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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.
