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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 P set. 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 uh 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.
This is footage taken from a helicopter
from folks that were either surveying or
dropping materials onto the reactor.
That was probably a bad idea.
Over where the smoke is. We'll get into
what the smoke was.
So, that
red stuff right there, that's actually
glowing graphite amongst other materials
from the graphite fire that resulted
from the R the RBMK reactor burning
after the Chernobyl accident caused by
both flaws in the physical design of the
RBMK reactor and absolute operator
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
radionuclides 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, or I
let's say 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
the Chernobyl reactor, which you guys
can read on the NEA or Nuclear Energy
Agency website, the same place that you
find Janice. And we're going to refer to
a lot of the Janice cross sections to
explain why these sorts of events
happened.
So, the whole point of what happened at
Chernobyl was it was desired to see if
you could use the spinning-down turbine
after you shut down the reactor to power
the emergency systems at the reactor.
Uh this would be following something
what's called a a loss of offsite power.
If the offsite 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 desired to find out can we
use the spinning-down turbine to power
the emergency equipment if we lose
offsite 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, a very low power, and see
what comes out of the turbine itself, or
out of the generator, rather.
Um 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 RBMKs, 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
uh light water reactor coolant loop
where a 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. Uh 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 Janice
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.
Z fission.
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, a thousand 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's
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,
those the tops of those pressure tubes,
despite being about 350 kilo chunks of
concrete,
you could walk on top of them. It'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.
Uh but the grid operator came back and
said, "No, you can't just cut the
reactor power to nothing. You have to
maintain it 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 2205
in neutron physics.
Black shirt really shows chalk well.
Okay.
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,
you know, fission as fission products
go, but it also absorbs lots and lots
and lots of neutrons.
Let's see if I can 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'll build up xenon 135 that you have
to account for in your sigma absorption
cross section.
Cuz like you guys saw on 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 zero cuz you don't
have any 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 in your K effective expression
where it's sources
over
absorption plus leakage
this has the effect of rise at raising
sigma absorption and lowering K
effective.
The trick is it doesn't last for very
long. It both decays with a half-life of
about 5 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 MW.
And they were like, "What is going on?
Why is the reactor power so low? We need
to increase the reactor power." So what
did they do? Couple things. One was
remove all but six or seven of the
control rods.
Going way outside the spec of the design
uh 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. Well,
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 that stuff
was 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
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 zero. It's small but fairly
significant. Let's actually take a look
at it cuz we can always see this in
Janus.
Go back down to hydrogen.
Hydrogen one.
And we look at the absorption
cross-section.
And of course it started us with a
linear scale. Let's go logarithmic. Ah.
So at low energies at you know, 10 to
the minus eight to 10 to the minus seven
it's around a barn.
Not super high but absolutely not
negligible.
Which meant that part of the normal
functionality of the RBMK depending on
depended on the absorption of the water
to help absorb some of those neutrons.
With those neutrons gone, I'm sorry,
with those 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 water the coolant to boil
which would cause less absorption which
would cause the power to increase. Yeah,
Charlie? So if they removed the water
from
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 denser
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 kg pressure tube
caps were just rattling. I mean imagine
something that weighs you know, 900 lb
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?"
Uh 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.
2 minutes later
when the power starts to become unstable
the caps on the fuel channels which
again are like 350 kg 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 kilo caps
and still what did they 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 4 seconds.
So it was normally a 1000 MW electric
reactor which is about 3200 MW thermal.
It was producing nearly
yeah, half a terawatt of thermal power
for a very short amount of time until it
exploded.
Now this is interesting. A lot of folks
called 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.
Should be further down. Yeah. To give
you a little sense of scale
the reactor cover which weighed about
1000 tons launched into the air
and landed above the reactor
sending most of the reactor components
up to a kilometer up in the air.
4 seconds later
that was followed by a hydrogen
explosion.
Let me get back down to that chronology.
So yeah.
At 1:23 and 40 seconds in the morning.
Oh yeah, so I should mention 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 were tipped with about
6 in 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 6 in 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's
cross-sections
I've plotted here the total
cross-section
the elastic scattering cross-section and
down here in the point 001 barn level is
the absorption cross-section. About 1000
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 ft 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? Um
so my understanding is that also one of
the designs was that the control rods
didn't like immediately drop down but
they were slowly lowered. Yep. Um so
They took they took 7 to 10 seconds.
Okay, if they had a system where they
did drop them would that have possibly
actually shut the system down properly?
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.
Um
whether or not, you know, so the extra
the extra abs- what is it? 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 4 seconds
later. Ah, to 120 times full power.
Getting towards a terawatt, so. 1 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, any- 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 above 4% in any of the labs
here because that reaches the
flammability or explosive limit.
So, we were doing some um 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 wall so 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. Have you guys ever seen
people making water from scratch?
Mix hydrogen and oxygen in the 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. Watch
them jump out of their chairs
to teach basic chemical reactions, but
it's pretty loud. Enough uh 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 on
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.
So, that smoke right there
is from the graphite fire.
Not normal smoke.
Spoke too soon.
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.
With 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 radionuclides are absorbed
by animals and the human body? And what
was the main fallout, let's say, in the
in the uh
colloquial sense and the actual sense
from the Chernobyl reactor.
Let's look a bit what they did next,
though.
That's not quite true. We'll see why.
That actually did happen.
I think that pretty much summarizes the
state of things now.
They uh 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 could see from the video,
where is that nice still of
mol- 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
about 100%.
And I can actually pull up those figures
when we talk about how much of which
radionuclide was released.
Uh that's also a typo. If somebody wants
to call in, there's no 33 isotope of
xenon. It's supposed to be 133.
Um that would be interesting if someone
wants to call in and say the NAA's 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 three exabecquerel activity.
So, we're talking like megacuries or
might be giga. I can't do that math in
my head. Lot a lot of radiation. And 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?
Uh no one does Oh, you have. Okay. What
happens when you play with it? I mean,
it just
absorbs the stuff like
for instance, everything
and it just reacts with like acids and
stuff. I haven't done very much with it,
so. Okay.
I happen to have extensive practice
playing with iodine in my home cuz I did
all the stuff you're not supposed to do
as a kid. Going to build your own
chemistry stuff, things that somehow,
you know, leak out of your local high
school, somehow.
Iodine's pretty neat.
Yeah, it happens sometimes. Um 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 the the leaves that 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 couple
days ago?
Yeah.
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 is more damaging to
biological organisms?
The beta or the gamma?
You say the gamma. Why do you say so?
Doesn't beta get stopped by like skin or
clothing? It does.
But, if cesium is better known as
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 piece at number eight.
You'll see. That's why you guys are
getting your whole body counts. Speaking
of, who's who's gotten their whole body
counts at DHS?
Awesome. So, that's almost everybody.
You will need that data for problem set
eight. 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, cuz you will. Okay.
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 six What is it? Water for 660 keV
gammas, let's find that.
Table three.
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 cm squared per gram, and with
a density of 1 g, 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?
Cuz it's going to come back up in our
discussion of dose and biological
effects.
Integral of
Yep, of stopping power to the negative
one.
And that stopping power
is this simple formula.
Let's see. What did that come out as?
log minus beta squared. Ah, 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 the 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, "Okay, at
the round
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.
It's 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. You know, it's it's uh
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 is preferentially uptaken by
the thyroid gland somewhere right about
here.
Um 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 uptaken 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
apply 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?
That's That's right. That's the first
thing I saw.
There are six different models for how
dose and increased risk of cancer
proceeds, and they all fall within
almost all the error bars of these
measurements.
I'll 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
that you all put on the Did you guys all
bring these uh
these like brass
pen dosimeters into the reactor? Anyone
look through them to see what the unit
of dose was?
It's going to be in roentgens cuz that's
directly correlatable 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 gamma
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?
Soft tissues like what? Cuz there's lots
of those.
Stomach lining, yep. Yep. Huh?
Lungs? Yep. What else?
Thyroid. Yep, there there is definitely
one for thyroid.
Bone marrow.
What other ones?
Y'all,
brain actually not so much. Eyes.
And where else do you find rapidly
dividing cells in your body?
Skin? Yep, the dermis.
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 anytime that radiation goes in and
damages or changes that DNA by either
causing what's called a thymine bridge
where two thymine 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 that this brings up a question,
when does a rapidly dividing cell become
cancer? Is it division number one 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 need to be replenished, gonads,
dermis, 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 an 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 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 in 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 ex was it
current or ex-KGB guy? One of the
Russian fellows?
No, it's the KGB guys that poisoned him,
right?
Yeah, you guys remember back in 2010 or
so?
There was a Russian
Was he a journalist?
Ex-KGB. So the current KGB somehow got
into London and slipped polonium into
his tea at a Japanese restaurant.
Uh, really?
What was his name?
Let's see.
Then polonium
poisoning. Did he actually die?
Poisoning of Alexander Litvinenko.
He's not doing too well.
Illness and poisoning, death and last
statement.
At the hospital in London. So, yeah.
Well
interesting. That probably has something
to do with it.
Yeah? Well
all right, we're not going to comment on
the politics, but the 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 intestine
die which are responsible for absorbing
nutrition. You can't uptake nutrition.
You basically starve. 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. Uh, 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 it should never happen to.
And effects were felt far away in towns
like Gomel and I can't read that one cuz
there's not enough pixels. Um, because
of the way that let's say rainwater
whisked or let's say um, the vapor cloud
from the reactor was whisked away,
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
should 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
cuz 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 leasing
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. Yeah, uh, could you explain the
quality factor or determine how you find
that? Yep, the quality factor Well,
there's two quality factors. There's the
quality factor for radiation which will
tell you how much 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 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. Can I ask uh
Why is Chernobyl still considered like
off-limits if most of the half-lives of
these things were like
on the range of like days to
like 2 years? I mean, it happened in
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 on Friday, we're going to have
Jake Heckler come in and give his
Chernobyl travelogue, cuz one of our
seniors has actually been to Chernobyl.
And his boots were so contaminated with
pluto- with plutonium that he can never
use them again. They 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,
uh 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 a 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. So, what what's like the process
now for like taking care of Chernobyl?
Like, what do they
do there?
The so so the sarcophagus around the
reactor has got to be shored up to make
sure that nothing else gets out, cuz
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.
Uh removal of any moss, lichen,
mushrooms, or anything with a sort of
radiation exposure has got to keep
going. But, this 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. It's like way worse than the
atomic bombs dropped on Hiroshima and
Nagasaki, because those are like fully
functioning cities at at this point.
Yeah, the number of deaths from the
atomic bombs way outweighed the number
of deaths that will ever happen from
Chernobyl. But, like, why is the
radiation from those bombs not
Oh, not that much of an issue? Yeah.
There wasn't that much material. The not
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?
4.7
cm.
Centimeters? Yeah. Yeah.
Doesn't take a lot. It takes, you know,
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 this the
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 okay.
The data's not out yet.
Hopefully, it never will be.
Any other questions?
All right. I'll see you guys on
Thursday.