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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.