,840 --> 00:19:45,780
And all this is happening
as a function of time.

426
00:19:45,780 --> 00:19:48,417
As you can see, this gets
pretty hairy pretty quick.

427
00:19:48,417 --> 00:19:50,250
That's why we put the
full equation for this

428
00:19:50,250 --> 00:19:51,877
on our department t-shirts.

429
00:19:51,877 --> 00:19:53,460
But no one ever
solves the full thing.

430
00:19:53,460 --> 00:19:55,050
What we're going to
be going over is,

431
00:19:55,050 --> 00:19:56,610
how do you simplify
it into something

432
00:19:56,610 --> 00:19:59,340
you can solve with a pen
and paper or possibly

433
00:19:59,340 --> 00:20:00,930
a gigantic computer?

434
00:20:00,930 --> 00:20:03,670
But it's not impossible.

435
00:20:03,670 --> 00:20:05,890
So inside this sigma
total, we talked

436
00:20:05,890 --> 00:20:07,570
about different scattering.

437
00:20:07,570 --> 00:20:15,310
And then you could
have absorption in

438
00:20:15,310 --> 00:20:17,860
all its different forms.

439
00:20:17,860 --> 00:20:20,330
What sort of reactions
with a neutron

440
00:20:20,330 --> 00:20:21,580
would cause it to be absorbed?

441
00:20:27,036 --> 00:20:27,980
AUDIENCE: Fission.

442
00:20:27,980 --> 00:20:29,580
MICHAEL SHORT: Yes, fission.

443
00:20:29,580 --> 00:20:31,130
Thank you.

444
00:20:31,130 --> 00:20:34,370
So there's going to be some
sigma fission cross section

445
00:20:34,370 --> 00:20:35,450
as a function of energy.

446
00:20:38,130 --> 00:20:42,260
And if it doesn't fizz,
but it is absorbed,

447
00:20:42,260 --> 00:20:43,260
we'll call that capture.

448
00:20:49,320 --> 00:20:51,750
But capture can mean a whole
bunch of different things

449
00:20:51,750 --> 00:20:52,505
too, right?

450
00:20:52,505 --> 00:20:53,880
There could be
also a whole bunch

451
00:20:53,880 --> 00:20:56,280
of other nuclear reactions.

452
00:20:56,280 --> 00:21:00,940
There could be a reaction
where one neutron comes in,

453
00:21:00,940 --> 00:21:05,500
two neutrons go out, like
we looked at with beryllium

454
00:21:05,500 --> 00:21:07,930
in the Chadwick paper
from the first day

455
00:21:07,930 --> 00:21:13,260
or like what actually
does exist for this stuff.

456
00:21:13,260 --> 00:21:15,790
So JANIS doesn't
like multi-touch,

457
00:21:15,790 --> 00:21:19,870
so you have to bear with me on
the small print on the screen.

458
00:21:19,870 --> 00:21:21,715
But there should
be-- yep, here it is.

459
00:21:21,715 --> 00:21:25,480
Cross section number 16,
there is a probability

460
00:21:25,480 --> 00:21:26,980
that one neutron goes in.

461
00:21:26,980 --> 00:21:31,060
That z right there is whatever
your incoming particle

462
00:21:31,060 --> 00:21:31,870
happens to be.

463
00:21:31,870 --> 00:21:34,180
And in this case, we know
it's a neutron, because we

464
00:21:34,180 --> 00:21:35,940
picked incident neutron data.

465
00:21:35,940 --> 00:21:38,920
And 2n means two
neutrons come out.

466
00:21:38,920 --> 00:21:42,090
Let's plot that cross section.

467
00:21:42,090 --> 00:21:46,890
You can see that the value is
0 until you hit about 4 or 5.

468
00:21:46,890 --> 00:21:52,290
Oh, it's actually 5.297781 MeV.

469
00:21:52,290 --> 00:21:54,960
So that's the q value at
which this particular reaction

470
00:21:54,960 --> 00:21:57,130
happens to turn on.

471
00:21:57,130 --> 00:22:00,370
Might be responsible for
a little bit of the blip

472
00:22:00,370 --> 00:22:02,200
in the total cross section.

473
00:22:02,200 --> 00:22:05,185
So technically, if we were
to turn on every single cross

474
00:22:05,185 --> 00:22:09,580
section in this database, it
should add up to that red line

475
00:22:09,580 --> 00:22:11,260
right there.

476
00:22:11,260 --> 00:22:14,500
So you can start to get an
idea for how much of all

477
00:22:14,500 --> 00:22:17,770
the reactions of uranium-235
are due to fission.

478
00:22:17,770 --> 00:22:20,550
That's the one we
want to exploit.

479
00:22:20,550 --> 00:22:24,690
So let's find fission,
right down there.

480
00:22:24,690 --> 00:22:26,420
Oh, wow, there's a 3n reaction.

481
00:22:26,420 --> 00:22:28,670
I want to see that.

482
00:22:28,670 --> 00:22:32,240
That doesn't happen
until 12 MeV.

483
00:22:32,240 --> 00:22:32,940
Yeah.

484
00:22:32,940 --> 00:22:35,700
So neutrons don't
typically tend to hit

485
00:22:35,700 --> 00:22:38,880
12 MeV in a fission reactor.

486
00:22:38,880 --> 00:22:41,400
So this is a perfect
flimsy pretext

487
00:22:41,400 --> 00:22:43,470
to bring in another variable.

488
00:22:43,470 --> 00:22:48,140
It's called the chi spectrum or
what's called the fission birth

489
00:22:48,140 --> 00:22:48,640
spectrum.

490
00:22:54,702 --> 00:22:55,202
Yeah.

491
00:22:59,090 --> 00:23:01,490
We've already talked about
the neutrons being born

492
00:23:01,490 --> 00:23:02,990
and how many there were.

493
00:23:02,990 --> 00:23:06,410
But we didn't say at
what energy they're born.

494
00:23:06,410 --> 00:23:11,590
In fusion reactors,
this is pretty simple.

495
00:23:11,590 --> 00:23:14,040
You've already
looked at this case.

496
00:23:14,040 --> 00:23:14,540
What is it?

497
00:23:14,540 --> 00:23:16,790
14.7 MeV.

498
00:23:16,790 --> 00:23:18,500
That's a lot simpler.

499
00:23:18,500 --> 00:23:20,810
That's the fusion.

500
00:23:20,810 --> 00:23:22,820
For fission, it's not so simple.

501
00:23:25,340 --> 00:23:29,450
For the case of fission, if
you draw energy versus this chi

502
00:23:29,450 --> 00:23:35,890
spectrum, it takes an
interesting looking curve

503
00:23:35,890 --> 00:23:42,590
from about 1 MeV to about 10
MeV with the most likely energy

504
00:23:42,590 --> 00:23:44,930
being around 2 MeV.

505
00:23:44,930 --> 00:23:48,050
So you aren't really
going to get neutrons

506
00:23:48,050 --> 00:23:52,040
at the energy required for a 3n
reaction in a regular fission

507
00:23:52,040 --> 00:23:54,790
reactor, just not
going to happen.

508
00:23:54,790 --> 00:23:57,140
But it's good that you
know that that exists.

509
00:23:57,140 --> 00:23:59,310
So let's go and answer
my original question.

510
00:23:59,310 --> 00:24:01,850
How much of the total cross
section is due to fission?

511
00:24:07,280 --> 00:24:10,850
Most of it, especially
at low energies.

512
00:24:10,850 --> 00:24:12,890
So let me get rid of
those 2n and 3n ones,

513
00:24:12,890 --> 00:24:16,110
because they're kind
of ruining our data.

514
00:24:16,110 --> 00:24:17,500
It's making it harder to see.

515
00:24:23,810 --> 00:24:25,730
That's better.

516
00:24:25,730 --> 00:24:30,860
So you can see at energies below
around, let's say, a keV or so,

517
00:24:30,860 --> 00:24:33,650
almost all of the reactions
happening with neutrons

518
00:24:33,650 --> 00:24:35,720
in uranium-235 are fission.

519
00:24:35,720 --> 00:24:37,970
This is part of what makes
it such a particularly good

520
00:24:37,970 --> 00:24:39,980
isotope to use in reactors.

521
00:24:39,980 --> 00:24:42,560
The other one is, you can
find it in the ground,

522
00:24:42,560 --> 00:24:46,440
unlike most of the other
fissile isotopes, unlike,

523
00:24:46,440 --> 00:24:49,020
I think, any of the
other fissile isotopes.

524
00:24:49,020 --> 00:24:52,450
Thorium you got to breed and
turn it into uranium-233.

525
00:24:52,450 --> 00:24:55,002
I'll have to think
about that one.

526
00:24:55,002 --> 00:24:56,460
But then you start
to look at, what

527
00:24:56,460 --> 00:24:59,190
are the other components
of this cross section,

528
00:24:59,190 --> 00:25:03,600
like zn prime, inelastic
scattering, which

529
00:25:03,600 --> 00:25:09,600
doesn't turn on until
about 0.002 MeV,

530
00:25:09,600 --> 00:25:12,180
but later on is one of
the major contributors

531
00:25:12,180 --> 00:25:13,810
and actually is
responsible for--

532
00:25:13,810 --> 00:25:15,750
wait, I've brought
this for a reason.

533
00:25:15,750 --> 00:25:18,855
--is responsible for that
little bump in the total cross

534
00:25:18,855 --> 00:25:19,710
section.

535
00:25:19,710 --> 00:25:22,740
So eventually all
these things do matter.

536
00:25:22,740 --> 00:25:25,520
But let's think about which
ones we actually care about

537
00:25:25,520 --> 00:25:30,080
at all, because what we
eventually want to do

538
00:25:30,080 --> 00:25:34,850
is develop some sort of
neutron balance equation.

539
00:25:34,850 --> 00:25:37,310
If we can measure the change
in the number of neutrons

540
00:25:37,310 --> 00:25:45,140
as a function of position,
energy, angle, and time,

541
00:25:45,140 --> 00:25:47,720
as a function of time,
and that would probably

542
00:25:47,720 --> 00:25:50,780
be a partial derivative, because
there are like seven variables

543
00:25:50,780 --> 00:25:52,550
here.

544
00:25:52,550 --> 00:25:54,270
Before I write any
equations, it's

545
00:25:54,270 --> 00:25:56,645
just going to be a measure of
the gains minus the losses.

546
00:26:00,070 --> 00:26:02,840
And while every particular
reaction has its own cross

547
00:26:02,840 --> 00:26:06,610
section, there's only going to
be a few that we care about.

548
00:26:06,610 --> 00:26:09,210
There will only be
one or two types

549
00:26:09,210 --> 00:26:12,360
of reactions that can result
in a gain of the neutron

550
00:26:12,360 --> 00:26:15,720
population into a certain
volume with a certain energy

551
00:26:15,720 --> 00:26:17,460
with a certain angle.

552
00:26:17,460 --> 00:26:22,140
And for losses, there's only
one we really care about, total,

553
00:26:22,140 --> 00:26:24,480
because any interaction
with a neutron

554
00:26:24,480 --> 00:26:27,240
is going to cause
that neutron to leave

555
00:26:27,240 --> 00:26:31,797
this little group of perfect
position, energy, and angle.

556
00:26:35,140 --> 00:26:36,670
So that's where we're going.

557
00:26:36,670 --> 00:26:39,520
We'll probably start down
that route on Tuesday,

558
00:26:39,520 --> 00:26:41,790
because I promised you
guys context today.

559
00:26:45,730 --> 00:26:47,660
You've all been to the
MIT Research Reactor.

560
00:26:47,660 --> 00:26:50,110
A couple of you-- are
you running it yet?

561
00:26:50,110 --> 00:26:50,752
AUDIENCE: Yeah.

562
00:26:50,752 --> 00:26:51,710
MICHAEL SHORT: Awesome.

563
00:26:51,710 --> 00:26:52,590
OK.

564
00:26:52,590 --> 00:26:54,110
Yeah.

565
00:26:54,110 --> 00:26:55,810
Yeah, so Sarah and
Jared are doing that.

566
00:26:55,810 --> 00:26:58,490
Anyone else training or trained?

567
00:26:58,490 --> 00:26:59,860
No.

568
00:26:59,860 --> 00:27:02,640
I'd say folks are usually
pretty scared when they find out

569
00:27:02,640 --> 00:27:03,670
MIT has a reactor.

570
00:27:03,670 --> 00:27:04,750
And they're even
more scared when

571
00:27:04,750 --> 00:27:06,000
they find out you guys run it.

572
00:27:06,000 --> 00:27:06,670
AUDIENCE: Yeah.

573
00:27:06,670 --> 00:27:08,253
MICHAEL SHORT: What
they don't realize

574
00:27:08,253 --> 00:27:10,965
is there's been basically
no problems since 1954.

575
00:27:10,965 --> 00:27:12,340
The only one I
know of is someone

576
00:27:12,340 --> 00:27:14,350
fell asleep at the
controls once and forgot

577
00:27:14,350 --> 00:27:16,480
to push the Don't
Call Fox News button,

578
00:27:16,480 --> 00:27:19,020
and it called Fox
News or something.

579
00:27:19,020 --> 00:27:22,000
So there was a big story
about, asleep at the helm,

580
00:27:22,000 --> 00:27:26,110
ignoring all of the alarms,
and passive safety systems,

581
00:27:26,110 --> 00:27:28,010
and backup operators,
and everything else

582
00:27:28,010 --> 00:27:30,550
that actually made sure
that nothing happened.

583
00:27:30,550 --> 00:27:32,137
But nowadays, correct
me if I'm wrong,

584
00:27:32,137 --> 00:27:33,970
you actually have to
get up every half hour,

585
00:27:33,970 --> 00:27:35,920
reach around a panel,
and hit a button, right?

586
00:27:35,920 --> 00:27:36,490
AUDIENCE: No.

587
00:27:36,490 --> 00:27:38,900
It's on console,
but it beeps at you.

588
00:27:38,900 --> 00:27:39,760
MICHAEL SHORT: Ah.

589
00:27:39,760 --> 00:27:41,870
AUDIENCE: Yeah,
it's pretty tiring.

590
00:27:41,870 --> 00:27:45,290
MICHAEL SHORT: So you want to
hit it before it beeps at you.

591
00:27:45,290 --> 00:27:47,650
AUDIENCE: It's reminding
you to take hourly logs.

592
00:27:47,650 --> 00:27:48,400
MICHAEL SHORT: OK.

593
00:27:48,400 --> 00:27:50,108
AUDIENCE: It does go
off every half hour.

594
00:27:50,108 --> 00:27:52,893
AUDIENCE: It is half hour, but
you we don't do [INAUDIBLE]..

595
00:27:52,893 --> 00:27:54,060
MICHAEL SHORT: Ah, OK, yeah.

596
00:27:54,060 --> 00:27:57,040
I'd heard the button's
every half hour.

597
00:27:57,040 --> 00:27:58,090
Gotcha.

598
00:27:58,090 --> 00:27:58,990
Cool.

599
00:27:58,990 --> 00:28:02,110
Yeah, so for all of you watching
on camera or whatever, just

600
00:28:02,110 --> 00:28:06,150
know that these guys
got it under control.

601
00:28:06,150 --> 00:28:08,400
So onto some gas cooled
reactors and to explain

602
00:28:08,400 --> 00:28:09,780
some of these acronyms.

603
00:28:09,780 --> 00:28:13,133
There are some that use natural
uranium, though pretty much all

604
00:28:13,133 --> 00:28:14,550
the ones in this
country, you need

605
00:28:14,550 --> 00:28:16,590
to enrich the
uranium to get enough

606
00:28:16,590 --> 00:28:19,350
U-235 to turn the reaction on.

607
00:28:19,350 --> 00:28:22,890
But you don't have to
do that in every case.

608
00:28:22,890 --> 00:28:26,550
And you'll also see these
acronyms, LEU, MEU, or HEU,

609
00:28:26,550 --> 00:28:30,420
standing for Low, Medium,
or High Enrichment.

610
00:28:30,420 --> 00:28:33,240
The accepted standard for
what's low enriched uranium

611
00:28:33,240 --> 00:28:35,780
is 20% or below.

612
00:28:35,780 --> 00:28:37,470
An interesting fact,
though, you can't

613
00:28:37,470 --> 00:28:42,930
have something at
19.99% enriched uranium

614
00:28:42,930 --> 00:28:45,245
and expect it to be
low enriched uranium,

615
00:28:45,245 --> 00:28:47,370
because every measurement
technique has some error.

616
00:28:47,370 --> 00:28:48,787
And what really
determines if it's

617
00:28:48,787 --> 00:28:51,870
LEU is when an inspector
comes and takes a sample,

618
00:28:51,870 --> 00:28:55,620
it better be below 20%
including their error.

619
00:28:55,620 --> 00:29:00,570
So you'll usually see 19.75%
given as the LEU limit,

620
00:29:00,570 --> 00:29:02,850
because there's always
some processing error,

621
00:29:02,850 --> 00:29:05,160
inhomogeneities,
measurement error.

622
00:29:05,160 --> 00:29:07,950
Hedge your bets, pretty much.

623
00:29:07,950 --> 00:29:10,770
Like in England or the UK,
the advanced gas reactors

624
00:29:10,770 --> 00:29:13,110
have been churning
along for decades.

625
00:29:13,110 --> 00:29:16,090
They actually use
CO2 as the coolant,

626
00:29:16,090 --> 00:29:18,000
which is relatively inert.

627
00:29:18,000 --> 00:29:20,240
And they use graphite
as the moderator.

628
00:29:20,240 --> 00:29:22,740
So in this case, the coolant
and the moderator are separate,

629
00:29:22,740 --> 00:29:25,230
unlike the light water
reactors we have.

630
00:29:25,230 --> 00:29:28,920
So this way, the graphite, right
here, just sits in solid form

631
00:29:28,920 --> 00:29:31,890
and slows down the neutrons,
not quite as good as water,

632
00:29:31,890 --> 00:29:33,210
but pretty good.

633
00:29:33,210 --> 00:29:38,100
There is an issue, though, that
CO2, just like anything, has

634
00:29:38,100 --> 00:29:44,410
a natural decomposition
reaction, where CO2 naturally

635
00:29:44,410 --> 00:29:49,390
is in equilibrium
with CO and O2.

636
00:29:49,390 --> 00:29:55,090
And O2 plus graphite
yields CO2 gas.

637
00:29:55,090 --> 00:29:57,647
Graphite was solid.

638
00:29:57,647 --> 00:29:59,980
In talking with a couple folks
from the National Nuclear

639
00:29:59,980 --> 00:30:02,470
Laboratory, they said
that 40 years later, when

640
00:30:02,470 --> 00:30:05,110
they took the caps
off these reactors,

641
00:30:05,110 --> 00:30:10,410
a lot of that graphite was just
gone with a good explanation.

642
00:30:10,410 --> 00:30:15,360
It vaporized very, very, very
slowly over 40 years or so due

643
00:30:15,360 --> 00:30:19,650
to this natural recombination
with whatever little bit of O2

644
00:30:19,650 --> 00:30:22,820
is in equilibrium with CO2
and possibly some other leaks.

645
00:30:22,820 --> 00:30:25,320
I'm sure I wouldn't have been
told that if there was a leak.

646
00:30:25,320 --> 00:30:27,510
So I'd say the
feasibility is high,

647
00:30:27,510 --> 00:30:30,300
because they've been running
for almost half a century.

648
00:30:30,300 --> 00:30:32,722
The power density is very low.

649
00:30:32,722 --> 00:30:34,305
Why do you guys think
that's the case?

650
00:30:37,548 --> 00:30:38,534
Yeah.

651
00:30:38,534 --> 00:30:43,464
AUDIENCE: [INAUDIBLE]

652
00:30:43,464 --> 00:30:44,450
MICHAEL SHORT: Mm-hm.

653
00:30:44,450 --> 00:30:47,057
AUDIENCE: [INAUDIBLE]

654
00:30:47,057 --> 00:30:48,140
MICHAEL SHORT: Absolutely.

655
00:30:48,140 --> 00:30:50,920
So well, let's say, you need
the same cooling capacity,

656
00:30:50,920 --> 00:30:51,870
but you're right.

657
00:30:51,870 --> 00:30:55,230
CO2, even if pressurized, is not
as good a heat transfer medium

658
00:30:55,230 --> 00:30:55,730
as water.

659
00:30:55,730 --> 00:30:57,200
Water is dense.

660
00:30:57,200 --> 00:30:59,700
It's also got one of the highest
heat capacities of anything

661
00:30:59,700 --> 00:31:01,050
we've ever seen.

662
00:31:01,050 --> 00:31:04,020
The other reason is right here.

663
00:31:04,020 --> 00:31:05,655
If you want enough
reaction density,

664
00:31:05,655 --> 00:31:09,240
then it not only matters
what the per atom density is,

665
00:31:09,240 --> 00:31:12,150
but what the number density is.

666
00:31:12,150 --> 00:31:14,490
And if you're using
gaseous CO2 coolant,

667
00:31:14,490 --> 00:31:17,310
even if it's pressurized,
there are fewer reactions

668
00:31:17,310 --> 00:31:19,440
happening per unit
volume, because there

669
00:31:19,440 --> 00:31:23,880
are few CO2 molecules per unit
volume than water would have.

670
00:31:23,880 --> 00:31:27,060
So that's why we pressurize
our light water reactors,

671
00:31:27,060 --> 00:31:28,560
to keep water in
its liquid state

672
00:31:28,560 --> 00:31:31,440
where it's a great heat
absorber, takes a lot of energy

673
00:31:31,440 --> 00:31:33,180
to boil it, and
it's really dense

674
00:31:33,180 --> 00:31:37,524
so it's a very effective
dense moderator.

675
00:31:37,524 --> 00:31:38,925
These have been around forever.

676
00:31:38,925 --> 00:31:39,540
Let me think.

677
00:31:39,540 --> 00:31:42,000
When did Windscale happen?

678
00:31:42,000 --> 00:31:45,870
Windscale was also the
source of an interesting fire

679
00:31:45,870 --> 00:31:47,560
that you guys might
want to know about.

680
00:31:47,560 --> 00:31:50,010
It's one of those
only nuclear disasters

681
00:31:50,010 --> 00:31:53,190
that hit 7 on the
arbitrary unit scale.

682
00:31:53,190 --> 00:31:56,010
I don't quite know how they
determine what's a seven.

683
00:31:56,010 --> 00:31:58,380
But there was a fire
at the Windscale plant

684
00:31:58,380 --> 00:32:01,750
due to the build up of
what's called Wigner energy.

685
00:32:01,750 --> 00:32:04,380
It turns out that when
neutrons go slamming around

686
00:32:04,380 --> 00:32:08,580
in the graphite, they leave
behind radiation damage.

687
00:32:08,580 --> 00:32:11,340
And when my family always
asks me to explain,

688
00:32:11,340 --> 00:32:12,540
what do you do for a living?

689
00:32:12,540 --> 00:32:15,780
And I can only think, well, they
don't know radiation damage.

690
00:32:15,780 --> 00:32:17,340
They've watched Harry Potter.

691
00:32:17,340 --> 00:32:20,760
I'd like to say, radiation,
like dark magic, leaves traces.

692
00:32:20,760 --> 00:32:22,380
Well, it leaves
traces in the graphite

693
00:32:22,380 --> 00:32:27,150
in the form of atomic defects,
which took energy to create.

694
00:32:27,150 --> 00:32:29,820
So by causing damage
to the graphite,

695
00:32:29,820 --> 00:32:32,850
you store energy in it, which
is known as Wigner energy.

696
00:32:32,850 --> 00:32:35,730
And you can store so
much that it just catches

697
00:32:35,730 --> 00:32:37,770
fire and explodes sometimes.

698
00:32:37,770 --> 00:32:40,080
That's what happened
here at Windscale.

699
00:32:40,080 --> 00:32:42,397
11 tons of uranium
ended up burning,

700
00:32:42,397 --> 00:32:44,730
because all of a sudden, the
temperature in the graphite

701
00:32:44,730 --> 00:32:47,280
just started going up
for no reason, no reason

702
00:32:47,280 --> 00:32:48,842
that they understood
at the time.

703
00:32:48,842 --> 00:32:51,300
It turns out that they had
built up enough radiation damage

704
00:32:51,300 --> 00:32:54,480
energy that it started
releasing more heat.

705
00:32:54,480 --> 00:32:57,240
And releasing more heat
caused more of that energy

706
00:32:57,240 --> 00:33:00,240
to be released, and it
was self-perpetuating

707
00:33:00,240 --> 00:33:03,930
until it just caught fire and
burned 11 tons of uranium out

708
00:33:03,930 --> 00:33:04,810
in the countryside.

709
00:33:04,810 --> 00:33:06,900
This was 1957.

710
00:33:06,900 --> 00:33:12,900
So again, a 7 on the scale with
no units of nuclear disasters.

711
00:33:12,900 --> 00:33:15,090
Argue it's probably not
as bad as Chernobyl,

712
00:33:15,090 --> 00:33:19,900
so they might want a little bit
of resolution in that scale.

713
00:33:19,900 --> 00:33:22,840
There's another type of gas cool
reactor called the Pebble Bed

714
00:33:22,840 --> 00:33:25,720
Modular Reactor, a much more
up and coming one, where

715
00:33:25,720 --> 00:33:28,570
each fuel element-- you
don't have fuel rods.

716
00:33:28,570 --> 00:33:30,370
You've actually
got little pebbles

717
00:33:30,370 --> 00:33:32,570
full of tiny kernels of fuel.

718
00:33:32,570 --> 00:33:34,630
So you've got a built-in
graphite moderator

719
00:33:34,630 --> 00:33:38,110
tennis ball sized thing with
lots of little grains of sand

720
00:33:38,110 --> 00:33:41,920
of UO2 cooled by
a bed of flowing

721
00:33:41,920 --> 00:33:44,050
helium or something like that.

722
00:33:44,050 --> 00:33:46,420
And then that helium,
or the other gas,

723
00:33:46,420 --> 00:33:49,660
transfers heat to water,
which goes in to make steam

724
00:33:49,660 --> 00:33:53,514
and goes into the turbine
like I showed you before.

725
00:33:53,514 --> 00:33:55,940
So this is what the fuel
actually looks like.

726
00:33:55,940 --> 00:33:58,190
Inside each one of
these tennis ball

727
00:33:58,190 --> 00:34:00,650
spheres of mostly
graphite, there's

728
00:34:00,650 --> 00:34:02,870
these little kernels
of uranium dioxide

729
00:34:02,870 --> 00:34:05,540
about a half a
millimeter across covered

730
00:34:05,540 --> 00:34:08,420
in layers of silicon
carbide, a really strong

731
00:34:08,420 --> 00:34:11,920
and dense material that keeps
the fission products in,

732
00:34:11,920 --> 00:34:14,790
because the biggest
danger from nuclear fuel

733
00:34:14,790 --> 00:34:18,510
is the highly radioactive
fission products that

734
00:34:18,510 --> 00:34:20,429
due to their instability
are giving off

735
00:34:20,429 --> 00:34:24,060
all sorts of awful, for
anywhere from milliseconds

736
00:34:24,060 --> 00:34:27,747
to mega years, after
reactor operation.

737
00:34:27,747 --> 00:34:29,580
And so if you keep those
out of the coolant,

738
00:34:29,580 --> 00:34:32,489
then the coolant stays
relatively nonradioactive.

739
00:34:32,489 --> 00:34:36,962
And it's safe to do things
like maintain the plant.

740
00:34:36,962 --> 00:34:38,920
Then there's the very
high temperature reactor,

741
00:34:38,920 --> 00:34:41,500
the ultimate in
acronym creativity.

742
00:34:41,500 --> 00:34:43,850
It operates at a very
high temperature,

743
00:34:43,850 --> 00:34:46,150
which has been
steadily decreasing

744
00:34:46,150 --> 00:34:50,440
over time, as reality has
caught up to expectations.

745
00:34:50,440 --> 00:34:52,630
When I first got into this
field, they were saying,

746
00:34:52,630 --> 00:34:55,272
we're going to run
this at 1100 Celsius.

747
00:34:55,272 --> 00:34:56,980
Then I started studying
material science.

748
00:34:56,980 --> 00:35:00,250
And I was like, yeah, nothing
wants to be 1100 Celsius.

749
00:35:00,250 --> 00:35:02,650
By that time, they
downgraded it to 1000.

750
00:35:02,650 --> 00:35:06,040
Now they've asymptoted
it at around 800 or 850

751
00:35:06,040 --> 00:35:10,570
due to some actual problems
in operating things in helium.

752
00:35:10,570 --> 00:35:13,070
It's not the helium itself, but
the impurities in the helium

753
00:35:13,070 --> 00:35:14,915
that could really mess you up.

754
00:35:14,915 --> 00:35:16,290
And the sorts of
alloys that they

755
00:35:16,290 --> 00:35:19,890
need to get this working,
these nickel superalloys,

756
00:35:19,890 --> 00:35:22,440
like Alloy 230,
they can slightly

757
00:35:22,440 --> 00:35:24,510
carburize or
decarburize depending

758
00:35:24,510 --> 00:35:26,760
on the amount of carbon
in the helium coolant.

759
00:35:26,760 --> 00:35:31,230
Either way you go, you lose
the strength that you need.

760
00:35:31,230 --> 00:35:33,100
So I'll say feasibility
is low to medium,

761
00:35:33,100 --> 00:35:37,122
because, well, we haven't
really seen one of these yet.

762
00:35:37,122 --> 00:35:38,455
Then onto water cooled reactors.

763
00:35:38,455 --> 00:35:40,240
Has anyone here
heard of the reactors

764
00:35:40,240 --> 00:35:43,340
they have in Canada,
the CANDU reactors?

765
00:35:43,340 --> 00:35:44,740
That's my favorite acronym.

766
00:35:44,740 --> 00:35:46,760
I hope that was intentional.

767
00:35:46,760 --> 00:35:47,260
It what?

768
00:35:47,260 --> 00:35:48,220
AUDIENCE: It's convenient.

769
00:35:48,220 --> 00:35:49,053
MICHAEL SHORT: Yeah.

770
00:35:49,053 --> 00:35:51,100
[LAUGHS] It's not like the--

771
00:35:51,100 --> 00:35:54,100
well, they're not sorry
about anything, but whatever.

772
00:35:54,100 --> 00:35:56,800
At any rate, one of the
nice features about this

773
00:35:56,800 --> 00:36:00,640
is you can actually use natural
uranium, because the moderator

774
00:36:00,640 --> 00:36:02,063
is heavy water.

775
00:36:02,063 --> 00:36:04,480
You have to look into what the
sort of cross sections are.

776
00:36:04,480 --> 00:36:09,490
Even though deuterium
won't slow down neutrons

777
00:36:09,490 --> 00:36:12,220
as much as hydrogen will--
where did my alpha thing-- oh,

778
00:36:12,220 --> 00:36:13,850
it was right here all along.

779
00:36:13,850 --> 00:36:17,070
Even though A is 2 instead
of 1 for deuterium,

780
00:36:17,070 --> 00:36:19,990
it's absorption cross section,
or specifically-- yeah,

781
00:36:19,990 --> 00:36:21,320
because it doesn't fission.

782
00:36:21,320 --> 00:36:26,257
Its absorption cross section is
way lower than that of water.

783
00:36:26,257 --> 00:36:28,090
It actually functions
as a better moderator,

784
00:36:28,090 --> 00:36:31,570
because fewer of those
collisions are absorption.

785
00:36:31,570 --> 00:36:34,150
And because you have a better
neutron population and less

786
00:36:34,150 --> 00:36:36,880
absorption, you don't need
to enrich your uranium.

787
00:36:36,880 --> 00:36:39,770
You also don't need to
pressurize your moderator.

788
00:36:39,770 --> 00:36:43,000
So you can flow some other
coolant through these pressure

789
00:36:43,000 --> 00:36:46,810
tubes and just have a big tank
of close to something room

790
00:36:46,810 --> 00:36:50,460
temperature unpressurized
D2O as your moderator.

791
00:36:50,460 --> 00:36:55,300
The problem with that
is D2O is expensive.

792
00:36:55,300 --> 00:36:59,415
Anyone priced out
deuterium oxide before?

793
00:36:59,415 --> 00:37:01,040
Probably have at the
reactor, because I

794
00:37:01,040 --> 00:37:03,619
know you have drums of it.

795
00:37:03,619 --> 00:37:07,323
AUDIENCE: It's like a couple
thousand per kilogram.

796
00:37:07,323 --> 00:37:08,990
MICHAEL SHORT: A
couple thousand a kilo,

797
00:37:08,990 --> 00:37:10,910
it's an expensive
bottle of water.

798
00:37:10,910 --> 00:37:14,260
It'll also mess you up if you
drink it, because a lot of it,

799
00:37:14,260 --> 00:37:16,150
even if it's crystal
clear, filtered

800
00:37:16,150 --> 00:37:19,690
D2O, a lot of what the
cellular machinery depends

801
00:37:19,690 --> 00:37:23,350
on the diffusion coefficients
of various things in water,

802
00:37:23,350 --> 00:37:25,540
those solutes in water.

803
00:37:25,540 --> 00:37:27,820
And if you change the
mass of the water,

804
00:37:27,820 --> 00:37:30,320
then the diffusion coefficients
of the water itself,

805
00:37:30,320 --> 00:37:32,400
as well as the things
in it, will change.

806
00:37:32,400 --> 00:37:35,200
And if you depend on, let's
say, exact sodium and potassium

807
00:37:35,200 --> 00:37:37,330
concentrations for your
nerves to function,

808
00:37:37,330 --> 00:37:39,310
a little change in
that can go a long way

809
00:37:39,310 --> 00:37:42,170
towards giving you a bad day.

810
00:37:42,170 --> 00:37:44,030
And actually, we have
a little piece of one

811
00:37:44,030 --> 00:37:47,100
of these pressure tubes upstairs
if anyone wants to take a look.

812
00:37:47,100 --> 00:37:49,100
There's all these
sealed fuel bundles

813
00:37:49,100 --> 00:37:51,770
inside what they call
a calandria tube,

814
00:37:51,770 --> 00:37:54,590
just a pressurized
tube that's horizontal.

815
00:37:54,590 --> 00:37:56,390
The problem with
some of these is

816
00:37:56,390 --> 00:37:58,730
if these spacers get
knocked out of place, which

817
00:37:58,730 --> 00:38:00,650
they do all the
time, those tubes

818
00:38:00,650 --> 00:38:03,050
can start to creep
downward and get

819
00:38:03,050 --> 00:38:06,830
a little harder to cool or touch
the sides and change thermal.

820
00:38:06,830 --> 00:38:09,240
And now I'm getting
into material science.

821
00:38:09,240 --> 00:38:11,320
It's a mess.

822
00:38:11,320 --> 00:38:16,000
Then there's the old RBMK, the
reactor that caused Chernobyl.

823
00:38:16,000 --> 00:38:20,230
You can also use natural uranium
or low enriched uranium here.

824
00:38:20,230 --> 00:38:22,660
The problem though that
led to Chernobyl-- one

825
00:38:22,660 --> 00:38:25,450
of the many problems that
led to Chernobyl was,

826
00:38:25,450 --> 00:38:27,380
you've got all this
moderator right here.

827
00:38:27,380 --> 00:38:30,340
So if you lose your coolant,
let's say you had a light water

828
00:38:30,340 --> 00:38:33,580
reactor and your coolant goes
away, your moderator also

829
00:38:33,580 --> 00:38:38,140
goes away, which means your
neutrons don't slow down

830
00:38:38,140 --> 00:38:39,190
anymore.

831
00:38:39,190 --> 00:38:41,680
That one reaction is messing up.

832
00:38:41,680 --> 00:38:42,580
There we go.

833
00:38:42,580 --> 00:38:45,140
Which means your neutrons
don't slow down anymore,

834
00:38:45,140 --> 00:38:47,140
which means the probability
of fission happening

835
00:38:47,140 --> 00:38:50,320
could be like
10,000 times lower.

836
00:38:50,320 --> 00:38:52,858
So losing coolant in
a light water reactor,

837
00:38:52,858 --> 00:38:54,650
temperature might go
up, but it's not going

838
00:38:54,650 --> 00:38:56,690
to give you a nuclear bad day.

839
00:38:56,690 --> 00:39:00,340
In the RBMK reactor,
it will and it did.

840
00:39:00,340 --> 00:39:02,860
And in addition,
the control rods,

841
00:39:02,860 --> 00:39:05,950
which were supposed to shut down
the reaction, made of things

842
00:39:05,950 --> 00:39:08,590
like boron 4 carbide,
or hafnium, or something

843
00:39:08,590 --> 00:39:12,390
with a really high
capture cross section

844
00:39:12,390 --> 00:39:15,150
were tipped with graphite
to help them ease in.

845
00:39:15,150 --> 00:39:17,760
So you've got
moderator tipped rods,

846
00:39:17,760 --> 00:39:20,490
which induce additional
moderation, which

847
00:39:20,490 --> 00:39:23,790
helps slow down the
neutrons even more to where

848
00:39:23,790 --> 00:39:25,087
they fission even better.

849
00:39:25,087 --> 00:39:27,420
And that's what led to what's
called a positive feedback

850
00:39:27,420 --> 00:39:28,560
coefficient.

851
00:39:28,560 --> 00:39:30,670
So the more you tried to
insert the control rods

852
00:39:30,670 --> 00:39:32,212
and the more you
tried to fix things,

853
00:39:32,212 --> 00:39:34,680
the worse things got
in the nuclear sense.

854
00:39:34,680 --> 00:39:37,320
And in something like
a quarter of a second,

855
00:39:37,320 --> 00:39:40,470
the reactor power went
up by like 35,000 times.

856
00:39:40,470 --> 00:39:43,350
And we'll do a millisecond by
millisecond rundown of what

857
00:39:43,350 --> 00:39:46,467
happened in Chernobyl after
we do all this neutron

858
00:39:46,467 --> 00:39:48,300
physics stuff when
you'll be better equipped

859
00:39:48,300 --> 00:39:49,050
to understand it.

860
00:39:49,050 --> 00:39:52,620
But suffice to say, there were
some positive coefficients here

861
00:39:52,620 --> 00:39:55,890
that are to be avoided at all
costs in all nuclear reactor

862
00:39:55,890 --> 00:39:58,410
design.

863
00:39:58,410 --> 00:40:00,480
In the actual reactor
hall you can go and stand

864
00:40:00,480 --> 00:40:01,560
on one of these things.

865
00:40:01,560 --> 00:40:04,020
It's a very different design
from what you're used to.

866
00:40:04,020 --> 00:40:05,687
I don't think anyone
would let you stand

867
00:40:05,687 --> 00:40:06,912
on top of a pressure vessel.

868
00:40:06,912 --> 00:40:09,120
First, your shoes would
melt, because they're usually

869
00:40:09,120 --> 00:40:11,418
at like 300 Celsius or so.

870
00:40:11,418 --> 00:40:13,710
And second of all, you'd
probably get a little too much

871
00:40:13,710 --> 00:40:14,790
radiation.

872
00:40:14,790 --> 00:40:17,190
But this is actually
what an RBMK reactor

873
00:40:17,190 --> 00:40:21,200
hall looks like for one of
the units that didn't blow up.

874
00:40:21,200 --> 00:40:23,970
There were multiple
units at that site.

875
00:40:23,970 --> 00:40:26,370
Then there's the
supercritical water reactor.

876
00:40:26,370 --> 00:40:28,830
Let's say you want to run
at higher temperatures

877
00:40:28,830 --> 00:40:31,200
than regular water
will allow you to.

878
00:40:31,200 --> 00:40:34,290
You can pressurize
it so much that water

879
00:40:34,290 --> 00:40:37,800
goes beyond the supercritical
point in the phase sense

880
00:40:37,800 --> 00:40:41,700
and starts to behave not
like liquid, not like a gas,

881
00:40:41,700 --> 00:40:44,400
but somewhere in between,
something that's really, really

882
00:40:44,400 --> 00:40:47,610
dense, so getting towards
the density of water, not

883
00:40:47,610 --> 00:40:50,130
quite, which means it's
still a great moderator,

884
00:40:50,130 --> 00:40:52,410
but still can cool the
materials quite well

885
00:40:52,410 --> 00:40:56,130
to extract heat to make
power and so on and so on.

886
00:40:56,130 --> 00:40:56,810
Yeah.

887
00:40:56,810 --> 00:40:59,260
AUDIENCE: So supercritical
refers to the coolant

888
00:40:59,260 --> 00:41:01,720
not the neutrons?

889
00:41:01,720 --> 00:41:03,310
MICHAEL SHORT: Good question.

890
00:41:03,310 --> 00:41:05,030
For a supercritical
water reactor,

891
00:41:05,030 --> 00:41:07,780
it most definitely
refers to the coolant.

892
00:41:07,780 --> 00:41:09,295
It's the phase of
the coolant where

893
00:41:09,295 --> 00:41:13,270
it's beyond the liquid
gas separation line,

894
00:41:13,270 --> 00:41:15,220
and it's just
something in between.

895
00:41:15,220 --> 00:41:17,920
Any of these reactors
can go supercritical,

896
00:41:17,920 --> 00:41:20,680
where you're producing more
neutrons than you're consuming.

897
00:41:20,680 --> 00:41:23,110
And that is a nuclear bad day.

898
00:41:23,110 --> 00:41:24,760
But the supercritical
water reactor

899
00:41:24,760 --> 00:41:28,480
does not refer to neutron
population, just a coolant.

900
00:41:28,480 --> 00:41:29,797
Good question.

901
00:41:29,797 --> 00:41:30,880
It's never come up before.

902
00:41:30,880 --> 00:41:34,630
But it's like, should
have thought of that.

903
00:41:34,630 --> 00:41:37,360
And so then my favorite,
liquid metal reactors,

904
00:41:37,360 --> 00:41:40,750
like LBE, or
Lead-Bismuth Eutectic.

905
00:41:40,750 --> 00:41:44,080
It's a low melting point
alloy of lead and bismuth.

906
00:41:44,080 --> 00:41:48,803
Lead melts at around 330
Celsius, bismuth 200 something.

907
00:41:48,803 --> 00:41:51,220
Put them together, and it's
like a low temperature solder.

908
00:41:51,220 --> 00:41:54,190
It melts at 123.5 Celsius.

909
00:41:54,190 --> 00:41:56,078
You can melt it in a frying pan.

910
00:41:56,078 --> 00:41:58,120
This is nice, because you
don't want your coolant

911
00:41:58,120 --> 00:42:01,460
to freeze when you're
trying to cool your reactor,

912
00:42:01,460 --> 00:42:04,570
because imagine something
happens, you lose power.

913
00:42:04,570 --> 00:42:06,610
The coolant freezes
somewhere outside the core.

914
00:42:06,610 --> 00:42:08,740
You can't get the
core cool again.

915
00:42:08,740 --> 00:42:10,360
That's called a loss
of flow accident

916
00:42:10,360 --> 00:42:12,250
that can lead to
a really bad day.

917
00:42:12,250 --> 00:42:14,620
And the lower your melting
point is the better.

918
00:42:14,620 --> 00:42:17,770
Sodium potassium is already
molten to begin with.

919
00:42:17,770 --> 00:42:20,030
Sodium melts at like 90 Celsius.

920
00:42:20,030 --> 00:42:22,000
And when you add two
different metals together,

921
00:42:22,000 --> 00:42:24,610
you almost always
lower the melting point

922
00:42:24,610 --> 00:42:25,852
of the combination.

923
00:42:25,852 --> 00:42:27,310
In this case,
forming what's called

924
00:42:27,310 --> 00:42:32,540
the eutectic, or a lowest
possible melting point alloy.

925
00:42:32,540 --> 00:42:35,690
The sodium fast reactor
has a number of advantages,

926
00:42:35,690 --> 00:42:37,760
like you don't really
need any pressure.

927
00:42:37,760 --> 00:42:40,280
As long as you have a cover
gas keeping the sodium

928
00:42:40,280 --> 00:42:43,940
from reacting with anything,
like the moisture in the air,

929
00:42:43,940 --> 00:42:45,980
or any errant water
in the room, you

930
00:42:45,980 --> 00:42:48,370
can just circulate
it through the core.

931
00:42:48,370 --> 00:42:51,500
And liquid metals are
awesome heat conductors.

932
00:42:51,500 --> 00:42:53,900
They might not have
the best heat capacity,

933
00:42:53,900 --> 00:42:56,930
as in how much energy per gram
they can store like water.

934
00:42:56,930 --> 00:42:58,520
But they're really
good conductors

935
00:42:58,520 --> 00:43:00,770
with very high
thermal conductivity.

936
00:43:00,770 --> 00:43:04,680
They also are really good at
not slowing down neutrons.

937
00:43:04,680 --> 00:43:07,700
So these tend to be what's
called fast reactors that

938
00:43:07,700 --> 00:43:11,960
rely on the ability of
other isotopes of uranium,

939
00:43:11,960 --> 00:43:15,540
like uranium-238, to undergo
what's called fast fission.

940
00:43:15,540 --> 00:43:18,100
And I want to show you
what that looks like.

941
00:43:18,100 --> 00:43:23,325
Let's pull up U-238 and look
at its fission cross section.

942
00:43:23,325 --> 00:43:24,700
And you might find
that it should

943
00:43:24,700 --> 00:43:28,340
look a fair bit different.

944
00:43:28,340 --> 00:43:37,220
So we'll go down to number
18 to fission cross section,

945
00:43:37,220 --> 00:43:38,960
very, very different.

946
00:43:38,960 --> 00:43:44,940
So U-238 is pretty terrible
at fission at low energies.

947
00:43:44,940 --> 00:43:46,870
It's pretty good at
capturing neutrons.

948
00:43:46,870 --> 00:43:48,750
This is where we
get plutonium-239,

949
00:43:48,750 --> 00:43:50,660
like you guys saw on the exam.

950
00:43:50,660 --> 00:43:53,160
But then you go to really high
energies and all of a sudden,

951
00:43:53,160 --> 00:43:58,630
it gets pretty good at
undergoing fission on its own.

952
00:43:58,630 --> 00:44:00,720
And so the basis behind
a lot of fast reactors

953
00:44:00,720 --> 00:44:03,990
is a combination of making
their own fuel and the fact

954
00:44:03,990 --> 00:44:08,010
that uranium-238 fast
fissions even better than it

955
00:44:08,010 --> 00:44:09,023
thermal fissions.

956
00:44:09,023 --> 00:44:10,440
So something good
for you to know,

957
00:44:10,440 --> 00:44:12,900
even though it's
not a fissile fuel,

958
00:44:12,900 --> 00:44:15,316
that's light water
reactor people talking.

959
00:44:15,316 --> 00:44:19,900
You can get it to fission if
the neutron populations higher.

960
00:44:19,900 --> 00:44:21,970
Now, there's some
problems with this.

961
00:44:21,970 --> 00:44:28,870
It takes some time for neutrons
to slow down from 1 to 10 MeV

962
00:44:28,870 --> 00:44:31,780
to about 0.025 eV.

963
00:44:31,780 --> 00:44:35,022
If your neutrons don't need to
slow down and travel anywhere,

964
00:44:35,022 --> 00:44:37,480
and pretty much all they have
to do is be born and absorbed

965
00:44:37,480 --> 00:44:40,990
by a nearby uranium
atom, the feedback time

966
00:44:40,990 --> 00:44:43,210
is faster in these
sorts of reactors.

967
00:44:43,210 --> 00:44:45,970
They're inherently more
difficult to control.

968
00:44:45,970 --> 00:44:49,690
And you can't use normal
physics like thermal expansion

969
00:44:49,690 --> 00:44:51,740
of things that might happen
on the order of micro

970
00:44:51,740 --> 00:44:55,810
to nanoseconds if it takes less
time than that for one neutron

971
00:44:55,810 --> 00:44:58,510
to be born and find
another uranium atom.

972
00:44:58,510 --> 00:45:01,670
You can still use it somewhat,
but not quite as much.

973
00:45:01,670 --> 00:45:05,790
So it's something to note
backed up by nuclear data.

974
00:45:05,790 --> 00:45:07,790
And that's what one of
them actually looks like.

975
00:45:07,790 --> 00:45:09,860
These things have
been built. That's

976
00:45:09,860 --> 00:45:14,280
a blob of liquid sodium on
the Monju reactor in Japan.

977
00:45:14,280 --> 00:45:16,190
And where I was all
last week in Russia,

978
00:45:16,190 --> 00:45:18,590
they actually have
fleets of fast reactors.

979
00:45:18,590 --> 00:45:24,410
Their BN-300 and BN-600 reactors
are 300 and 600 megawatt sodium

980
00:45:24,410 --> 00:45:25,910
cooled reactors.

981
00:45:25,910 --> 00:45:27,560
One of them in the
Chelyabinsk region

982
00:45:27,560 --> 00:45:29,822
they use pretty much
for desalination

983
00:45:29,822 --> 00:45:31,280
down in the center
of Russia, where

984
00:45:31,280 --> 00:45:35,180
there's no oceans nearby
and probably dirty water.

985
00:45:35,180 --> 00:45:38,655
They actually use that
to make clean water.

986
00:45:38,655 --> 00:45:40,280
They also use this
for power production

987
00:45:40,280 --> 00:45:42,470
and for radiation
damage studies.

988
00:45:42,470 --> 00:45:45,800
So when it comes to
radiation material science,

989
00:45:45,800 --> 00:45:49,990
these fast reactors are
really where it's at.

990
00:45:49,990 --> 00:45:52,920
Yeah, you just
noticed the bottom.

991
00:45:52,920 --> 00:45:55,290
I went to Belgium, to their
national nuclear labs,

992
00:45:55,290 --> 00:45:57,330
where they have a
slowing sodium test loop.

993
00:45:57,330 --> 00:45:59,580
It's not a reactor, but it's
like a thermal hydraulics

994
00:45:59,580 --> 00:46:01,430
and materials test loop.

995
00:46:01,430 --> 00:46:03,000
And I asked a simple question.

996
00:46:03,000 --> 00:46:04,790
Where's the bathroom?

997
00:46:04,790 --> 00:46:06,420
And they started laughing at me.

998
00:46:06,420 --> 00:46:09,370
And they said, we're not putting
any plumbing in a sodium loop

999
00:46:09,370 --> 00:46:10,540
building.

1000
00:46:10,540 --> 00:46:12,373
You'll have to go to
the next building over.

1001
00:46:12,373 --> 00:46:14,957
And that's when I noticed, there
weren't any sprinkler systems

1002
00:46:14,957 --> 00:46:15,900
or toilets.

1003
00:46:15,900 --> 00:46:19,650
But every 15 or 20 feet, there
was a giant barrel of sand.

1004
00:46:19,650 --> 00:46:22,530
That's the fire extinguisher
for a liquid metal fire

1005
00:46:22,530 --> 00:46:24,720
is you just cover it with
sand, absorb the heat,

1006
00:46:24,720 --> 00:46:28,125
keep the air out, the moisture
out, wick away the moisture

1007
00:46:28,125 --> 00:46:29,430
or whatever else sand does.

1008
00:46:29,430 --> 00:46:30,920
I don't know.

1009
00:46:30,920 --> 00:46:33,090
But you can't use normal
fire extinguishers

1010
00:46:33,090 --> 00:46:34,810
to put out a sodium fire.

1011
00:46:34,810 --> 00:46:37,765
AUDIENCE: When you said sand,
I thought of kitty litter.

1012
00:46:37,765 --> 00:46:38,760
MICHAEL SHORT: Ah.

1013
00:46:38,760 --> 00:46:40,093
I don't know if that would work.

1014
00:46:40,093 --> 00:46:42,197
[LAUGHTER]

1015
00:46:42,197 --> 00:46:43,280
I guess it's worth a shot.

1016
00:46:43,280 --> 00:46:44,760
[LAUGHTER]

1017
00:46:44,760 --> 00:46:47,460
With glasses, and safety,
and stuff, of course.

1018
00:46:47,460 --> 00:46:49,610
And the ones that I spent
the most time working on,

1019
00:46:49,610 --> 00:46:51,440
like I showed you in
the paper yesterday,

1020
00:46:51,440 --> 00:46:53,970
is the lead or
lead-bismuth fast reactor.

1021
00:46:53,970 --> 00:46:56,300
This one does not
have the disadvantages

1022
00:46:56,300 --> 00:46:57,802
of exploding like sodium.

1023
00:46:57,802 --> 00:47:00,260
It does have the disadvantage,
like I showed you yesterday,

1024
00:47:00,260 --> 00:47:03,720
of corroding everything,
pretty much everything.

1025
00:47:03,720 --> 00:47:07,083
And so the one thing keeping
this thing back was corrosion.

1026
00:47:07,083 --> 00:47:08,500
And I say the
ultimate temperature

1027
00:47:08,500 --> 00:47:10,340
is medium, but higher soon.

1028
00:47:10,340 --> 00:47:11,840
Hopefully, someone
picks up our work

1029
00:47:11,840 --> 00:47:14,215
and is like, yeah, that was
a good idea, because we think

1030
00:47:14,215 --> 00:47:16,340
it can raise the
outlet temperature

1031
00:47:16,340 --> 00:47:19,180
of a lead-bismuth reactor
by like 100 Celsius

1032
00:47:19,180 --> 00:47:21,860
as long as some other unforeseen
problem doesn't pop up,

1033
00:47:21,860 --> 00:47:24,360
and we don't quite know yet.

1034
00:47:24,360 --> 00:47:27,240
These things also
already exist in the form

1035
00:47:27,240 --> 00:47:30,720
of the Alfa Class attack
submarines from the Soviet

1036
00:47:30,720 --> 00:47:31,770
Union.

1037
00:47:31,770 --> 00:47:34,592
These are the only subs
that can outrun a torpedo.

1038
00:47:34,592 --> 00:47:37,550
So you know that old algebra
problem, if person A leaves

1039
00:47:37,550 --> 00:47:39,380
Pittsburgh at 40 miles
an hour and person

1040
00:47:39,380 --> 00:47:41,750
B leaves Boston at
30 miles an hour,

1041
00:47:41,750 --> 00:47:45,620
where do the trains collide or
I forget how it actually ends?

1042
00:47:45,620 --> 00:47:48,650
Well, in the end, if a
torpedo leaves an American sub

1043
00:47:48,650 --> 00:47:52,640
at whatever speed and the Alfa
Class submarine notices it,

1044
00:47:52,640 --> 00:47:55,700
how close do they have to be
before the torpedo runs out

1045
00:47:55,700 --> 00:47:56,970
of gas?

1046
00:47:56,970 --> 00:48:00,020
So what I was told by the
designer of these subs,

1047
00:48:00,020 --> 00:48:03,463
a fellow by the name of Georgy
Toshinsky, when he came here

1048
00:48:03,463 --> 00:48:05,630
to talk about his experience
with these lead-bismuth

1049
00:48:05,630 --> 00:48:09,020
reactors is, there is a
button on the sub that's

1050
00:48:09,020 --> 00:48:12,790
the Forget About Safety,
It's a Torpedo button.

1051
00:48:12,790 --> 00:48:15,940
Because if you're underwater
in a lead-bismuth reactor

1052
00:48:15,940 --> 00:48:17,860
and a torpedo is
heading at you, you

1053
00:48:17,860 --> 00:48:21,730
have a choice between maybe
dying in a nuclear catastrophe

1054
00:48:21,730 --> 00:48:24,650
and definitely dying
in a torpedo explosion.

1055
00:48:24,650 --> 00:48:29,060
Well, that button is the
I Like Those Odds button.

1056
00:48:29,060 --> 00:48:31,400
And you just give full
power to the engines

1057
00:48:31,400 --> 00:48:33,590
and whatever else
happens, happens.

1058
00:48:33,590 --> 00:48:37,340
The point is, you may be
able to outrun the torpedo.

1059
00:48:37,340 --> 00:48:40,800
And quite popular nowadays,
especially in this department,

1060
00:48:40,800 --> 00:48:44,070
is molten salt cooled reactors
that actually use liquid salt,

1061
00:48:44,070 --> 00:48:47,960
not dissolved, but molten
salt itself as the coolant.

1062
00:48:47,960 --> 00:48:50,940
It doesn't have as many of
the corrosion problems as lead

1063
00:48:50,940 --> 00:48:53,610
or the exploding
problems as sodium.

1064
00:48:53,610 --> 00:48:56,160
It does have a high melting
point problem though.

1065
00:48:56,160 --> 00:48:59,340
They tend to melt at
around 450 degrees Celsius.

1066
00:48:59,340 --> 00:49:01,110
But there's one
pretty cool feature.

1067
00:49:01,110 --> 00:49:03,600
You can dissolve
uranium in them.

1068
00:49:03,600 --> 00:49:05,250
So remember how in
light water reactors

1069
00:49:05,250 --> 00:49:07,620
the coolant is
also the moderator?

1070
00:49:07,620 --> 00:49:09,570
In molten salt
reactors, the coolant

1071
00:49:09,570 --> 00:49:12,790
is also the fuel, because
you can have principally

1072
00:49:12,790 --> 00:49:15,400
uranium and lithium
fluoride salt

1073
00:49:15,400 --> 00:49:17,008
co-dissolved in each other.

1074
00:49:17,008 --> 00:49:18,550
And the way you make
a reactor is you

1075
00:49:18,550 --> 00:49:22,630
just flow a bunch of that
salt into nearby pipes.

1076
00:49:22,630 --> 00:49:26,280
And then you get less, what's
called, neutron leakage,

1077
00:49:26,280 --> 00:49:28,520
where in each of these pipes
once in a while uranium

1078
00:49:28,520 --> 00:49:29,728
will give off a few neutrons.

1079
00:49:29,728 --> 00:49:32,228
Most of them will just come out
the other ends of the pipes,

1080
00:49:32,228 --> 00:49:33,880
and you won't have a reaction.

1081
00:49:33,880 --> 00:49:36,010
When you put a whole bunch
of molten salt together,

1082
00:49:36,010 --> 00:49:38,230
most of those neutrons
find other molten

1083
00:49:38,230 --> 00:49:41,715
salt. And the reaction proceeds.

1084
00:49:41,715 --> 00:49:43,340
And it's got some
neat safety features.

1085
00:49:43,340 --> 00:49:46,780
Like if something goes wrong,
just break open a pipe.

1086
00:49:46,780 --> 00:49:50,770
All the salt spills out,
becoming subcritical,

1087
00:49:50,770 --> 00:49:52,240
because leakage goes up.

1088
00:49:52,240 --> 00:49:55,460
It freezes pretty quickly, and
then you must deal with it.

1089
00:49:55,460 --> 00:49:58,180
But it's not a big deal
to deal with it if it's

1090
00:49:58,180 --> 00:50:01,660
already solid and not critical.

1091
00:50:01,660 --> 00:50:03,100
So it's actually five of.

1092
00:50:03,100 --> 00:50:04,660
It's zero of five of.

1093
00:50:04,660 --> 00:50:06,340
I'll stop here.

1094
00:50:06,340 --> 00:50:08,410
On Tuesday, we'll
keep developing

1095
00:50:08,410 --> 00:50:10,003
the many, many
different variables

1096
00:50:10,003 --> 00:50:11,920
we'll need to write down
the neutron transport

1097
00:50:11,920 --> 00:50:13,523
equation, at which
point you'll be

1098
00:50:13,523 --> 00:50:15,940
qualified to read the t-shirts
that this department prints

1099
00:50:15,940 --> 00:50:16,720
out.

1100
00:50:16,720 --> 00:50:18,637
And then we'll simplify
it so you can actually

1101
00:50:18,637 --> 00:50:20,400
solve the equation.