Chernobyl Visually Explained

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