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MICHAEL SHORT: All right.

So like I told you guys, Friday marked the end of the hardest

part of the course.

And Monday marked the end of the hardest Pset.

So because the rest of your classes

are going full throttle, this one's

going to wind down a little bit.

So today, I'd say, sit back, relax,

and enjoy a nuclear catastrophe because we

are going to explain what happened at Chernobyl now

that you've got the physics and intuitive background

to understand the actual sequence of events.

To kick it off, I want to show you guys

some actual footage of the Chernobyl reactor

as it was burning.

So this is the part that most folks know about.

[VIDEO PLAYBACK]

- [NON-ENGLISH SPEECH]

MICHAEL SHORT: This is footage taken from a helicopter

from folks that were either surveying or dropping materials

onto the reactor.

- [NON-ENGLISH SPEECH]

MICHAEL SHORT: That was probably a bad idea.

"Hold where the smoke is."

We'll get into what the smoke was.

- [NON-ENGLISH SPEECH]

[END PLAYBACK]

MICHAEL SHORT: So that red stuff right there,

that's actually glowing graphite amongst other materials

from the graphite fire that resulted from the RBMK reactor

burning after the Chernobyl accident, caused by both flaws

in the physical design of the RBMK reactor

and absolute operator of stupidity and neglect

of any sort of safety systems or safety culture.

We're lucky to live here in the US

where our worst accident at Three Mile Island

was not actually really that much of an accident.

There was a partial meltdown.

There was not that much of a release of radio nuclides

into the atmosphere because we do

things like build containments on our reactors.

If you think of what a typical reactor looks like,

like if you consider the MIT reactor as a scaled-down

version of a normal reactor--

let's say you have a commercial power reactor.

You've got the core here.

You've got a bunch of shielding around it.

And you've got a dome that's rather thick

that comprises the containment.

That would be the core.

This would be some shielding.

So this is what you find in US and most other reactors.

For the RBMK reactors, there was no containment

because it was thought that nothing could happen.

And boy, were they wrong.

So I want to walk you guys through a chronology of what

actually happened at that the Chernobyl reactor, which

you guys can read on the NEA, or Nuclear Energy Agency, website,

the same place that you find JANIS.

And we're going to refer to a lot of the JANIS cross sections

to explain why these sorts of events happened.

So the whole point of what happened at Chernobyl

was it was desire to see if you could

use the spinning down turbine after you shut

down the reactor to power the emergency

systems at the reactor.

This would be following something,

what's called a loss of off-site power.

If the off-site power or the grid

was disconnected from the reactor,

the reactor automatically shuts down.

But the turbine, like I showed you a couple weeks ago,

is this enormous spinning hulk of metal and machinery

that coasts down over a long period of, let's say, hours.

And as it's spinning, the generator coils

are still spinning and still producing electricity,

or they could be.

So it was desire to find out, can we

use the spinning down turbine to power the emergency equipment

if we lose off-site power?

So they had to simulate this event.

So what they actually decided to do

is coast down the reactor to a moderate power level

or very low power and see what comes out

of the turbine itself, or out of the generator rather.

Now, there were a lot of flaws in the RBMK design.

And I'd like to bring it up here so we

can talk about what it looks like

and what was wrong with it.

So the RBMK is unlike any of the United States light

water reactors that you may have seen before.

Many of the components are the same.

There's still a light water reactor coolant

loop where water flows around fuel rods,

goes into a steam separator, better known

as a big heat exchanger.

And the steam drives a turbine, which produces energy.

And then this coolant pump keeps it going.

And then the water circulates.

What makes it different, though, is that each of these fuel rods

was inside its own pressure tube.

So the coolant was pressurized.

And out here, this stuff right here

was the moderator composed of graphite.

Unlike light water reactors in the US,

the coolant was not the only moderator in the reactor.

Graphite also existed, which meant

that, if the water went away, which would normally shut down

a light water reactor from lack of moderation,

graphite was still there to slow the neutrons down

into the high-fission cross-section area.

And I'd like to pull up JANIS and show you

what I mean with the uranium cross section.

So let's go again to uranium-235 and pull up its fission cross

section.

Let's see fission.

I can make it a little thicker too.

So again, the goal of the moderator

is to take neutrons from high energies like 1 to 10 MeV

where the fission cross section is relatively low

and slow them down into this region where fission is,

let's say, 1,000 times more likely.

And in a light water reactor in the US,

if the coolant goes away, so does the moderation.

And there's nothing left to slow those neutrons down

to make fission more likely.

In the RBMK, that's not the case.

The graphite is still there.

The graphite is cooled by a helium-nitrogen mixture

because the neutron interactions in the graphite that's slowing

down--

we've always talked about what happens from the point of view

of the neutron.

But what about the point of view of the other material?

Any energy lost by the neutrons is gained

by the moderating material.

So the graphite gets really hot.

And you have to flow some non-oxygen-containing gas

mixture like helium and nitrogen, which

is pretty inert, to keep that graphite cool.

And then in between the graphite moderator

were control rods, about 200 of them or so, 30 of which

were required to be down in the reactor at any given time

in order to control power.

And that was a design rule.

That was broken during the actual experiment.

And then on top of here, on top of this biological shield,

you could walk on top of it.

So the tops of those pressure tubes,

despite being about 350 kilo chunks of concrete,

you could walk on top of them.

That's pretty cool, kind of scary too.

So what happened in chronological order was,

around midnight, the decision was made to undergo this test

and start spinning down the turbine.

But the grid operator came back and said, no, you can't just

cut the reactor power to nothing.

You have to maintain at a rather high power for a while,

about 500 megawatts electric or half the rated power

of the reactor.

And what that had the effect of doing

is continuing to create fission products, including xenon-135.

We haven't mentioned this one yet.

You'll talk about it quite a lot in 22.05 in neutron physics.

Black shirt really shows chalk well.

What xenon-135 does is it just sits there.

It's a noble gas.

It has a half-life of a few days.

So it decays on the slow side for as fission products go.

But it also absorbs lots and lots and lots of neutrons.

Let's see if I could find which one is the xenon one.

There we go.

So here, I've plotted the total cross-section

for xenon-135 and the absorption cross-section.

And notice how, for low energies,

pretty much the entire cross section of xenon

is made up of absorption.

Did you guys in your homework see anything that

reached about 10 million barns?

No.

Xenon-135 is one of the best neutron absorbers there is.

And reactors produce it constantly.

So as they're operating, you build up xenon-135

that you have to account for in your sigma absorption cross

section.

Because like you guys saw in the homework,

if you want to write what's the sigma absorption cross

section of the reactor, it's the sum

of every single isotope in the reactor of its number

density times its absorption cross section.

And so that would include everything for water

and let's say the uranium and the xenon

that you're building up.

When the reactor starts up, the number density of xenon

is 0 because you don't have anything to have produced it.

When you start operating, you'll reach the xenon equilibrium

level where it will build to a certain level that

will counteract the reactivity of the reactor.

And then your k-effective expression,

where it sources over absorption plus leakage,

this has the effect of raising sigma absorption

and lowering k effective.

The trick is it doesn't last for very long.

It built decays with a half-life of about five days.

And when you try and raise the reactor power,

you will also start to burn it out.

So if you're operating at a fairly low power level,

you'll both be decaying and burning xenon

without really knowing what's going on.

And that's exactly what happened here.

So an hour or so later--

let me pull up the chronology again.

A little more than an hour later,

so the reactor power stabilized at something like 30 megawatts.

And they were like, what is going on?

Why is that reactor power so low?

We need to increase the reactor power.

So what did they do?

A couple of things.

One was remove all but six or seven of the control rods

going way outside the spec of the design

because 30 were needed to actually maintain

the reactor at a stable power.

All the while, the xenon that had been building up

is still there keeping the reactor from going critical.

It's what was the main reason that the reactor didn't even

have very much power.

But it was also burning out at the same time.

So all the while--

let's say if we were to show a graph of two things, time,

xenon inventory, and as a solid line

and let's say control rod worth as a dotted line.

The xenon inventory at full power

would have been at some level.

And then it would start to decay and burn out.

While at the same time, the control

rod worth, as you remove control rods from the reactor--

every time you remove one, you lose some control rod worth,

would continue to diminish leading to the point where

bad stuff is going to happen.

Let me make sure I didn't lose my place.

So at any rate, as they started pulling the control rods out,

a couple of interesting quirks happened in terms of feedback.

So let's look back at this design.

Like any reactor, this reactor had

what's called a negative fuel temperature coefficient.

What that means is that, when you heat up the fuel,

two things happen.