Placeholder Image

字幕表 動画を再生する

  • The following content is provided under a Creative

  • Commons license.

  • Your support will help MIT OpenCourseWare

  • continue to offer high quality educational resources for free.

  • To make a donation or to view additional materials

  • from hundreds of MIT courses, visit MIT OpenCourseWare

  • at ocw.mit.edu.

  • 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.

  • One, the cross section for anything, absorption

  • or fission, would go up.

  • But the number density would also go down.

  • As the atoms physically spaced out in the fuel,

  • their number density would go down,

  • lowering the macroscopic cross section for fission.

  • And that's arguably a good thing.

  • The problem is, at below about 20% power,

  • of the reactor had what's called a positive void coefficient,

  • which meant that, if you boil the coolant,

  • you increase the reactor power.

  • Because the other thing that--

  • I think I mentioned this once.

  • And you calculated in the homework the absorption cross

  • section of hydrogen is not 0.

  • It's small, but fairly significant.

  • Let's actually take a look at it.

  • We can always see this in JANIS.

  • Go back down to hydrogen, hydrogen-1.

  • Then we look at the absorption cross section.

  • And of course, it started us with the linear scale.

  • Let's go logarithmic.

  • Oh!

  • OK!

  • So at low energy, at 10 to the minus 8 to 10 to the minus 7,

  • it's around a barn.

  • Not super high, but absolutely not negligible,

  • which meant that part of the normal functionality

  • of the RBMK depended on the absorption of the water to help

  • absorb some of those neutrons.

  • With that water gone, there was less absorption.

  • But there was still a ton of moderation in this graphite

  • moderator.

  • So they still could get slow.

  • But then there'd be more of them.

  • And that would cause the power to increase.

  • And then that caused more of the coolant

  • to boil, which would cause less absorption, which would

  • cause the power to increase.

  • Yeah, Charlie?

  • AUDIENCE: So did they remove the water from the reactor?

  • MICHAEL SHORT: They did not remove the water

  • from the reactor.

  • However, as the power started to rise, some of the water

  • started to boil.

  • And so you can still have, let's say, steam flowing through

  • and still remove some of the heat.

  • However, you don't have that dense or water

  • to act as an absorber.

  • And that's what really undid this reactor.

  • In addition, they decided to disable

  • the ECCS, or the Emergency Core Cooling System,

  • which you're just not supposed to do.

  • So they shut down a bunch of these systems

  • to see if you could power the other ones

  • from the spinning down turbine.

  • And then, as they noticed that the reactor was

  • getting less and less stable, they

  • had almost all the rods out.

  • Some of these pressure tubes started to bump and jump.

  • These 350-kilogram pressure tube caps were just rattling.

  • I mean, imagine something that weighs 900

  • pounds or so rattling around.

  • And there's a few hundred of them.

  • So there was someone in the control room that said,

  • the caps are rattling.

  • What the heck?

  • And didn't quite make it down the spiral staircase

  • because, about 10 seconds later, everything went wrong.

  • And so I want to pull up this actual timeline

  • so you can see it splits from minutes to seconds.

  • Because the speed at which this stuff started to go wrong

  • was pretty striking.

  • So for example, the control rods raised at 1:19 in the morning.

  • Two minutes later, when the power starts to become

  • unstable, the caps on the fuel channels-- which, again,

  • are like 350-kilogram blocks--

  • start jumping in their sockets.

  • And a lot of that was--

  • we go back to the RBMK reactor.

  • As the coolant started to boil here, well,

  • that boiling force actually creates huge pressure

  • instabilities, which would cause the pressure

  • tubes to jump up and down, eventually rupturing

  • almost every single one of them with enough force to shoot

  • these 350-kilogram caps.

  • And what did they say?

  • I like the language that they used--

  • jumping in their sockets.

  • So 50 seconds later, pressure fails

  • in the steam drums, which means there's been

  • some sort of containment leak.

  • So all the while, the coolant was boiling.

  • The absorption was going down.

  • The power was going up.

  • Repeat, repeat, repeat.

  • And the power jumped to about 100 times the rated power

  • in something like four seconds.

  • So it was normally 1,000-megawatt electric

  • reactor, which is about 3,200 megawatts thermal.

  • It was producing nearly half a terawatt

  • of thermal power for a very short amount of time

  • until it exploded.

  • Now, it's interesting.

  • A lot of folks call Chernobyl a nuclear explosion.

  • That's actually a misnomer.

  • A nuclear explosion would be a nuclear weapon, something

  • set off by an enormous chain reaction principally heated

  • by fission or fusion.

  • That's not actually what happened at Chernobyl,

  • nor at Fukushima, nor was that the worry at Three Mile Island.

  • Not to say it wasn't a horrible thing,

  • but it wasn't an actual nuclear explosion.

  • At first, what happened was a pressure explosion.

  • So there was an enormous release of steam

  • as the power built up to 100 times normal operating power.

  • The steam force was so large that it actually

  • blew the reactor lid up off of the thing.

  • And I think I have a picture of that somewhere here too.

  • It should be further down.

  • Yeah, to give you a little sense of scale.

  • The reactor cover, which weighed about 1,000 tons,

  • launched into the air and landed above the reactor

  • sending most of the reactor components

  • up to a kilometer up in the air.

  • Four seconds later, that was followed

  • by a hydrogen explosion.

  • Let me get that down to that chronology.

  • So yeah.

  • At 1:23 and 40 seconds in the morning--

  • oh, yeah.

  • So I should mentioned why this happened-- emergency insertion

  • of all the control rods.

  • The last part that this diagram doesn't mention is these

  • control rods-- and I'll draw this up here--

  • we're tipped with about six inches of graphite.

  • So if these were two graphite channels--

  • let's say these are carbon--

  • and this is your control rod, the goal

  • was to get this control rod all the way into the reactor.

  • One part they didn't mention was they

  • were tipped with about six inches of graphite, which

  • only functions as additional moderator.

  • Graphite is one of the lowest absorbing materials

  • in the periodic table, second, I think, only to oxygen.

  • And if we pull up graphite cross sections,

  • I've plotted here the total cross section,

  • the elastic scattering cross section.

  • And down here, in the 0.001 barn level,

  • is the absorption cross section, about 1,000 times lower

  • than water.

  • So you're shoving more material in the reactor that slows down

  • neutrons even more, bringing them

  • into the high-fission region without absorbing anything.

  • And they jammed about halfway down,

  • about 2 and 1/2 feet down, leaving the extra graphite

  • right in the center of the core where

  • it could do the most damage.

  • And it didn't take that much time.

  • Yeah?

  • AUDIENCE: So my understanding is that, also, one of the designs

  • is that the control rods didn't immediately drop down.

  • But they were slowly lowered.

  • MICHAEL SHORT: Yep.

  • They took 7 to 10 seconds.

  • AUDIENCE: If they had a system where they did drop,

  • would that have possibly actually set

  • the system down properly?

  • MICHAEL SHORT: I'm not sure.

  • I don't know whether lowering control rods into something

  • that was undergoing steam explosions

  • would have actually helped.

  • I mean, to me, by this point, it was all over.

  • So the extra moderator that was dumped in

  • was the last kick in the pants this thing

  • needed to go absolutely insane.

  • And if we go back to the timeline on the second level,

  • control rods inserted at 1:23 and 40 seconds.

  • Explosion, four seconds later, to 120 times full power,

  • getting towards a terawatt or so.

  • One second later, the 1,000-ton lid launches off from the first

  • explosion.

  • Very shortly after that, second explosion.

  • And that happened because of this reaction.

  • Well, just about anything corroding with water

  • will make pretty much anything oxide

  • plus hydrogen, the same chemical explosion that

  • was the undoing of Fukushima and was the worry

  • at Three Mile Island that there was a hydrogen bubble

  • building because of corrosion reactions

  • with whatever happened to be in the core.

  • This happens with zirconium pretty vigorously.

  • But it happens with other materials too.

  • If you oxidize something with water,

  • you leave behind the hydrogen. And the hydrogen,

  • in a very wide range of concentrations in the air,

  • is explosive.

  • We're actually not allowed to use hydrogen at about 4%

  • in any of the labs here because that reaches the flammability

  • or explosive limit.

  • So for my PhD, we were doing these experiments

  • corroding materials in liquid lead.

  • And we wanted to dump in pure hydrogen

  • to see what happens when there's no oxygen. We were told,

  • absolutely not.

  • We had to drill a hole in the side of the walls

  • that the hydrogen would vent outside and do

  • some calculations to show if the entire bottle of hydrogen

  • emptied into the lab at once, which it could

  • do if the cap of the bottle breaks off,

  • it would not reach 4% concentration.

  • So hydrogen explosions are pretty powerful things.

  • You guys ever seen people making water from scratch?

  • Mix hydrogen and oxygen in a bottle and light a match?

  • We've got a video of it circulating somewhere around

  • here because for RTC, for the Reactor Technology Course,

  • I do this in front of a bunch of CEOs

  • and watch them jump out of their chairs to teach basic chemical

  • reactions.

  • But it's pretty loud.

  • About enough hydrogen and oxygen to just fill this cup

  • or fill a half-liter water bottle

  • makes a bang that gets your ears ringing.

  • Not quite bleeding, but close enough.

  • So that's what happened here, except at a much

  • more massive scale.

  • So there was a steam explosion followed

  • seconds later by a hydrogen explosion from hydrogen

  • liberated from the corrosion reaction of everything

  • with the water that was already there.

  • And that's when this happened.

  • [VIDEO PLAYBACK]

  • - [NON-ENGLISH SPEECH]

  • MICHAEL SHORT: So that smoke right there

  • is from a graphite fire, not normal smoke.

  • - [NON-ENGLISH SPEECH]

  • MICHAEL SHORT: Yeah.

  • Spoke too soon.

  • - [NON-ENGLISH SPEECH]

  • [END PLAYBACK]

  • MICHAEL SHORT: This actually provides a perfect conduit

  • to transition from the second to the third parts of this course.

  • A lot of you have been waiting to find out

  • what are the units of dose and what

  • are the biological and chemical effects of radiation.

  • Well, this is where you get them.

  • From neutron physics, you can understand

  • why Chernobyl went wrong.

  • Honestly, you've just been doing this for three or four weeks.

  • But with your knowledge of cross sections, reactor feedback,

  • and criticality, you can start to understand why Chernobyl

  • was flawed in its design.

  • And what we're going to teach you in the rest of the course

  • is what happens next, what happens when radio nuclides are

  • absorbed by animals of the human body,

  • and what was the main fallout, let's

  • say, in the colloquial sense and the actual sense

  • from the Chernobyl reactor.

  • [VIDEO PLAYBACK]

  • Let's look a bit at what they did next though.

  • - [NON-ENGLISH SPEECH]

  • MICHAEL SHORT: That's not quite true.

  • You'll see why.

  • - [NON-ENGLISH SPEECH]

  • MICHAEL SHORT: That actually did happen.

  • - [NON-ENGLISH SPEECH]

  • [END PLAYBACK]

  • MICHAEL SHORT: I think that pretty much summarizes

  • the state of things now.

  • They built a sarcophagus around this reactor, a gigantic tomb,

  • which, according to some reports,

  • is not that structurally sound and is

  • in danger of partial collapse.

  • So yeah, more difficult efforts are ahead.

  • But let's now talk about what happened next.

  • I'm going to jump to the very end of this.

  • The actual way that the accident was noticed was the spread

  • of the radioactive cloud to not-so-close-by Sweden.

  • So it was noticed that folks entering a reactor in Sweden

  • had contaminants on them, which they thought was

  • coming from their own reactor.

  • Good first assumption.

  • When it was determined that nothing

  • was amiss at the reactor in Sweden,

  • folks started to analyze wind patterns

  • and find out what happened.

  • And then it was clear that the USSR

  • had tried to cover up the Chernobyl accident.

  • But you can't cover up fallout.

  • And it eventually spread pretty wide,

  • covering most of Europe and Russia

  • and surprisingly not Spain, lucky them

  • for the wind patterns that day, or those few days.

  • So what happened is a few days after the actual accident,

  • a graphite fire started to break out.

  • Because graphite, when exposed to air, well,

  • you can do the chemistry.

  • Add graphite plus oxygen, you start making carbon dioxide.

  • So graphite burns when it's hot.

  • And as you can see from the video--

  • where is that nice still of burning graphite?

  • Yeah.

  • That graphite was pretty hot.

  • So a lot of that smoke included burning graphite

  • and a lot of the materials from the reactor itself.

  • Now, when you build up fission products in a reactor

  • and they get volatilized like this,

  • the ones that tend to get out first

  • would be things like the noble gases.

  • So the whole xenon inventory of the reactor was released.

  • It's estimated at about 100%.

  • And I can actually pull up those figures.

  • When we talk about how much of which radionuclide

  • was released.

  • That's also a typo.

  • If somebody wants to call in, there's no 33 isotope of xenon.

  • It's supposed to be 133.

  • That would be interesting if someone wants to call in

  • and say the NEA has got a mistake.

  • So 100% of the inventory released.

  • That should be pretty obvious because it's a noble gas.

  • And it just kind of floats away.

  • The real dangers, though, came from iodine-131, about 50%

  • of a 3-exabecquerel activity.

  • So we're talking like megacuries.

  • It might be giga.

  • I can't do that math in my head.

  • A lot of radiation.

  • The problem with that is iodine behaves

  • just like any other halogen. It forms salts.

  • It's rather volatile.

  • Have any of you guys played with iodine before?

  • No one does-- oh, you have.

  • OK.

  • What happens when you play with it?

  • AUDIENCE: I mean, just throw some stuff--

  • like, it turns everything yellow and it just

  • reacts with acids and stuff.

  • I haven't really done very much with it.

  • So--

  • MICHAEL SHORT: OK.

  • I happen to have extensive practice playing

  • with iodine in my home because I did all the stuff you're not

  • supposed to do as a kid, kind of build your own chemistry

  • stuff things that somehow leak out to your local high school

  • somehow.

  • Iodine's pretty neat.

  • Yeah, it happens sometimes.

  • If you put iodine in your hand, it actually sublimes.

  • The heat from your hand is enough to directly go

  • from solid to vapor.

  • And so the iodine was also quite volatile.

  • Some of it may have been in the form of other compounds.

  • Some of it may have been elemental--

  • probably not likely.

  • But there was certainly some iodine vapor.

  • And about half of that was released.

  • The problem is then it condenses out

  • and falls on anything green, anything with surface area.

  • So the biggest danger to the folks living nearby

  • was from eating leafy vegetables because leaves

  • got lots of surface area.

  • Iodine deposits on them.

  • And it's intensely radioactive for a month or so.

  • Or depositing on the grass that cows eat,

  • which led to the problem of radioactive milk.

  • And so that's why milk in the Soviet Union

  • was banned for such a long time because this

  • was one of the major sources of iodine contamination.

  • The other one, which we're worrying about now

  • from Fukushima as well, is cesium,

  • which has similar chemistry to sodium and potassium-- again,

  • a rather salty compound, or rather salty element.

  • But it's got a half-life of 30 years.

  • And if we look it up in the table of nuclides,

  • we'll see what it actually releases.

  • Oh, good.

  • It's back online.

  • Anyone else notice this broken a couple days ago.

  • AUDIENCE: Yeah.

  • MICHAEL SHORT: Well, luckily, Brookhaven National Lab

  • has a good version up too.

  • But let's grab cesium.

  • Yeah, there's plenty out there.

  • Cesium-137.

  • Beta decays to barium but also gives off gamma rays.

  • And most of the decays end up giving off

  • one of those gamma rays, let's say a 660-keV gamma ray.

  • So it's both a beta and a gamma emitter.

  • Now, which of those types of radiation

  • do you think it's more damaging to biological organisms?

  • The beta or the gamma?

  • AUDIENCE: Gamma?

  • MICHAEL SHORT: You say the gamma.

  • Why do you say so?

  • AUDIENCE: Doesn't beta get stopped

  • by the skin and clothing?

  • MICHAEL SHORT: It does.

  • But if cesium is better known as--

  • AUDIENCE: [INAUDIBLE]

  • MICHAEL SHORT: Yes.

  • That's right.

  • So did I get to tell you guys this question,

  • the four cookies question?

  • Yeah.

  • You eat the gamma cookie because most gammas that

  • are emitted by the cookie simply leave you and irradiate

  • your friend, which is going to be the topic of pset number 8.

  • You'll see.

  • That's why you guys are getting your whole body counts.

  • Speaking of, who's gotten their whole body counts at EHS?

  • Awesome.

  • So that's almost everybody.

  • You will need that data for problem set 8.

  • So do schedule it soon, preferably before Thanksgiving

  • so that you'll be able to take a look at it.

  • Has anyone found anything interesting in your spectra?

  • Good.

  • Glad to hear that.

  • But you do see a potassium peak that you can probably

  • integrate and do some problems with, right?

  • Yeah, because you will.

  • OK.

  • Anyway, yeah.

  • It's the betas.

  • That's the real killer.

  • The gammas are going to leave the cesium, enter your body,

  • and most likely come out the other side.

  • Because the mass attenuation coefficient of 6-- what is it?

  • Water for 660-keV gammas.

  • Let's find that.

  • Table 3.

  • Let's say you're made mostly of water.

  • Water, liquid, that's pretty much humans.

  • 660 keV is right about here leading to about 0.1

  • centimeter squared per gram.

  • And with a density of 1 gram, that's

  • a pretty low attenuation of gammas.

  • So this chart actually shows why most

  • of the cesium gammas that would be produced from ingestion

  • just get right out.

  • But it's the betas that have an awfully short range.

  • Anyone remember the formula for range in general?

  • So this is going to come back up in our discussion of dose

  • and biological effects.

  • Integral, yep, of stopping power to the negative 1.

  • And that's stopping power is this simple formula.

  • Let's see.

  • What did that come out as?

  • Log minus beta squared.

  • That simple little formula, which

  • I'm not going to expect you guys to memorize.

  • So don't worry about it.

  • But if you integrate this, you find out

  • that the range of electrons, even 1 MeV electrons, in water

  • is not very high.

  • So most of them are stopped near or by the cells

  • that absorb them doing quite a bit of damage to DNA, which

  • is eventually what causes mutagenic effects--

  • cancer, cell death, what we're going

  • to talk about for the whole third part of the course.

  • There's also a worry about which organs actually

  • absorb these radionuclides.

  • And iodine in particular is preferentially

  • absorbed by the thyroid.

  • So when we started looking at the amount

  • of radioactive substances released--

  • remember they said, OK, at around the 26th of April

  • or the 2nd of May or so the release was stopped?

  • Not according to our data.

  • That's when the graphite fire picked up again.

  • In addition, the core of Chernobyl,

  • which had undergone a mostly total meltdown,

  • was sitting in a pool on top of this concrete pad.

  • So let's just call this liquid stuff--

  • the actual word that we use in parlance is called corium.

  • It's our tongue-in-cheek word for every element

  • mixed together in a hot radioactive soup.

  • First of all, it started to redistribute,

  • reacting with any water that was present, flashing it to steam.

  • And the steam caused additional dispersion of radionuclides.

  • And eventually, it burrowed its way

  • through and into the ground, releasing more.

  • It's the worst nuclear thing that's

  • ever happened in the history of nuclear things.

  • Quite a mess.

  • And luckily, it did sort of taper off after this.

  • But let's now look into what happens next.

  • And this is the nice intro to the third part of the course.

  • Iodine is preferentially uptaken by the thyroid gland

  • somewhere right about here.

  • So has anyone ever heard of the idea

  • of taking iodine tablets in the case of a nuclear disaster?

  • Anyone have any idea why?

  • If you saturate your thyroid with iodine,

  • then if you ingest radioactive iodine,

  • it's less likely to be permanently taken

  • by the thyroid.

  • So this actually provided some statistics

  • on the probability of getting thyroid cancer

  • from radioactive iodine ingestion.

  • Luckily, the statistics were quite poor,

  • which means that not many people were exposed.

  • It was somewhere around 1,300 or so, not like millions.

  • Yeah, 1,300 people total.

  • But what I want to jump to is the dose-versus-risk curve.

  • And this is going to belie all of our discussion

  • about the biological long-term effects of radioactivity.

  • What's the most striking thing you see as part of this curve?

  • AUDIENCE: Error bars.

  • MICHAEL SHORT: That's right.

  • That's the first thing I saw.

  • There are six different models for how dose an increased

  • risk of cancer proceeds.

  • And they all fall within almost all the error

  • bars of these measurements.

  • I say, again, thank God that the error

  • bars are so high because that means that the sample size was

  • so low.

  • So when folks say we don't really

  • know how much radioactivity causes how much cancer, they're

  • right because, luckily, we don't have enough data

  • from people being exposed to know that really, really well.

  • So some folks say we should be cautious.

  • I kind of agree with them.

  • Some folks say the jury's still out.

  • I also agree with them.

  • But you can start to estimate these sorts of things

  • by knowing how much radiation energy was absorbed

  • and to what organ.

  • So I think the only technical thing I want to go over today

  • is the different units of dose.

  • Because as you start to read things

  • in the reading, which I recommend

  • you do if you haven't been doing yet,

  • you're going to encounter a lot of different units of radiation

  • dose ranging from things like the roentgen, which responds

  • to a number of ionizations.

  • You won't usually see this one given

  • in sort of biological parlance.

  • Because it's the number of ionizations

  • detected by some sort of gaseous ionization detector.

  • So the dosimeters is that you all put on--

  • did you guys all bring these brass pen dosimeters

  • in through the reactor?

  • Did anyone look through them to see what the unit of dose was?

  • It's going to be in roentgens because that's

  • directly corelatable to the number of ionizations

  • that that dosimeter has experienced.

  • You'll also see four dose units, two of which

  • are just factors of 100 away from each other.

  • There is what's called the rad and the gray.

  • And there's what's called the rem and the sievert.

  • You'll see these approximated as gray.

  • You'll see these as R. And these are just

  • usually written as rem.

  • So a rad is simple.

  • Let's see.

  • 100 rads is the same as 1 gray.

  • And 100 rem is the same as 1 sievert.

  • And for the case of gamma radiation,

  • these units are actually equal.

  • I particularly like this set of units

  • because this is the kind of SI of radiation units

  • because it comes directly from measurable calculatable

  • quantities.

  • Like the gray, for example, the actual unit of gray

  • is joules absorbed per kilogram of absorber.

  • It's a pretty simple unit to understand.

  • If you know how many radioactive particles or gammas

  • or whatever that you have absorbed,

  • you can multiply that number by their energy,

  • divide by the mass of the organ absorbing them,

  • and you get its dose in gray.

  • Sievert is gray times some quality factor

  • for the radiation times some quality

  • factor for the specific type of tissue.

  • What this says is that some types of radiation

  • are more effective at causing damage than others.

  • And some organs are more susceptible to radiation damage

  • than others.

  • Does anyone happen to know some of the organs that

  • are most susceptible to radiation damage?

  • AUDIENCE: Soft tissues.

  • MICHAEL SHORT: Soft tissues like what?

  • Because there's lots of those.

  • AUDIENCE: Stomach lining.

  • MICHAEL SHORT: Stomach lining.

  • Yep.

  • Yeah?

  • AUDIENCE: Lungs.

  • MICHAEL SHORT: Lungs.

  • Yep.

  • What else?

  • AUDIENCE: Thyroid.

  • MICHAEL SHORT: Thyroid.

  • Yep, there is definitely one for thyroid.

  • AUDIENCE: Bone marrow.

  • MICHAEL SHORT: Bone marrow.

  • What other ones?

  • Brain, actually not so much.

  • The eyes.

  • And where else do you find rapidly

  • dividing cells in your body?

  • AUDIENCE: Skin.

  • MICHAEL SHORT: Skin.

  • Yep, the dermis.

  • AUDIENCE: The liver?

  • MICHAEL SHORT: I don't know about the liver.

  • I would assume so.

  • Yeah, it's a pretty active organ.

  • But when folks are worried about birth defects,

  • reproductive organs.

  • The link here that, for some reason,

  • is not said in the reading, and I've never figured out

  • why, is the more often a cell is dividing, the more susceptible

  • it is to gaining cancer risk.

  • Because every cell division is a copy of its DNA.

  • And any time that radiation goes in and damages or changes

  • that DNA by either causing what's

  • called a thiamine bridge where two thiamine bases get linked

  • together or damaging the structure in some other way,

  • that gene is then replicated.

  • And the faster they're replicating,

  • the more likely cancer is going to become apparent.

  • I guess this brings up a question.

  • When does a rapidly dividing cell become cancer?

  • Is it division number 1 or is it when you notice it?

  • I guess I'll leave that question to the biologists.

  • But if you notice, in the reading,

  • you'll see a bunch of different tissue equivalency factors.

  • And you'll just see them tabulated and say,

  • there they are.

  • Memorize them.

  • I want you to try and think of the pattern between them.

  • The tissues that basically don't matter,

  • like the non-marrow part of the bone, dead skin cells, muscles,

  • things that basically aren't listed that much,

  • they're not dividing very fast.

  • But anywhere where you find stem cells, the lining

  • of your intestine, your lungs which

  • undergo a lot of environmental damage

  • and need to be replenished, gonads, dura, skin--

  • what was the other one that we said?

  • Eyes.

  • These are places that are either sensitive tissues

  • or they're rapidly dividing.

  • And so the sievert is kind of in a unit of increased equivalent

  • risk so that, if you were to absorb one gray of gamma rays

  • versus one gray of alphas, you'd be about 20 times more likely

  • to incur cancer from the alphas than the gammas because

  • of the amount of localized damage that they do to cells.

  • And we'll be doing all this in detail pretty soon.

  • And then for tissue equivalency factor, if you absorb one gray

  • and your whole body, which means one joule per kilogram

  • of average body mass, versus one gray directly

  • to the lining of your intestine by,

  • let's say, drinking polonium-laced tea

  • like happened to a poor-- who was it?

  • Current or ex-KGB guy or the Russian fellas?

  • No, it was the KGB guys that poisoned him, right?

  • Yeah.

  • Do you guys remember back in 2010 or so?

  • There was a Russian--

  • was he a journalist?

  • AUDIENCE: Actually, he was ex-KGB.

  • MICHAEL SHORT: Ex-KGB.

  • So the current KGB somehow got into London

  • and slipped polonium into his tea at a Japanese restaurant.

  • AUDIENCE: [INAUDIBLE]

  • MICHAEL SHORT: Really?

  • AUDIENCE: I think so, right?

  • [INAUDIBLE] It was unsuccessful.

  • MICHAEL SHORT: What was his name?

  • Let's see.

  • The polonium poisoning.

  • Did he actually die?

  • Poisoning of Alexander Litvinenko.

  • AUDIENCE: That's pretty close to dead.

  • MICHAEL SHORT: He's not doing too well.

  • Illness and poisoning, death, and last statement

  • at the hospital in London.

  • So yeah.

  • AUDIENCE: He probably said something awesome.

  • AUDIENCE: What did he say?

  • MICHAEL SHORT: Well, interesting.

  • That probably has something to do with it.

  • AUDIENCE: That's a lot of-- a really long last--

  • MICHAEL SHORT: Yeah?

  • Well, we're not going to comment on the politics.

  • But the radiation effect worked, clearly, unfortunately.

  • So polonium is an alpha emitter.

  • And that caused a massive dose of alphas

  • to his entire gastrointestinal tract.

  • And that caused a whole lot of damage to those cells.

  • No time for cancer.

  • It actually killed off a lot of those stem cells.

  • And the way that radiation poisoning would work

  • is that, if you kill off the stem cells,

  • the villi in your intestines die,

  • which are responsible for absorbing nutrition.

  • You can't uptake nutrition.

  • You basically starve.

  • It doesn't matter what you eat.

  • It's messed up.

  • Yeah.

  • That's a really bad way to go.

  • It's called gastrointestinal syndrome.

  • And we'll be talking about the progressive effects

  • of acute radiation exposure where

  • you have immediate effects mostly relating

  • to the death of some organ that is responsible for either cell

  • division to keep you alive or, in extreme cases,

  • your neurological system.

  • And nerve function just stops at the highest levels of dose.

  • And that corresponds to doses of around 4 to 6 gray.

  • 4 to 6 joules per kilogram of villi, or body mass,

  • will kill you pretty quickly with very little chance

  • of survival as what happened here.

  • And so this was the problem.

  • With all the folks living around and near Chernobyl and Ukraine

  • and Belarus and everywhere was the contamination

  • was pretty extensive.

  • About 4,000 people are estimated to have died

  • or contracted cancer from this.

  • I can't believe how low that number is.

  • But it's still 4,000 people that should've never happened to.

  • And effects were felt far away in towns like Gomel

  • and-- can't read that one because there's not

  • enough pixels.

  • Because of the way that, let's say, rainwater--

  • or let's say the vapor cloud from the reactor was--

  • the way rainwater caused it to fall

  • on certain places, which still, to this day,

  • can have a really large contamination area.

  • And this brings me a little bit into what should we be

  • worried about from Fukushima--

  • a whole lot less than Chernobyl.

  • And the reason why is Fukushima did

  • undergo a hydrogen explosion and did

  • and still continues to release cesium-137 into the ocean.

  • Luckily, for us, the ocean is big.

  • And except for fish caught right near around Fukushima,

  • even though concentrations can be measured at hundreds

  • to thousands of times normal concentrations,

  • they can still be hundreds to thousands of times

  • lower than the safe consumption.

  • So a lot of the problems you see in the news today,

  • I'm not going to call them lies.

  • But I'm going to call them half truths.

  • Folks will show the radiation plume of cesium-137 escaping

  • from Fukushima.

  • And that's true.

  • There is radiation escaping.

  • The question is, is it high enough

  • to cause a noticeable increased risk of cancer?

  • That's the question that reporters

  • shouldn't be asking themselves.

  • When they only tell the half of the story that

  • gets them viewers and they don't tell the half of the story

  • to complete the story and tell you,

  • should you be afraid or not?

  • Because unfortunately, fear brings viewers.

  • This is the problem--

  • and I'm happy to go on camera saying this.

  • This is the problem with the media today is,

  • with a half truth and with a half story,

  • you can incite real panic over non-physical issues

  • that may not actually exist.

  • And so it's important that the media tell the whole story.

  • Yes, it's true that Fukushima's releasing cesium-137.

  • How much though is the question that people and the media

  • should be asking themselves.

  • And in the rest of this course, we're

  • going to answer the question, how much is too much?

  • So I'm going to stop here since it's 2 of 5 of

  • and ask you guys if you have any questions

  • on the whole second part of the course

  • or what happened in Chernobyl.

  • Yeah.

  • AUDIENCE: Yeah.

  • Could you explain the quality factor term

  • and how you find that?

  • MICHAEL SHORT: Yeah.

  • Well, there's two quality factors.

  • There is the quality factor for radiation, which will tell you,

  • let's say, how much more cell damage

  • a given amount of a given type of radiation of the same energy

  • will deposit into a cell.

  • And the tissue equivalency factor

  • tells you, well, what's the added

  • risk of some sort of defect leading to cell death or cancer

  • or some other defect from that radiation absorption.

  • So to me, the tissue equivalency factor

  • is roughly, but not completely, approximated

  • by the cell division rate.

  • And the radiation quality factor is

  • going to be quite proportional to the stopping power.

  • You'll see a term called the Linear Energy Transfer, or LET.

  • This is the stopping power unit used in the biology community.

  • It's stopping power.

  • And luckily, the Turner reading actually

  • says it's somewhere buried in a paragraph.

  • LET is stopping power.

  • So if you start plotting these two together,

  • you might find some striking similarities.

  • I saw two other questions up here.

  • Yeah?

  • AUDIENCE: Why is Chernobyl still considered off limits

  • if most the half-lives of these things

  • are on the range of days to two years?

  • I mean, it happened--

  • MICHAEL SHORT: Let's answer that with numbers.

  • So most of the half-lives were on the range of days to hours.

  • But still, cesium-137, with a half-life of 30 years,

  • released a third of an exabecquerel.

  • That's one of the major sources of contamination

  • still out there.

  • In addition, if we scroll down a little more,

  • there was quite a bit of plutonium inventory

  • with a half-life of 24,000 years.

  • So on Friday, we're going to have Jake Hecla come in

  • and give his Chernobyl travelogue

  • because one of our seniors has actually been to Chernobyl.

  • And his boots were so contaminated with plutonium

  • that he could never use them again.

  • They've got to stay wrapped up in plastic.

  • So some of these things last tens of thousands of years.

  • And even though there weren't a lot

  • of petabecquerels of plutonium released,

  • they're alpha emitters.

  • And they're extremely dangerous when ingested.

  • So greens and things that uptake radionuclides

  • from the soil like moss and mushrooms are totally off

  • limits in a large range of this area.

  • You will find the video online, if you

  • look, of a mayor from a nearby town saying,

  • oh, they're perfectly safe to eat.

  • Look, I eat them right here.

  • And I just say read the comments for what

  • people have to say about that.

  • Not too smart.

  • Yeah.

  • AUDIENCE: So what's the process now

  • for taking care of [INAUDIBLE]?

  • MICHAEL SHORT: So the sarcophagus around the reactor

  • has got to be shored up to make sure

  • that nothing else gets out.

  • Because most of the reactor is still there.

  • And let's say rainwater comes in and starts

  • washing away more stuff into the ground or whatever.

  • We don't want that to happen.

  • Soil replacement and disposal as nuclear waste

  • is still going on.

  • Removal of any moss, lichen, mushrooms,

  • or anything with a sort of radiation exposure

  • has got to keep going.

  • But the area that it covers is enormous.

  • I don't know if we're ever going to get rid of all of it.

  • The question is, how much do we have to get rid of

  • to lower our risk of cancer in the area to an acceptable rate?

  • There will likely be parts of this

  • that are inaccessible for thousands to tens of thousands

  • of years unless we hopefully get smarter

  • about how to contain and dispose of this kind of stuff.

  • We're not there yet.

  • So right now, the methods are kind of simple.

  • Get rid of the soil.

  • Fence off the area.

  • Some folks have been returning.

  • And they do get compensation and free medical visits

  • because the background levels there

  • are elevated but not that high.

  • So folks have started to move back to some of these areas.

  • But there's a lot that are still off limits.

  • Any other questions?

  • Yeah.

  • AUDIENCE: It's way worse than the atomic bombs dropped

  • on Hiroshima and Nagasaki because those

  • are full-functioning cities at this point.

  • MICHAEL SHORT: Yeah.

  • The number of deaths from the atomic bombs way

  • outweighed the number of deaths that will ever

  • happen from Chernobyl.

  • AUDIENCE: But why is the radiation

  • from those bombs not--

  • MICHAEL SHORT: Oh, not that much of an issue?

  • There wasn't that much material.

  • There wasn't that much nuclear material in an atomic bomb.

  • What did you guys get for the radius of the critical sphere

  • of plutonium?

  • AUDIENCE: [INAUDIBLE] centimeters.

  • MICHAEL SHORT: Centimeters?

  • Yeah.

  • It doesn't take a lot.

  • It takes 10, 20 kilos to make a weapon.

  • Now, we're talking about tons or thousands

  • of tons of material released.

  • So an atomic weapon doesn't kill by radiation.

  • It kills by pressure wave, the heat wave.

  • The fallout is not as much of a concern.

  • And we'll actually be looking at the data from Hiroshima

  • and Nagasaki survivors to see who

  • got what dose, what increased cancer risk did they get,

  • and is the idea that every little bit of radiation

  • is a bad thing actually true.

  • The answer is you can't say yes or no.

  • No one can say yes or no because we don't have good enough data.

  • The error bars support either conclusion.

  • So I'm not going to go on record and say

  • a little bit of radiation is OK.

  • They data is not out yet.

  • Hopefully, it never will be.

  • Any other questions?

  • All right.

  • I'll see you guys on Thursday.

The following content is provided under a Creative

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B1 中級

26.チェルノブイリはどのようにして起こったのか (26. Chernobyl — How It Happened)

  • 2 0
    林宜悉 に公開 2021 年 01 月 14 日
動画の中の単語