good morning John a nuclear reactor at
Chernobyl exploded this kind of
explosion had never happened before and
hasn't happened since and it wasn't
supposed to be possible but why why was
it supposed to be impossible and why
wasn't it there are simplified answers
out there I'm not gonna get deep into
physics but I am gonna be a little bit
more robust in how I talk about it
because I think it's really interesting
to actually understand what happened
here and why but first an introduction
of terms nuclear pertaining to the
nucleus which is the center of the atom
that contains the protons and the
neutrons protons the nuclear particle
that has a positive charge the number of
protons defines what element an atom is
neutrons the neutral particle why does
it even exist well why does anything
exist so basically for it like subatomic
particle physics reasons if you have too
few neutrons and your nucleus the
nucleus will fall apart if you have too
many nucleus will fall apart well I
forces I guess isotope so the number of
protons an atom has always determines
what element that atom is six protons
its carbon every time now carbon usually
has six neutrons but sometimes it has
five or seven or eight or nine and so we
have this word isotope to describe these
different forms of carbon so the normal
carbon is carbon-12 and then those
carbon 13 and carbon 11 etc unstable
isotope now almost all naturally
existing elements on earth are stable
isotopes that's because they're stable
unstable isotopes generally have too
many or too few neutrons and so they
will emit energy or particles on their
path to becoming a more stable nucleus
and that process of emitting energy or
particles is called radioactive decay
radioactive decay is one kind of nuclear
reaction there are two other kinds
there's fusion where two atoms come
together to become one atom and then
there's fission when one atom breaks
into two atoms during fission the
leftover fission products which is all
the particles and atoms that are left
over are always a little bit lighter
than the original atom what happened to
that mass well it became energy as
described by a pretty famous equation
that energy is most of what we
ultimately capture in a nuclear reactor
and turn into electricity the products
of fission themselves are unstable
isotopes so they continue to decay
inside the nuclear reactor so heat and
nuclear react
is actually created in two ways one the
vast majority of it is created through
the fission of the atoms into smaller
atom secondarily though heat is produced
by the radioactive decay of these
unstable isotopes that are the product
of the fission reaction so even after
the nuclear reaction stop this
radioactive decay continues to produce
heat that heat has to go somewhere it
goes into the fuel and if you can't keep
cooling off the fuel it will melt itself
and then it keeps getting hotter and
hotter it makes itself hotter as long as
these isotopes are unstable and emitting
radiation
they heat themselves up and that can
melt through just about anything which
is what we call a meltdown now there is
one single naturally occurring isotope
on our planet that has a very special
ability rhenium 235 if you hit it with a
neutron not only will it split apart but
it will create more neutrons and those
neutrons if they hit another uranium 235
that will split apart and it will cause
this nuclear chain reaction this is the
reason why we have nuclear power on this
plan now u-235 is a rare isotope it's
about 0.7% of naturally occurring
uranium uranium 238 is much more common
and much more stable so usually when
we're making nuclear power or a nuclear
bomb we have to enrich the uranium so
there's much more u-235 and like a
nuclear bomb it's like 80% u-235 in that
case pretty much every Neutron flying
around and the thing is causing another
fission reaction which is causing more
neutrons so this instantaneously all of
the uranium fizzes fizzes it becomes a
bomb like it's a bomb that's the whole
point of the bomb this is of course not
what we want in nuclear reactors we want
to be able to very carefully control the
speed of the chain reaction usually a
super important part of this is
enriching the uranium not all the way up
to 80 percent but to some more enriched
version than naturally occurring uranium
just that enriching uranium it turns out
is extremely expensive so the genius ish
thing about the RBMK reactors design the
thing that people liked about it was
that it was a cheap way to use
unenriched uranium or very slightly
enriched uranium to create a
self-sustaining nuclear reaction to do
this because the fissionable atoms of
u-235 are farther apart and there's
other stuff in there that can absorb new
Tron's you have to do more with the
neutrons you have primary goal of all
nuclear reactors is controlling neutrons
and you can do this in two ways you can
absorb them which slows the reaction
down or you can moderate them which
speeds the reaction up that sounds a
little counterintuitive it is I'll
explain why when a uranium atom splits
apart the neutrons that come out are
going extremely fast they're going too
fast
actually for quantum mechanics reasons
that I don't understand to actually hit
another uranium atom and break it apart
mostly it will just bounce off and then
it'll fly out of the reactor and it
won't get used so counter intuitively
you have to slow down the neutrons to
speed up the rate of reaction to do this
you use something called a moderator if
they moderate the speed of neutrons they
do not moderate the speed of the
reaction they increase the speed of the
reaction now there are some reactors
that are able to do this with heavy
water which is like isotopically
enriched water that actually makes it
really good at neutrons bouncing off of
it and slowing down some that's
expensive because heavy water is
expensive but the nice thing is if the
reactor starts to get hot the heavy
water starts to boil and then there's
less of it around to slow down the
neutrons so the reaction starts to slow
down it's a negative feedback loop which
is the kind that you want inside of a
nuclear reactor
now the RBMK reactors used regular water
as a coolant because that's cheaper but
regular water is actually a neutron
absorber that's gonna become really
importantly so instead of heavy water
RBMK reactors use big heavy blocks of
graphite to slow down the neutrons
that's the neutron moderator so to sum
up you've got moderators which help
speed things up and you've got absorbers
which help slow things down
these are your gas pedal and your brakes
in your nuclear reactor and you need
these things because there's other stuff
that affects the rate of the reaction
that changes as the fuel is used it
changes depending how long the reaction
has been going and then all reactor is
like in the fuel you have the build up
of different byproducts of the fission
and some of those absorb neutrons some
of them slow neutrons down there's a big
one xenon 135 which is part of the decay
pathway after uranium breaks apart so it
doesn't show up immediately it just like
ramps up slowly and it starts absorbing
neutrons so you have to pull your
control rods out some there's ways to
control all this stuff it's complicated
so you need ways to be able to put on
the brakes or put on the accelerator
see anon thing is what happened on the
day of the accident they had basically
run the reactor all day in a sort of a
perfect procedure for what you would do
if you wanted to increase the amount of
xenon-135 in your reactor they didn't do
this on purpose it's just what happened
and so when they started to decrease the
power for the test the xenon started to
suck up all the neutrons and the reactor
dropped into this situation where they
just weren't getting any power out of
the reactor now the normal thing to do
with an RBMK reactor because of all
these positive feedback loops is to very
slowly bring the power back up but they
wanted to get the power back up fast so
what they did is they pulled all of the
control rods out pretty much all the
control rods out of the reactor
the only things left absorbing neutrons
in this reactor now our water and
xenon-135 if one or both of those things
go away then you would have a nuclear
reactor that is in a very difficult to
control state they talk in the show and
a lot of science communication talks
about and I had completely understand
this because like do you don't have
however long this video is to explain
all of this they talked about how the
control rods were tipped with graphite
so when they came back into the reactor
the first thing that happened is that
this moderator the things that speed up
the reaction was the first thing that
came into the reactor now you might
having heard that version which is the
simplified version be like why on earth
would you tip a control rod with
graphite like there's easier cheaper
stuff to tipic control rod with the
simple answer to that question is that
they didn't let's explain so when you're
using normal water as your coolant it
also is absorbing neutrons so if you
take your control rod out which is the
thing that absorbs neutrons and all
that's left like water is just taking
that space you have a less good but
still a control rod made of water in
that space absorbing neutrons so in RBMK
reactors they have control rods that
control in both directions the top of
the control rod is made to absorb
neutrons and then as that pulls out it
pulls in a graphite rod and that is made
to increase the reaction this is the
graphite tip that's not actually a
graphite tip it's part of what makes the
reactor work so you have in every
control rod a brake and an accelerator
and you can't do one without the other
so on the day of the accident they
just pulled out all of the brakes they'd
pulled in all of their accelerators
pretty much all of them this is not the
biggest problem the biggest problem is
neutron flux which is basically the
movement of neutrons throughout
different parts of the reactor and you
want to control the movement of neutrons
throughout the reactors so that all of
the fuel burns evenly so you don't waste
anything and so there aren't areas of
the reactor that are hotter than some
other areas to accomplish this the
graphite moderator rods are actually
shorter both at the top and the bottom
than the fuel rods so in those spaces
there's some water and that water is
absorbing neutrons and in the middle you
get a nice flat curve of Neutron flux
evenly distributed throughout the
reactor and this is fine this is great
unless you take all of the control rods
out at the same time and then put them
all back in at the exact same time and
we'll see why so on the day of the
accident they had pulled out all of the
absorbers pulled in all of the
accelerators and then you have your
positive feedback loops kicking at
specifically because the flow of water
through the reactor had been slowed down
the water started to boil water is
usually there absorbing neutrons so it's
boiling lis there isn't as much water
there absorbing neutrons which is
increasing the rate of the reaction and
finally all these neutrons that are
suddenly flowing around eats up all of
the xenon-135 so that control that had
been there the whole time is suddenly
gone and then you have the final fatal
decision the thing that you're supposed
to do in this situation when the control
rod started to come back to the reactor
the graphite rods displaced the water at
the bottom of the reactor until suddenly
there was nothing in the bottom row of
the reactor that in any way throughout
the entire reactor was controlling
neutrons at all so the reactor is
already dangerously overpowered and then
localized in the bottom of this reactor
you have the spike of Neutron flux
that's beyond anything the reactor is
designed for that very rapidly increased
the amount of energy being produced and
something somewhere broke it could have
been a lot of different things that in
this situation broke it might have been
multiple things at once but when it
broke it locked those graphite rods into
this most dangerous of positions
and now it's time for a sidenote on
explosions an explosion generally seems
like it has something to do with fire to
us fire is often a component of
explosions it's why we can see them so
you know that's what the movie
explosions like very fiery explosions
actually happened usually when a solid
or a liquid converts into a gas and
gases take up much more space than
liquids and as that rapid expansion
happens that is your explosion that
sounds wild but it's the case this is
all about phase change and the hotter
your gas is the more space it takes up
and so and so and so the water and the
reactor got so hot that it turned into
this extremely pressurized vat of steam
so hot that in fact it dissociated into
hydrogen and oxygen which then becomes
itself a fuel the top popped off the
reactor the graphite in this environment
became basically fuel and so it caught
on fire
and short exploded now Fukushima also
exploded it exploded in a much different
way instead of the reactor itself
exploding hydrogen escaped from the
reactor and that hydrogen gas exploded
and that damaged the reactor had but
much different from what happened to
Chernobyl all those isotopically
unstable fission products were then
either started melting because there was
no cooling left or they burned and went
into the air for people to breathe for
hundreds of miles it sounds like a
worst-case scenario and in some ways it
is this is as bad as it can get when it
comes to a reactor exploding but it is
not the worst-case scenario when it
comes to the impact that it could have
on people there were ways after the fact
that this could have gone much worse if
it were not for a lot of hard work and
clever engineering and bravery the
reactor design is all about creating
balance even with all of these problems
the design of the reactor prevented
anything catastrophic from happening
unless you did a very specific set of
things the things that they did in 1986
if you want to get a little deeper into
the physics of this Scott Manley's video
on this topic is amazing and if you want
to watch the HBO miniseries I found that
very good and enlightening in terms of
the human and political causes of this
disaster as in in addition to the
engineering and physical causes of the
disaster John I'll see you on Tuesday