Chernobyl: Worst Accident Ever
accidentin history happened in 1986 in what is now the Ukraine the town and the reactor of
Chernobylthere are some remarkable differences between the our MBK reactor that was used in the Soviet Union actually they had maybe a dozen of them and Western reactors or I should say the reactors in the rest of the world this first slide illustrates the difference here is your standard light-water reactor right we have a pressure vessel we've got the control rods and the nuclear fuel inside of it this is a Marin BK and you might say it first oh I don't know why is this you know different than this that part's not all that different a Canadian deuterium reactor has a similar type of geometry here the thing that's missing it's this thick line this is a containment building and a containment building is expensive this is a three-foot thick concrete this is the stuff that a jet airplane can hit into and the jet airplane disappears in a cloud of vapor the building structure the three-foot thick reinforced concrete with the steel liner stays sitting right there the purpose of a containment building is if it all your other safety systems somehow go wrong the radioactive debris the fission products the things that are inside the nuclear fuel the bad stuff the nuclear wastes will not get out to the general public the reactors in the Soviet Union are MBK reactors did not have a containment building this flimsy little white line here it's merely a...
normal building cement block something else nothing whatso
everdesigned to hold the material inside so when people say Oh concern or will happen here the first answer is absolutely not we and by we I mean the entire world has containment buildings that's not the only difference there's another very important difference and that has to do with moderation it has to do with the neutrons we've drawn this graph before and it's very illustrative this is the chance of a fission event okay and this is the speed of the neutron it's a log graph and it looks something like this and this might be one and this is a tenth and this is a hundredth and then this side it goes by nine orders of magnitude right this could be point o 1e V and we have a whole bunch of these and we come up to something like a million electron volts the neutrons are born fast that's when they're created by the fission but the chance of creating a fission only happens when they're slow so you need to have something that will slow down the neutrons something called a moderator all right you say oh yeah I heard your earlier lecture I know all about fission wonderful what does a moderator need to be needs to be something similar to the mass of a neutron so one of the very best ones is the element hydrogen he's got one proton in the nucleus about the same mass as one Neutron they bounce into each other they slow down you can't put hydrogen gas there one that's explosive and two...
it's a gas not going to hit very many things in a gas but if you make this into water and you simply have h2o water as your moderator you've got a wonderfully useful and good Neutron slower you could use other things right carbon graphite not quite as good as water but it will slow down neutrons and it's a solid and you put blocks of it there the thing is that the purpose of all these visions is to boil water you want to boil water you want to turn it to steam and have the steam spin a turbine the turbine spins a generator in all of the reactors except for the kind the
Chernobylwas the water is the moderator and it's exactly the same water that you're trying to boil and what's beautiful about this is if somehow this system breaks a pipe break someone turns off the valve the steam all runs out earthquake you name it if you lose the water the reaction stops because there's no more moderator the fast neutrons have a very low chance of making another fission event so the chain reaction stops let's think about the situation on the other hand if carbon was your moderator but you still cooled the reactor by boiling water okay if you lose your coolant after all this is why you're trying to do it but basically you might be boiling the water but you're cooling the reactor if you lose your coolant your reactor gets hotter and hotter and hotter and the reaction does not stop because the carbon is still there that's the second reason why...
Chernobylcannot happen here we use the same water as the moderator as the coolant if you lose one the reaction stops so let's take a closer look at the our MBK
Chernobylreactor here is a drawing and I want to illustrate a couple points it still has pressure tubes right and loops the boil water the moderator are these blocks of carbon which are always there you might say oh well great the moderator can't leave but remember that means the chain reaction is only controlled by the control rods control rods which you could lift out of the reactor - of course not have as many neutron absorptions so this reactor doesn't have the failsafe that if you lose your coolant the reaction stops
every now and then and you need to refuel a nuclear reactor uranium has an awful lot of energy content
every year or so you're going to take out some of the used fuel rods and put in new ones that month in 1986 the
Chernobylreactor was scheduled to shut down to be refueled they only do this once a year and of course it's making electricity the city of Kiev needs the power needs the electricity we come up to the scheduled shutdown of the reactor and one of the engineers not a normal reactor operator but someone that wants to test out a safety system that's what's so ironic the
accidentwas caused by humans not by mechanical failure and it was caused by humans because they wanted to test out a safety system you see a reactor needs electricity to operate...
so reactors have many many backup electricity systems usually there's a diesel generator someplace at Fukushima in case the diesel generator was flooded by a tidal wave that's what happened they had batteries which would backup stuff the first line of defense and being able to create enough electricity to control things is to use the same generator that you have been using now maybe you're not going to power the whole city of Kiev but maybe you can have just enough power as your turbans are spinning down as you've turned off your water coolant to be able to run the safety systems to run the reactor itself for some period of time and that's what they wanted to test they wanted to turn off the reactor and in this turning off phase they wanted to see how much steam they could still generate and therefore still be able to run safety systems just in case the diesel generators didn't kick in or maybe the time when they should kick in and so forth so we come to that evening all right and here is the reactor power level and here's time and the reactors humming and humming along right and then the middle of the night of course
everything bad happens in the middle of night and it's Balban plan then they tell you of okay guys we're going to shut off the power get your electricity somewhere else hopefully the grid can do it all right and the plan was to turn the power down to something around half because then from this half power they were gonna start...
their experiment where they were gonna turn off the cooling water they were gonna see what happened if they could spin this down doesn't sound all that great in experiment but that was their plan but an operator hit the wrong button in the control room here's a picture of the control room lots of buttons might be a fairly common mistake and what happened as you can see to my power level here that instead of going here very quickly went to a very low power at this point I should have just called it a night hey you know we got the power way too low this means the xenon content is going to come up the xenon is a reactor poison it's something that will not allow you to control the reactor well they didn't do that we didn't know the cardinal rule if you
everturn off or turn way down the power and a reactor you can't turn it on for three days so they try to do things that they should n
everdo hey let's pull out some control rods when you do that nothing happens it doesn't budge pull off some more control rods a regulation say you should n
everpull out this many control rods ah who cares so they pull out more control rods and more control rods and more control rods and as you do this the reactor becomes unstable the xenon poisoning is building up and building up it's becoming more and more over these few hours all of a damper on the reaction the reaction wants to simply decay away and stop but they're having to do extraordinary measures to...
get the power up to their half level so they can do their experiment so finally they release so many control rods their power starts going up and then when they say okay let's try to do the experiment you know let's let's see even the power out they try putting the ron's back in but there's another special thing that happens when they first insert one of their rods it actually acts as a little bit better of a moderator poorly designed control rods so when they start doing that they get up around this level they're hoping to turn it off but instead it goes through the roof an uncontrolled chain reaction releasing enormous amounts of heat this is not a nuclear explosion mind you this is just making a lot and a lot of heat wher
everI said the moderator was carbon like charcoal briquets indeed the carbon got red hot water hydrogen and oxygen under enough heat decomposes into hydrogen and oxygen red hot carbon that is the recipe for a very large chemical explosion like dynamite and indeed what happens is that the reactor became uncontrollably hot and it blew off the roof and because there was no containment building all of these fission products just blew through the roof and up into the surrounding countryside this is a picture of what the core looked like from the air no building in the way nothing to stop those debris from going it's another aerial picture right after the
accidentof course this was quite a disaster and the world noticed some of the...
first people to notice we're scientists I think in southern Germany near Switzerland they had some detectors set up and they said oh it's gonna rain sometimes you can see the fallout from earlier atomic weapons tests and they're very sensitive detectors and when they were looking at this experiment and the rain was coming down they said oh my god this isn't from some ten year ago nuclear experiment this looks like the entire core contents of a nuclear reactor a nuclear reactor in Sweden with very sensitive radiation detection monitoring because of course you want to make sure nothing goes wrong in your own reactor the alarm starts going off because of the stuff coming in the air from outside the Soviet Union was at first very tight-lipped about this no Free Press no great way to try to describe what's happening in the world they mobilize their own Fire Department of course right in the local
ChernobylFire Department and those were the fatalities the firefighters wanted to put out the fire on the building and maybe they were heroes or maybe they needed more training because by putting out those fires they absorbed enormous radiation doses and over the next few days got radiation sickness and maybe over the next few weeks perished 30 people the reactor right next door to
Chernobylit was a twin unit kept operating so it's not a nuclear explosion this is a chemical explosion it's like somebody put a whole bunch of dynamite into that reactor and sent...
it off and it blows up that building doesn't blow up the building right next to it and that reactor was still making electricity so firefighters come in they have problems they need to try to turn this off then over the next day or two the world starts to know what's happened because we notice from the stuff in the air and the Russians mobilize a defense to try to stop it one of the key things of course from the air was to dump in shielding there's no containment so one we have to make sure there's no more chain reaction I don't think there was at all at this point but there certainly still fire burning so you need to dump stuff on top of it and besides just dumping things like sand you want to dump stuff that will also absorb some of the neutrons that are left and the radioactive product so they dumped lead heavy materials then over the next few days they need to actually take this structure the blown out roof remember non mushroom cloud but a chemical explosion blowing off the roof of the building and they need to build the containment building after the fact classic example of after the horses go out you close the barn door and they did they've built a very impressive containment building they call it the sarcophagus and this had to have very solid sides very solid roof very good containment and of course you also had to build under it you had to build a containment building a three-foot thick concrete structure beneath the reactor they mobilized...
volunteers from their army the volunteers to get a minimal radiation dose how to Russia and work for a few hours and be able to come out tunneled under the building with miners poured all of this concrete to get the base to try to stabilize the reactor so much easier to build a containment building first