What Technically Happened at Chernobyl

what what happened at Chernobyl I think a lot of people seen the show a lot of interest for that but why am I here tonight why am i giving this presentation so I am a nuclear engineer and I became a nuclear engineer because I'm passionate about climate change I'm passionate the nuclear power is one of the most powerful tools we have to fight climate change and Chernobyl in that context is really difficult it was the worst nuclear disaster that's ever happened and so I'm here tonight because I believe that one of the problems nuclear power has is a lack of trust it's a complicated system it has all these interconnected effects and ultimately you have to trust that engineers structures and you know the government the governance of society is looking out for your interest there's plenty of historical evidence that that's not true if if I tell you that Chernobyl can't happen again in the West it's impossible with our reactor designer if I tell you that there's you know more debate about the health effects of low doses of radiation or that we can deal with nuclear waste why should you believe me why should you trust me when there's been so much mistrust before and so that's why I do these presentations that's why I go out I'm a nuclear engineer I see myself as someone who can advocate for the nuclear industry and I think the Chernobyl is such an important topic to understand but the second reason I'm here is I'm a huge nerd Chernobyl is fascinating as a nuclear engineer in the West you always understand and you're always trying to understand how far away you are from something really bad happening because when something bad is happening it's really complicated and difficult to follow difficult to model difficult to predict and so in the West we've done a really good job of staying out of these really crazy regimes and so when I first kind of flipped open a textbook and saw the Chernobyl accident I was like oh that's what a prompt reactivity insertion looks like it's not this is bad stay away from it but this is what happens and so I'm a huge nerd I've been I've studied Chernobyl a lot for a long time it's a really interesting way to learn the physics but it's I think it's more important for that first reason so so just to be it's full transparency about about why I'm here but so I'm gonna kind of get nerdy a little bit and so I wanted to make sure that we understand that Chernobyl was a human tragedy and so I'd like to take a moment of silence before we begin because as with so many tragedies the people who are most impacted had the least ability to prevent it the people in Pripyat the people surrounding Ukrainian the reactor operators even really had no way to prevent what was gonna happen that night there were 31 prompt fatalities from Chernobyl hundreds of cleanup workers who had no understanding of the radiation effects that could happen to them hundreds and thousands of people who were exposed to radiation without their consent so so 1966 17 years before true Nobel exploded as part of a 10-year plan the Soviet Union decided to add 12 gigawatts of nuclear power into the grid nuclear power was seen as an important prestigious symbol in addition to the coal and the oil that they had and also as part of their weapons development program a nuclear power station was plopped on a map given its sensuality in the grid near a small town of cappucci the town of Pripyat many people are familiar with it was really the workers village it was designed to house the construction and operations crews when they were thinking about building the reactor they actually did a trade study on multiple different reactor types that were available there was a hard BMK but there was also the pressurized water reactor that the West was building as well as a gas cooled reactor that had started to have experimental work they selected the RBMK RBMK 1000 is the the name of the reactor that was built and if you read the Soviet report you'll have this interesting acknowledgement they say while the engineering factors were the worst the availability of supply and the ease of construction where why it was selected so they built this plant there and they called it turn noble so RBMK was not a dumb design by any means it's actually sort of beautiful in one sense and in the way and the trades that had to be made in its design so it's huge for the reactor the day when it was built it was the largest most powerful reactor constructed reactor is being built in the United States at the time were a third of this size it was cheap and simple to build so it was not simple to operate and I'll talk about that in a second but all of the components and we're actually gonna get a pretty good understanding of what's going on in that plant if you follow with me through the end of this it was pipes it was graphite blocks it was Val I mean it was small components there was no huge steel welded vessels no large diameter welding this was easy to build for local tradesmen in the Soviet Union it also used very low enrichment fuel the Soviet Union didn't have the enrichment capacity the industrial base at the United States had at the time so they were able to use very low enriched fuel in this reactor design had continuous refueling so rather than in the u.s. plants shut down every 18 months to refuel but this plant could run continuously refueling now those last three points actually have a very important tie together so this reactor produce high-quality weapons-grade plutonium and yeah that was a design feature and actually if you look at this design the u.s. also had water-cooled graphite moderated reactors evaporated at very low power and they only produce weapons-grade plutonium these produce commercial power and just the purest so so what were the problems you know the biggest problem we're gonna talk about this a lot is positive void reactivity and that goes that's inherent in a graphite moderated reactor if I do my job you will understand what that sentence means by the end of this it had complex and non-intuitive nuclear physics fine but even some nuclear engineers that I know looking at what's going on here requires really careful understanding of a lot of physics that aren't intuitive codes that were in place at the time could not model what was going on in Chernobyl and they knew this and that's you know they thought they understood it well enough but it was definitely hard to operate at power there are sixteen hundred and fifty individual reactors in a lot of ways going on inside there and it was huge and the size of the core doesn't necessarily make a lot of sense to you but it doesn't make a lot of sense if you don't think about Neutron physics but a score this large this is a PWR core the blue cylinder in the center this is what a light water reactor in the West looks like so compared to that you're you can have multiple cores the neutrons are so far apart they can't talk to each other so you can have power in one part of the core and be shut down in another part of the core so let's understand how an RBMK works and you know kind of a generic nuclear reactor in general so let's start with the pump so pump circulates water it goes through a main distribution header and then up through the fuel next to the fuel it boils and turns into steam every fuel tube has its own steam header we're gonna see on the next slide how many fuel tubes those actually are this is a very simplified schematic those steam tubes go into a steam drum steam separator where the steam goes to a turbine and the onion so there's water droplets in the steam and so the water droplets go back to the coolant pump but the steam goes to a turbine it can it expands and cools that produces electricity which goes off to the grid then the pump the water that pumps back goes to the pump if you think that animation was cool I've had a lot of flights recently just wait so this is first of all this picture is only available on Pinterest I have no idea why i trolled every page of google looking RBMK RBMK 1,000 chernobyl and this was like on page 43 on Pinterest I have no idea why anyway awesome pictures so this is a 3d piping diagram of an RBMK so in the center you have the core every one of those grey lines contains fuel and coolant and also you know we're not going to go into it because it's not really germane but at the top is where the refueling machine connects so you can individually add power pull a fuel bundle out put in a new one so after you generate steam those theme lines every single one of those steam lines comes from its own fuel tube going into for moisture separators from the moisture separators goes to the turbine which is off off side there's a turbine on each side of the reactor so in that previous picture there's one turbine there's a left turbine and a right turbine there's left coolant pumps right coolant pumps drawing from the left condenser right condenser and from those main coolant pumps back to the inlet water back to the core that's the loop I'm not really gonna pause for questions cuz I talk a lot and I'll leave a lot of time for questions at the end but if I really mess something up and throw a hand up in Ala I'll try and I'll try and go back and cover it so just to give you a sense we're gonna go through the physics in a second and so I want you to have a sense of what the whole core looks like so everyone kind of get a sense of what that picture on the left shows so there's all these fuel tubes each one's surrounded by graphite so I kind of blew that up just to show you what a cell looks like so the orange is fuel in an annulus around each fuel element there's water and then an annulus around that there is graphite so it's fuel water graphite water fuel fuel water graphite make sense cool okay so de ke the fundamental challenge of reactor design and I've thought about this a lot because I wanted to say that sentence if this weren't true we would have a hundred percent nuclear power and I believe that a hundred percent when you turn off a nuclear reactor it does not go off it goes to five percent what is five percent of 3,600 megawatts so after one second five but for the fuel consumption of a 747 in-flight it's about 150 megawatts a decent-sized natural gas plan just producing heat in the core that you thought was off now that goes away pretty quickly so after a month or sorry after a day it's the fuel consumption of the most powerful locomotive in the world pulling steel through the Siberian like tundra that's where the most powerful locomotive in the world is a lot of people like locomotives a lot of internet stuff out there and then after a month it's the ninth most powerful locomotive so this is he it's like taking that fuel and just lighting it on fire and you have to remove that heat so there's a fundamental challenge of reactor design and so I have a shutting down a reactor is not that hard generally in the West shutting down the reactors assumed hit the scram button shutdown right one exception was Chernobyl so this starts to go into the why the why of Chernobyl right I'm mostly gonna talk about the wet but I want to touch on why so Chernobyl when it was designed they pointed out that the design didn't have a way to deal with a loss of off-site power you know and there was some debate of how probable is it for you to lose the whole grid and also have to shut down but they're like this is a feature we should have every plan in the West has this this is something that we should do so there's one sentence in the original operating procedure that says on a loss of off-site power use this use the steam still on the generator to spin the turbine make electricity for the pumps period that was it and they knew that they had this gap now there is some debate and in the shoter Noble they talk about this very differently this is I'll come back to this at the end but from all the literature I can tell certainly never at Chernobyl and almost certainly never at any other RBMK had this test been successfully run they had tried this three times and they were coming up to the fourth attempt analog electronics is apparently really hard I'm a mechanical engineer so t-minus four years four years before the accident another RBMK ignore Lena one ruptures a fuel rod and they blame it on operator error but operator error and then the Commission recommends a whole host of design changes to the control rods of our BM case so again there was some operator error but we need to redesign all the control rods except none of those changes were implemented not one it was a commission report it wasn't even circulated to other reactor operators amazingly one and a half years later sorry one and half years before the accident not later a similar event happened at Chernobyl didn't rupture a fuel rod they were doing a routine shutdown they hit the scram button and they saw a slight power increase couldn't explain it and that was it couldn't explain it moved on so let's go back to the basics we're gonna build up from here so what is fission a uranium atom is heavy if it gets hit by a neutron it splits it releases it releases fission project products and on average 2 point 3 negative 2 3 on average the the fission products are actually what produce the heat so it's these heavy isotopes slowing down hitting other things produces heat and excess neutrons keep the chain reaction going so this is sort of a similar chart a lot of people have seen before one neutron comes in hits uranium atom two neutrons fly out it's another uranium atom two neutrons from up in the moon so that time period is about 0.0001 seconds fast at a reactor period at 0.001 seconds that doubling time if you have a slight increase in reactivity you start doubling at that rate you will increase power by 2 million percent in one second oh I screwed up my transitions but so critical means increasing or sorry supercritical means increasing so this is super critical here critical is stable subcritical means decreasing so obviously you can't run a reactor with a period of 0.001 seconds any time you in any reactivity increase it would be bad so what happens is you trust that some of your neutrons aren't gonna hit anything summary neutrons are gonna get absorbed somewhere some of your neutrons are gonna get lost and so you'll kind of get this 1 1 1 thing going ok nice and stable but then if one Neutron hits something you'd have that whole chain reaction running away so your reliance line called delayed neutrons I may have spoken through that a little fast how many people followed that I'm going to do it one more time kind of quickly with knowing what the slides are going to click through just because it's a little important so super critical this is super critical one neutrons causing this chain reaction Neutron population is increasing population one two four eight but if you're stable right you're just producing constant power your Neutron population is constant that means most of your neutrons are flying off and not causing a chain reaction but if one of those extra neutrons hit something you'd go back to that chain reaction increasing a little more sense okay so when in a fission when on uranium atom splits it creates fission products those fission products in nuclear physics whatever they will release a neutron spontaneously randomly seconds two minutes later and so what you do is you get your chain reaction almost constant and then you have 0.64% of your Neutron population coming from those spontaneously release neutrons but they're very predictably not spontaneous they're very predictably released they're just a distribution of the time so you get this lag you're you're almost critical and then this old Neutron comes in and I are critical again you're almost critical old Neutron comes you're just bouncing along and you get this lag because if your power goes up you're only getting delayed neutrons from the power you used to be producing so this is how you operate a reactor by keeping the population critical and relying on delayed neutrons so reactivity talked about we're going to talk about this a lot so reactivity is a change in your multiplication factor so a 1 to 2 population multiplication factor of 2 your population doubled I promise it gets more exciting I just want you guys have some understanding when I'm talking about later so a positive reactivity your Neutron populations increasing let's say you had a control rod in the core it was absorbing neutrons you took it out that's a positive reactivity you're going towards supercritical and then something happens your control rod goes in you're leakage increases something happens that's a negative reactivity you go towards subcritical so these things can cancel out you stay critical you have some factors that are positive reactivity some that are negative they're balanced harmony but if you lose one of your negative reactivities now you still have the positive activity that might be your fuel it might be whatever so you lose a negative reactivity you still have the positive reactivity in the core balance is critical this was a pun that I've made unintentionally and it made me really happy okay so there's five reactivity effects that we're going to talk about control rods coolant voids xenon and a little bit less moderation in fuel temperature so we're gonna go and dab through this slide I'm just kidding so I'm gonna go through it really quickly though so this green line is the population of neutrons as energy decreases high energy low energy neutrons are born fast but to create fission they have to be going slow so they have to slow down the orange is uranium-235 238 uranium 238 is not fizzle it doesn't produce energy when it captures the neutron uranium-235 is so we talked about in rich geranium it's how much 235 do you have the blue line so and the important point if you can sort of capture that there's gonna be a better slide the next slide is that as the temperature of uranium 238 goes up I'm not gonna explain why but I would love to afterward these bands expand they get wider and so you're capturing more neutrons and so as a neutrons trying to slow down it has to go through this valley of fission resonances the fission resonance of death people make really like weird jokes about those and those expand and those will capture your neutrons and so the hotter your fuel gets the more you're stealing neutrons in that resonance region so there's a negative thermal feedback from your fuel temperature the hotter your fuel is the less power you're producing it's good one right nice and stable also there's other things in here that are moderators they're helping you slow this down the neutrons are bouncing off of water atoms carbon atoms slowing down slowing down if they get captured while they're trying to slow down then they die okay let's go less complicated a whole bunch of different things so after the orange and the green the y-axis is cross-section so it's the probability of an interaction and for the green it's population so neutrons normalized to the total Neutron flux how many neutrons are in any energy group it is log scale okay so the Modi slider so we're gonna seem content as the last slide hopefully a little easier to follow so neutrons are born fast and they hit stuff they bounce around they slow down bounce around bounce around and eventually if they avoid getting captured by water if they escape the uranium resonances and they miss the control rods they'll get old and slow they'll have a party they'll make baby new neutrons and they can fly off have a happy life this is somewhere over Wisconsin okay so this is what the RBMK control rods look like so this is this middle picture control rods fully inserted boron is a very strong Neutron poison if you have boron in your core it's absorbing a ton of neutrons graphite is a moderator Graphite's one of those things that helps slow you down most of the time the core has a lot of graphite in it that's what's slowing the neutrons down in an RBMK almost all of your moderation occurs in graphite the control rods will absorb heat when they're absorbing neutrons neutrons have some power it's you need to cool them down so in the RBMK the control rods are in water tubes okay water in the RBMK is a Neutron poison in a PWR in a western reactor and there's russian PWRs but we're a Western reactor of the time water was the only moderator so all the moderation happened in the water and all the poisoning moderation was weaker than the poisoning so in a PWR water is is basically a moderator but here it's graphite okay so what happened you know we talked about graphite tips if you've if you've seen the show this is what the control actually looked like it's roughly to scale so there's a graphite tip in the core and it's not crazy that this is there because water is a poison if you replace a poison with another poison like a stronger poison right you're only getting that little Delta in reactivity you had poison you put a stronger poison little bilder on the other hand if you have graphite the moderator you're replacing a moderator with a poison you're placing a positive reactivity with a negative reactivity so it's a much more effective control rod you get much more work is the is the term we use so I'm gonna come back to this later but this is what the post Chernobyl control rod modifications look like this is also the post ignant Lina design recommendation but so anyway active core here's the fuel this is where the critical region is but the general idea control rods absorb neutrons a little bit about the Hardy m'kay we're gonna come back to it explain it more later or right here so we're gonna talk about positive reactivity later but but this is what was going on this is the bottom of the core when the graphite tip moved down there's water in the bottom of the core it's like a little reactor down there right it's a huge core this bottom can be critical just by itself and so you're displacing water with graphite in the bottom of the core first at the top you got these two negative right so this is as you insert from left to right so negative negative nice top your reactor shutting down bottom gets us little spike right you took water out replace it with graphite so what did water do in the RBMK so slower neutrons lead to more efficient the slower your neutrons are the more fission you have so captured neutrons leaves a less fission this is poisoning what's poisoning means water can be an absorber or a moderator water can slow you down or it can be a poison and the net effect can be either one so this is kind of that unit cell before you have a neutron being born here it goes through some water doesn't get captured it slows down as it bounced around the graphite makes it through the water hits another fuel rod chain reaction on the other hand if it didn't have a such a lucky life born in the fuel rod and they died in the water so cool and reactivity avoid reactivity this is a good one too okay so you have fuel in the core and you have water coming in right the neutron being born here makes it to the water it dies I removed the graphite just simplified its drying so now you start coming up in power your fuels making more heat now your water is gonna start to boil and so as the water boils you have water in the bottom steam at the top steam is less dense in water by about a thousand times so where you used to have water that was blocking a neutron now you have steam not blocking the neutrons fly right through hits the graphite bounced around cuz the graphite doesn't boil lands on the other side and has you know makes more neutrons so this is called the void reactivity if you boil you produce heat you make more steam your fuel gets a little hotter but you also lose all the absorption in the water and in the RBMK there's so much absorption in the water that it just totally dominates the fuel so this is how it's supposed to work if you have a reactor transit when they designed it they knew they knew about this they didn't realize how strong it was and then he realized how strong it could be in different operating modes but they knew about positive roid reactivity so if something happens to increase power there's increased boiling if you were sitting at power there's decreasing xenon we'll talk about xenon in a second your fuel gets hotter that's a negative reactivity your control rods move negative reactivity and you get a new equilibrium you're good so 24 hours before the accident scheduled to be offline the next day and they say all right we have a shutdown tomorrow we're gonna run this test we only shut down very infrequently because we have online refueling we need to run this test tomorrow this is their window now we talked about decay power this is a decay power test it's supposed to run from full power they don't know when they're gonna lose power to the grid but the more power you're sitting at the worst your decay power is that first seconds really hot you know time after that so they wanted to go as low power as possible but the way this is supposed to work is they have excess steam right they have excess steam in the in the boiler and that's what's allowing them to spin the turbine and power the pumps so if they're too low power there's not enough steam to power the pumps so they can't be too low so they needed to be between about 700 thousand megawatts that was what they showed so test procedure what was supposed to happen close the right-sized turbine valve excess steam and the turbine used to power the pumps and as a steam ran down the the generators would turn on oh sorry see that's messed up as the steam ran down the pumps would increase flow rate through that or they would keep flowing water through the core and meanwhile someone would go turn on the diesels the diesels took a long time to turn on so and when they ran the test they knew this was a test so they left the left side coolant pumps on full-blast plenty of cooling water not supposed to be a big issue so 11 hours beforehand they've been sitting down in power for a little bit they get a call from grid operators saying hey don't go down in power sidenote this is like a really interesting thing for me I hope grids run so today in the US still if a nuclear power plant has to change power there's a phone call the grid operator calls him and says like hey you can't go down in power it's like no digital control and it was the same back then so they said and actually there's some little interesting tidbit that it was May Day the next day and so there was end of month production quotas that had to be met so people were working overtime that's why they couldn't reduce power at least according to one random internet article I read so so they say okay towers held steady at 1600 megawatts I think I mentioned this before but they had turned the emergency cooling system off to prepare for this test and they left it off and maybe I didn't say that but that really goes to something I'm gonna talk about later which is a safety culture going on in the Soviet nuclear industry at the time now that had no impact on the test the the emergency cooling system would have done nothing but it's the concept that hey I'm gonna run my reactor for 11 hours at power on the grid anything could happen with the emergency cooling system like unplugged physically unplugged Oh so as they come down in power and they're sitting there and they wait and they wait xenon is accumulating so would xenon so when uranium fission's instantly it creates a lot of things one of them is ice one of the isotopes it creates as iodine iodine is fine it's actually like a personally toxic right iodine is a chemical and radiation hazard but it doesn't affect reactor power but xenon does so over six hours six hour half-life iodine decays to xenon and over nine hours that xenon will just go away by itself but instantly xenon is a very strong Neutron poison it will absorb a lot of neutrons and so as those neutrons flying around they'll hit a xenon and the xenon will turn into something else and not be xenon anymore and so there's this balance that gets formed you're you're creating xenon and you're destroying xenon but what's tricky is that the iodine creation right it was create and then it decays over six hours depends on the power you used to have and the uranium the the neutrons destroying the xenon depends on your power now so the amount of xenon in your core it increases with the power you had before and decreases with the power now so if you have a reactor and you're sitting at 50% power and you go up you burn off xenon because you were only producing 50% worth of xenon before now you're burning a hundred percent of it right goes down eventually you'll stabilize and now if you go down in power back to 50% you have all of this xenon it this iodine that's building up waiting to decay into xenon now your neutrons go away that iodine is still going to decay doesn't care it would produce this xenon peak so in a nuclear plant in the US and in most of the world it this is like not time scale well actually a prism but this is three days so if you shut down a nuclear reactor all the way all that iodine decays and it's worth way more than your control rods can ever take out and it's actually by design and so you're shut down plants down for three days till the xenon decays and you can start back up so as they're coming down to 1600 megawatts they're building up xenon and as they reduce power even further to the target power xenon accumulates even more and so every time every time to get that to get your power constant right if we talked about the the neutron chain reaction before so if some of your neutrons are getting hit by xenon if they were also getting hit by control rods your population will be decreasing so you start pulling control rods out because the xenon has taken their place right the difference between xenon and control rods is that xenon can go away and not come back so two hours before the test they go down from 1600 megawatts and they're going down to about 700 megawatts and so for unclear reasons there was there's disagreement there's no consistency in reports post accident reports operator error did they fail to set the computer did something happen they lost control the reactor a little bit and says the xenon runaway took them down to shut down so went to 30 megawatts thermal all decayed power there was no chain reaction going on in the core and if they knew that they sat there for long they were never gonna be able to start up because all add xenon was gonna start building up so they start quickly trying to pull control rods up get power up burn off that xenon almost all the control rods in the core withdrawn so you know I'm gonna talk about that so 200 megawatts that's where they were able to get to they need 700 megawatts of steam to run the test if they can only get to 200 megawatts this test is gonna fail there is no chance of success at 200 megawatts and they they shouldn't know this and they decide to go forward anyway yeah so they were told not to reduce power oh maybe I skipped that slide at 11 hours before they said you're good we have enough power on the grid you can proceed with your test importantly there was a shift change so this is made a big deal a midnight shift change they did the the next day shift change did not get the operating procedure they walked in here's a procedure this is what we're running go do it the other thing too is this was an electrical system test it was designated an electrical system test so only the electrical engineers had to approve it no nuclear engineers no reactor designers no general design bureau it's fully within the the electrical engineering division so almost all the control rods are withdrawn from the core and there's a computer that was running there an analog computer that was calculating how many control rods they need in the core for the control rods to be stronger than the the water evaporating I want to explain all that physics is gonna be a little complicated but there's a computer I can calculate you need this many control rods in the core period that number was 15 that's a minimum you could get 2 if you ever had less than 15 your reactor was highly highly unstable but they were manually pulling out control rods they were not using the computer so they got down to 8 this computer was 50 yards from the operator console and it printed results every 15 minutes so there's this wouldn't have helped them but is it was collecting the data afterward that they were able to recreate that and they also didn't know they didn't know they had more than eight control rods in the core they had effectively eight control rods but they assumed that all eight control rods were equal but they're not the control rods in the center of the core worth a lot more control rods in cold part of the fuel are worth a lot more so they had no information about that also this reactor was never designed to run below 50% power it's very very complicated every one of those fuel tubes has its own coolant valve that's controlling the flow rate to each fuel tube those are controlled by a computer about 50% and by hand below 50% so 20 minutes before the test so now they're sitting at 200 megawatts and their current trying to come up in power anymore but the test says turn all the main coolant pumps on full there is no boiling almost no boiling going on instructor anymore just a little bit at the top so all the voids are gone right you're pumping in so much water it's just going so quickly it's cooling itself down there's no boiling going on so that's negative reactivity it's pushing their power down even more they pull out more control rods trying to pull up power the other thing is doing is there's almost no void in the core so we talked that slide before that arrow right when it disappeared that negative arrow is all of your water holding you down water unlike graphite unlike control rods can boil it can be gone in an instant and that water is the only thing holding them down that in Zena so this reactor was now highly unstable they're manually controlling the flow rate through the core they're mean annually controlling the pump speed their control rods are almost all the way out of the core then they decide to start the test they close the turbine so at this point it really doesn't matter what they would have done from before this point it would have been hard to shut this reactor down they probably could have done it they would have had to carefully slowly reinsert control rods keep the pumps running at full speed once I got enough control rods and they could start to back off the flow rate it would have been a delicate operation if they understood everything that was exactly going on that didn't happen 45 seconds before the accident they start the test cut the turbine power turbine starts spinning producing steam it's producing less less steam right because it's spinning down and it didn't have that much steam to begin with because there's only producing 200 megawatts so pumps Coast down this leads to increased boiling now now their power starts to go up actually right there they're losing coolant in the core so the Zenon that they'd accumulate it starts to burn away now again the fuel it used to be hot and these two have control rods that used to leave to a new equilibrium fuels cold there are no control rods no equilibrium instead the power keeps increasing that leads to more boiling now they're in a positive feedback loop so the power starts to rise they see this and the monitors and they go that's weird we're shutting down the reactor why is the power going up so they hit the they hit the scram button they think this is the button and there's actually you know part of the thought is they thought that button was failsafe they thought that button that's our get-out-of-jail-free card whatever we're doing to the reactor hit the button we go home so it drops all the control rods but those water cooling channels that the control rods in are actually pretty tight around the control rod so it's like a piston you're compressing so these and the in the reactors huge so those two effects I mean these control rods take forever to get down in the core 18 seconds they wait after six seconds there's a shock control rod stop moving so what happened that positive reactivity in the bottom of the core started boiling as a control rods got to the bottom of the core about six seconds in that inserted a small positive reactivity on an already positive feedback loop so control rods start to go in the core displacing water in the bottom this causes fuel this was localized this wasn't everywhere in the core there was very different cooling channels going through the core but a few of them had a little bit lower flow rate so the fuel started to get hotter this this fuel is getting really hot so this is a picture of a fuel failure test done in I&L different fuel Idaho National Lab in the US different fuel different different values but full power for the RBMK was about 100 90 Cal program right now those fuel rods are about 600 Cal program so it is dust it is glowing dust it disintegrates there's a cladding that zirconium cladding that separates the fuel from the water got vaporized now the fuels direct contact with the steam what happens if you put glowing plasma sand into water flashes to steam control so this coolant channel is all full of water now it's all full of steam instantly over pressures the tube the tube ruptures so I described what's about to happen to a senior design engineer and I said they knew that this would happen they thought that welds could fail at the bottom of the fuel and the steam would flash anode family - and so they've designed this cavity to be safe if two fuel channels fail because like one's unlikely and so what are the odds of - he looks at me he goes brave so this happens in about five fuel channels three to four at least five so the fuel tube ruptures very very high pressure all that steam escapes into the cavity space now what's holding it in a 2,000 ton welded steel lid that every control rod and every fuel channel goes through they knew when they design the RBMK that if more than two fuel channels failed the pressure increase would be enough to separate the upper shield and pull out all the control rods that's what he said it was braid and that's what's about to happen so the steam pressure increases inside the core it lists 2,000 ton lid and all of the control rods up into the air that actually wasn't that bad because the control rods on see I'm serious like the control rods were already out so lifting all the control rods out of the core didn't do much of anything but what it did do was all the remaining 16 hundred and thirty fuel channels that hadn't flash to steam were now exposed to atmospheric pressure they were running about seven mega Pascal's and so as soon as that lid lifted every one of those those fuel channels flashed his team it's like you're use a pressure cooker as you open it all that water in there turns to steam is it pressurizes so now there's a all of them are open to the atmosphere at the top they all flashed a steam positive void feedback like so fast it's so there's a number one dollar that's the amount of delayed neutrons in the cord anytime if you insert one dollar you are prompt critical you are X you are increasing power at a period of 0.0001 seconds five dollars of reactivity is the estimate of what flashing all of the water instantly in the core was this wasn't a nuclear bomb it was not if you tell a nuclear engineer was a nuclear explosion to get really really upset but it's like hey this was an explosion caused by nuclear things forgive the semantics so so then what happened all this fuel is now producing five 600 Cal per gram and now you have air in there water and zirconium the alloy that used to clad the fuel at very very high temperature produced hydrogen this is actually what happened in Fukushima it's unfortunate draconian is far and away the best metal you can use in a reactor core because it's transparent to neutrons from that perspective they are looking at other alloys to use but everything else absorb neutrons darkonians almost completely transparent to neutrons and so it's not a poison it's everywhere in your core so if it was even a little bit of a poison it would make your reactor really inefficient but this problem very very very hot fuel water instantly flashing the steam reacting with the Zircaloy produces hydrogen hydrogen a lot of heat and a lot of oxygen this is what chernobyl looked like after the explosion 2,000 ton lid was thrown up in the air and landed back down almost the entire rest of the core was ejected most of it stayed within the building plenty of it left through the plume this is kind of the end of the cool physics from here on there was an open pit fire in the reactor burning graphite burning fuel sending radionuclides into the environment so why did Chernobyl happen I think if you follow it along and I did a good enough job you now understand what happened but why why was this allowed to happen why did engineers design something that this could go wrong how do we know this can't happen in the West and even more importantly we're designing complex engineered systems that go wrong when they fail 737 is a really good example of that there's a lot of parallels it was designed in stages over time by multiple different design groups who didn't know each other it was modified over time by people who didn't know the original designers and it was complicated it's impossible it's almost impossible to really regulate something like that in a system where the designer regulates him or herself so why did it happen I don't know what I know is that this requires human nature will make things like this happen if you don't put systems in place to prevent it if you just let people be lazy and if you let people be calm and comfortable things like this will happen the Soviet Union also had a very authoritarian government where people couldn't express safety concerns but things like this have happened in the West Challenger explosion the 737 comes to mind and so why did you know will happen sure it will happen because of a lack of good engineering process and we can't just trust it a good engineering process will happen we have to make it happen we have to be vocal about it and we have to look for our failures and examine ourself the nuclear industry that I'm a part of today has learned a lot from these lessons and is one of the most like rigorously regulated industries in the world today and that's really difficult being in the industry but it's sort of hard when you see something like this to not say hey that's kind of a good thing but the other thing you can do is not design systems that are unstable not design systems that get to the point where you don't quite understand the physics they knew their codes couldn't predict what was going on here they had very simple reactor codes at the time they knew they were making something unstable there's always a drive for money it's always a drive for authoritarians to say we need to do something so it's not like a really rosy rosy picture I mean Chernobyl was bad I think obviously we've learned a lot we continue to do better the the US has put a really rigorous nuclear regulatory program in place and had one at the time but I think it's important as a nuclear engineer to be upfront about the failures it happened and make sure that we don't recreate those mistakes and that's why I go and do these talks because I get that thing like that's why I go get these talks because I this is something that we are very concerned about and something we designed safer or better technology to avoid these even happening new reactors have new features that prevent this stuff from being possible and to not kind of go off on a long tangent that's sort of why I'm why I'm more calmed about nuclear power going forward RBMK shouldn't have been built but we need nuclear power to fight climate change may be a weird conclusion but that's sort of the reality we live with so ever since I've been a nuclear engineer in Chernobyl comes out I get this question all the time was the show accurate I know a lot of nuclear engineers who don't think the show was accurate and I sort of take a little issue with that because it's an accurate portrayal of a lot of things that happened and a lot of ways that characters probably talked so there's a scene where the the chief engineer he was named I'm forgetting you know he's talking to Gorbachev and saying the Corbitt of Khrushchev start gonna Khrushchev and saying like Gorbachev sorry yeah he's talking to Gorbachev and he's describing radiation is bullets now if I were to describe radiation health physics to you I would use the like a slightly more nuanced analogy but if I gave the the analogy and the description and written you know the level of risk that I would describe to you right now to someone saying there's nothing we need to do about an open-pit reactor burning he might not do anything and so it's like fair in the moment that he exaggerated s' right and it's just tough as a nuclear gene you're watching to know I don't know when they're exaggerating for effect because that's what the characters would have said and I know that people watching it can't tell the difference between they're exaggerating for effect and they're exaggerating because that's reality because he's a scientist standing up and talking so I highly recommend as an NPR podcast if you've seen the show and have like detailed step-by-step questions the broad sequence of event is accurate there's lots and lots of true very specific details in quotes radiation is portrayed accurate to the moment and what I mean by that is people were afraid and people didn't know what was going on and radiation is scary and so the the fear that they portray capture than that it doesn't I wouldn't say is not a radiation health physics class like the baby so there's a baby that gets radiation poisoning and like it's really counter to how radiation physics works but I don't want to kind of like go through that point by point now it's just something to kind of look into and some dramatization and it's also hard to tell when characters were lying like so the guy says like I was lying in the presentation because he had to lie right so when he points at Dyatlov and says it was a routine test was not a routine test never been done successfully before is that the show making a mistake is that the app there's that him lying or that in inaccuracy I don't know and you as a viewer don't know and that's sort of why it's difficult okay so I know that was a lot I tried to go through everything [Applause] actually before I take questions that I did go quickly I have some cool pictures of RBMK is that like didn't really fit in here so this is the top of an RBMK that was operating until 2013 and so each one of these is the top of a fuel tube except for these boxes are the control rods and so that just kind of gives you a an idea of what's going on this is like well above the upper shield of the reactor yeah and so this is each one yeah so these are so RB m K's are still running there's about 13 still running they're the three other units so Chernobyl one two and three ran until 2005 through 2010 so 2003 or unit 3 shared a wall with Chernobyl unit 4 and kept operating until T doesn't find for a Power provide power to Ukraine and that's I mean because of the cleanup you know the radiation from the core once they did that initial cleanup like workers wearing booties and things walking through it it's like the radiation that's left is if you drink it eat it that is where you can sort to have a health effect but you know coming into the town and working it's pretty it's pretty low relative to the dose you get working in a plan which is already low so this is what the bottom of a reactor looked like so all those individual tubes coming up and you see these are like small diameter tubes the small the tubes in are the pipes in a normal reactor like 16 18 inches require very special welding 2-inch tubes can be welded by you know much less strain mechanic and so this is something I didn't really talk about but the the fuel handling machine this is a schematic of how that works so it comes down and latches with a fuel tube in operation while the reactor is running at full power removes this nut pulls out this plug sets it aside pulls a fuel rod out sets it aside while it's pressurized with seven mega Pascal's of water puts another two tube in screws the cap back on goes on to the next one there's this huge crane I'll come back to in a second any slides are this is an actual picture of the control rods there's a control rod diagram so it's just showing this is the post turn Obul modification so I didn't talk about that but after turn Obul this picture where is it no this one yeah so this is the the control rod fully withdrawn and so they extended this telescope by a metre and a quarter so that when the control rod was fully withdrawn the graphite was at the bottom of the course you only got a negative reactivity insertion that was it they knew that a TIG Melina one that was a design recommendation and changing a few other things which they didn't change and so this is just showing you these control rods are in the core and this red line so this is the upper shield plate this is what came off this is a graphite blocks you were seeing on that previous picture this is the core and this is the biological shield this was above ground so like the control room was well so a turbine Hall was over here I think the control room was like it wasn't that far away just a ton of concrete in the way and yeah this is a full plant schematic I found I was like laughs that's really nice this is from one of the operating reactors it's not true noble but so this is the turbine hall these are the left and right coolant pumps separators and so these are the pumps that they shut off while these were supplying excess power to the core yeah I took that slide out I thought I'd have fewer side [Applause]

Channel: Ethan Chaleff

Views: 453,993

Rating: 4.6957774 out of 5

Keywords: chernobyl, rbmk, science, technology, nuclear, engineering, nuclear power

Id: YRPuO1RhbKo

Length: 49min 25sec (2965 seconds)

Published: Sat Jun 15 2019

Reddit Comments

[u/ppitm]

One of the actually good video explainers of the accident. With caveats of course:

25:00: Actually the test program specified that only enough power for the 'self-sufficiency' of the reactor was necessary for the test. The turbine had enough inertia to provide sufficient coolant even at 200 MW. The 700-1000 MW limit was set by an electrician, since the reactor was viewed as not even participating in the test.

29:57: Just like the drop in power, there is no clear indication on whether the reactor was fully shut down or just stalled a very low neutron power.

30:35: The test succeeded(!) at 200(!) MW! After the accident the core parameters were analyzed, and it was found that there was only a steady 10-20% reduction in coolant flow, more than sufficient to handle decay heat and bridge the gap. When the reactor exploded, the diesel generators were already almost at full electrical load.

31:00: The test was approved and signed by nuclear engineers Dyatlov, Kryat and Lyutov, who also had input on the procedures. The test was essentially identical to the tests that had been run previously at Chernobyl in years past, and had been submitted to at least regulatory body. But no one in the scientific community really cared about the rundown principle at this point.

33:45: The reactor could have been shut down safely after the beginning of the test too, by dropping control rods in groups (according to the private letter sent to plant directors, this mitigated the tip effect), re-enabling the other 4 MCPs or by inserting an auxiliary set of absorber rods from underneath the reactor. Assuming a time machine where you could warn the operators, of course.

34:18: Read your INSAG-7. It will tell you straight-up that there was no increase in reactor power during run-down. I'm no physicist, but I highly doubt that the small reactivity insertion at this point could have had the slightest impact on xenon concentrations (over just 10 seconds!), which would still be increasing overall. This factor seems to a sort of fan fiction for Western engineers, and to my knowledge is not demonstrated in any actual documents or calculations.

35:00: INSAG-7 will also confirm that it is unknown why AZ-5 was pressed. Given the calm atmosphere in the room at the time, it appears just as likely that it was pressed once the shift supervisor realized that the reactor should have been tripped automatically at the start of the test.

38:30: It's really anyone's guess whether the entire core flashed to steam before or after containment was ruptured. The reactivity insertion of the tip effect and void coefficient was more than enough to cause a prompt criticality. At this point a small nuclear explosion (sending short-lived isotopes to high altitudes) is also a respected theory. The explosion can also be explained by steam pressure alone, without much in the way of hydrogen deflagration.