Introduction The Chernobyl accident the physics clearly
explained the worst nuclear accident in history but why did it happen today I'll show
you step by step I'll explain the relevant Nuclear Physics I'll Implement that in code and
make a basic model of the Chernobyl reactor then I'll use the model to explain and redo the same
terrible decisions in the Chernobyl control room and you'll be able to see how the reactor responds
and exactly what went wrong a very simple reactor but let's start with the basics in any nuclear
reactor you must bring the activity up to power Basic Fission and keep it balanced there the power from the
nuclear reactor heats the water into steam which then drives a turbine to generate power which
is done by fission I do recommend watching my video about critical mass where simulate nuclear
reactor and also an atomic bomb while going into deeper details about fission but don't worry I'll
assume you lazy and quickly recap now I'll also start building up the model so we'll be able to
simulate the Chernobyl reactor the nuclear reactor uses uranium of the isotope 235 this element
has the property of undergoing induced fission which is when the element absorbs a neutron the
element will split i.e. fissioning into two new lighter daughter elements and release three
new neutrons now these secondary elements are not important for the bigger picture so not to
confuse yourself visually in the model instead of showing two daughter elements I'll simply
just show whether the element is uranium 235 or not blue for uranium235 gray if it isn't we
can stack many uranium 235 in a metal grid and fire a single neutron we just created a nuclear
Chain Reaction this is what we want in a nuclear reactor although we want it a bit more controlled
and not this explosive in the reactor we will have different isotopes of uranium in the fuel and only
a few percentages is the fuel type uranium 235 this is how the metal is extracted from the ground
having highly enriched fuel in reactor is not only extremely expensive but also way too dangerous
you can see in this reaction with a lot less fissionable material the reaction seems way more
stable the goal of course is to get the reactor up to some amount of reactions per second and once
you are there you keep it stable by ensuring each loose Neutron on average only hits a single
uranium235 now I want to add two more things to the model before continuing these are just for
helping my model and not really of any importance in actual reactors the first thing I want to add
is the ability to replace used of fuel since I only have around a thousand simulated nuclei
I need to be able to magically pop in new and enriched uranium the second thing I want to add
in the model is the radioactive decay spontaneous Neutron emission this is when a radioactive
element suddenly releases a neutron a naturally occurring process let's give this property to all
non uranium nuclei i.e. the gray circles building the simulation control rods I want to simulate
the same type of of reactor used in the Chernobyl Control Rods accident the R.B.M.K reactor which apparently is
not an abbreviation for a ridiculously badly made Kettle the first thing I want to add is control
rods control rods have the property to absorb neutrons and just to state the obvious you can
cool down a reactor (Blue) by inserting control rods and you can heat up a reactor (Red) by
removing the rods allowing neutrons to flow freely we don't have everything yet to fully simulate the
R.B.M.K reactor and its downfall but for now let's we have it's a very basic nuclear reactor model
normally reactor reactivity is measured in megawatts but I'll just count the number of
neutrons present in the reactor you can see that number on top here I've coded some very
basic automatic control of the control rods every second control rod will go up if there's
under 40 neutrons inside the reactor if there's above 40 the control rods will be inserted
instead as you can see by slowly pulling out control rods I'm able to increase reactivity
safely whereas reinserting the control rod decrease the reactivity building the simulation
water all right let's keep adding real physics Water concept to the model as I briefly mentioned the
reactor is surrounded by water which seats up and drives a turbine I'll depict water as a small
box the water has a low chance of absorbing a neutron whenever a neutron scatter with water
it will transmit kinetic energy heating up the water in the model let's add whenever a neutron
is inside the water it will heat it up when the water is cold I'm coloring it blue and then
fading it into red as as it gets hotter but I also want to add when the water gets too hot
it evaporates when this happen the water is gone and can no longer absorb neutrons in my model
this is going from red to completely gone when water cools off it condensates and reappears
let's try it again and fire even more neutrons than last time the front of the burst is mostly
absorbed while heating up the water this means the backside can easily propagate without being
absorbed in reality of course there will be some temperature exchange between each water lock but
let's just ignore that fact you can also see the water reappears so for our reactor that means
presence of a lot of water will keep reactivity down whereas heating up the water will lead
to holes or voids and the reactivity will go up even further reactors with this property are
set to have a positive void coefficient let's see that in action from the example before with the
control rods notice that we know how to lift the control rods even up further because the water
is helping to absorb the neutrons but because control rods keep the reactivity down to 40
neutrons inside you won't see enough neutrons to evaporate the water for now you'll have to wait
a bit to see the positive void coefficient in action building the simulation on for this chapter
I'll keep it short and concise and don't explain Xenon135 that much after absorbing a neutron at a later
time the nuclei will have a chance to Decay into the isotope Xenon 135 I'll denote Xenon with the
same dark gray color as the control rods Xenon has the property to strongly absorb neutrons once
it's a Xenon element the only way to undo that is for the element to absorb a neutron this is what
is referred to as burning away Xenon now let's run the model again with the Xenon property you'll
see once again it's now hard of the reactor and we must raise the control rods even further
to get the same reactivity level around 40 neutrons building the simulation moderation last
thing to add when neutrons are released from the Moderation nuclei they fly out with around 5% the speed
of light let's add the property to the model now neutrons sent out from nuclei are fast I will
denote that with a white dot in the neutron this speed is way too fast for fission absorption it
turns out the chance of interaction is incredibly low then this is called the nuclear cross-section
it can be thought of as the probability of a reaction when they are going fast I'll set the
interaction chance to 0% for fission you must first use the so-called moderator something
for the neutron to scatter on and absorb some kinetic energy in the R.B.M.K reactor all fuel
rods were surrounded by graphite for moderation in the model I'll depict a moderator as a white
rod with a gray border after collision with the moderator the neutrons are no longer too fast
and are instead called thermal neutrons which basically just means slow neutrons let's have
these thermal neutrons behave just as before with an interaction chance of 100% when the neutrons
and nuclei touch when thermal neutrons collide with the moderator we will just set them to fly
right through to make sure we on the same page I'll run the model without control rods water or
the Xenon on property only with the fission fuel and moderators slowly being inserted hopefully
this depicts the influence of moderation and the nuclear cross section this is yet another
control of reactivity removing moderation means neutrons will have a much lower chance of
undergoing fission while more moderation means more neutrons will have a high probability
of fission finally let's see everything together a lot is going on so for this model
I added a legend of all the different objects below now we have everything we need to recreate
the Chernobyl accident I'll recreate the same events as in the control room here's the first
event day of the accident event one reactor normal Event 1: Reactor normal the simulation I'm showing right now is showing
how the reactor was running stable just before the accident it was running at around 50% power to
meet power demands for the grid line the Xenon was being burned off just as quickly it was being
made reactor is stable we can pretend the 50% power corresponds to 40 active neutrons which is
what the control here are trying to stay stable at to illustrate the states we just saw here
I'm plotting the reactivity of the reactor and according to the events event one reactor
normal for this event the reactor was running at 50% on this part here I'll show how many of
the control rods was present how much water was in the reactor the water was stable cooling the
reactor down some voids here and there I'll also plot how much Xenon was present not much day
of the accident event 2 power reduction let's Event 2: Power Reduction continue the simulation from where we stopped
last time we'll do that from now on well what happened then a safety test was scheduled later
this night this test required the power to be reduced to around 30% power we can present 30%
power correspond to 20 active neutrons and set the automatic control rod to try and hit this
number what we can see here is the Xenon is still being generated from a delayed reaction
when the reaction ran at 50% power but only now 30% power burns the Xenon away let's see the state
of this simulation we just saw the reactivity well around 30% control rods were inserted in order
to try and reach this reduced power level for some reason they also increased the water flow
cooling meaning control rods has to be raised even further up however the lower power means
not as many neutrons are present to burn the Xenon away day of the accident event 3 power drop
after that the power unexpectedly dropped to 1% Event 3: Power drop because of the high Xenon buildup we are reaching
very low levels I set the control rod to try and reach 20 active neutrons but in my simulation
we are reaching lower values because of the high Xenon buildup this is me trying to show you
a reactor stalling and stuck in the Xenon pit this is the 1% unexpected power drop I'm trying
to show that happened the day of the accident so the power level is plummeted to 1% they
didn't change anything to the control rods of course the reactor was simply stalling because
of the extra high water flow with no voids and the Xenon poison continually building up from
the reactor running at 50% power day of the accident event fall power up attempted now many
safety system were turned off safety protocols Event 4: Power up attempted ignored automatic control of the control rods
was turned off in an attempt to power up the reactor they were only able to get the reactor up
running at 7% power by pretty much removing all control rods notice the control rods are raised
but the water and Xenon keeps the reaction in check so the state is 7% power all control rods
raised water level is still cooling the core and Xenon keeps building up from the delayed reaction
day of the accident event five test starts the reactor was still stuck at 7% power the computer
wants them to shut down immediately so they turn Event 5: Test starts off the computer instead they could not get
the power higher than 7% and yet they started the test pretty much all control rods are still
raised the test included switching off half the re-circulation pumps temporarily to reflect this
change in the water pumps I'll simply allow the water to cool off much slower the effect is
of course more water boils away causing more Xenon to burn off and this is a positive feedback
loop causing ever more water to evaporate causing ever more s to burn away right so the reactor
is stuck in 7% power pretty much all control rods are still raised water is boiling away
quickly forming voids causing more Xenon to burn the day of the accident event 6 scram now here's
something I hadn't added to the simulation the Event 6: SCRAM control rods actually had graphite displaces
hanging below looking something like this commonly said graphite tips so actually this
whole time the scenario should have looked like this instead now I'll add that to the simulation
let's continue the simulation with these added changes let's pause the simulation suddenly with
the water and the Xenon disappearing the reactor increases unexpectedly fast in the reactor and The
Operators notice this and presses the scram button AKA az-5 a safety switch scram button's job is to
put in all of the control rods as absolutely fast as possible it was a lifeline The Operators always
thought they had in case something went wrong in the test anyway pressing this scram button it
will take some time to deploy the control rods they are slow the operator didn't know what they
just had done because normally the top had a lot more stopping power than the accelerating bottom
graphite but the reactor was in such an unstable state that the bottom had much much stronger
accelerating power than the top had a stopping power as a result the scram actually increased
the reaction rate after that the control rods and graphite displacers were stuck control rods
didn't get far enough in the reactor to do any difference let's just quickly see the states
here and the pause of the simulation the power had unexpectedly risen a lot control rods were set
in by the scram but it was stuck and all of the sudden all the water flash boils away and all of
the free neutrons burn away all of the Xenon now there's nothing to stop the reactor the reactor
is just set in such a crazy state it's just all gas and no brakes let's see that here you see
the control rods with graphite displacers got stuck extremely heavy metal rods fuel rods
and the graphite and the control rods are jumping up and down in the sockets due to
the extreme pressure it has been estimated the power was over 100,000% its design
capacity and of course the core could not hold that together explosion occurred so
hopefully now you have an intuition on what went wrong and also learned a little Nuclear
Physics I basically only cover the physics and the incredibly dangerous design of the
R.B.M.K. reactor but there are tons of other factors that also played a role which we didn't
cover such as human errors and unexperienced team the lack of any safety culture the ability
to turn off safety system cheap reactors over safety the Soviet Union and the corrupt system
and more and now I don't want you to walk away with the feeling that nuclear reactors are unsafe
this is just one reactor design and many more safe reactor design exists for example ones with the
negative void coefficient they are impossible to blow up I could simulate that for you one
day and lastly even including all of the death of this horrible accident nuclear reactors are
still much much more safe thank you for watching