Chernobyl: Worst Accident Ever
Feb 27, 2020The
worst
nuclearaccident
in history occurred in 1986 in what is now Ukraine, the city and reactor of Chernobyl. There are some notable differences between our MBK reactor that was used in the Soviet Union, they actually had maybe a dozen of them, and Western reactors. or should I say the reactors in the rest of the world. This first slide illustrates the difference. Here is the standard light water reactor. We have a pressure vessel, we have the control rods and the nuclear fuel inside. This is a Marin. BK and could you say it first oh I don't know why this is different than this that part is not that different a Canadian deuterium reactor has a similar type of geometry here what's missing is this thick line this is a containment building and a containment building it's expensive this is three foot thick concrete this is what a jet plane can hit and the jet plane disappears in a cloud of steam the structure of the building three foot thick reinforced concrete with the steel cladding remains sitting there the purpose of a containment building is if all its other safety systems somehow fail radioactive waste fission products things that are inside nuclear fuel bad things nuclear waste won't reach the general public reactors in the Soviet Union they are MBK, the reactors did not have a containment building, this flimsy little white line here is just a cement block of normal construction, something else, nothing at all designed to contain the material inside, so when people say: Oh, worry or it will happen here, the first response.It's not us at all and by us I mean the entire world has containment buildings. That's not the only difference. There is another very important difference and it has to do with moderation. It has to do with neutrons. We have drawn this graph before and it is very illustrative. this is the probability of a fission event, okay, and this is the speed of the neutron, it's a logarithmic graph and it looks like this and this could be one and this is a tenth and this is a hundredth and then on this side goes through nine orders of magnitude, right, this could be a 1e V point and we have a bunch of these and we get to something like a million electron volts, neutrons are born fast, that's when they are created by fission, but the possibility of creating a fission only happens when they are slow, so you need to have something that slows down the neutrons, something called a moderator, okay, you say, oh yeah, I heard your previous lecture.
I know all about fission. Wonderful, what does a moderator need? be something similar to the mass of a neutron, so one of the best is the element hydrogen. It has a proton in the nucleus of approximately the same mass as a neutron. They bounce off each other and slow down. You can't put hydrogen gas in there. one that is explosive and two is a gas that won't hit many things in a gas, but if you turn it into water and just have water as a moderator, you will have a wonderfully useful Neutron and well, slower, you could use other things.
Correct carbon graphite is not as good as water, but it will slow down neutrons and it is a solid and you put blocks in there. The thing is, the purpose of all these visions is to boil water, you want to boil water, you want to spin it. vaporize and make the steam turn a turbine, the turbine turns a generator in all reactors except the Chernobyl type, water is the moderator and it is exactly the same water that you are trying to boil and the beautiful thing about it is if somehow this system breaks, a pipe breaks, someone closes the valve, the steam runs out, earthquake, what
ever
, if you lose the water, the reaction stops because there is no more moderator, the fast neutrons have very little chance of causing 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 coolant after all, this is why you're trying to do it, but you could basically be boiling. the water but you are cooling the reactor if you lose the coolant your reactor gets hotter and hotter and hotter and hotter and the reaction doesn't stop because the carbon is still there that's the second reason why Chernobyl can't happen here we use the same water than the moderator as a coolant, if you lose one the reaction stops, so let's take a closer look at our Chernobyl MBK reactor. Here is a drawing and I want to illustrate a couple of points.
It still has pressure tubes and makes a circuit to boil the water. These are the moderator. blocks of carbon that are always there, you might say, "Oh, great, the moderator can't leave, but remember that means the chain reaction is only controlled by the control rods, control rods that you can take out of the reactor; Of course, they don't have that many." neutron absorptions, so this reactor does not have the security that if it loses the coolant, the reaction stops from time to time and needs to refuel in a nuclear reactor. Uranium has a huge amount of energy content
ever
y year or so.I removed some of the used fuel rods and put in new ones that month in 1986. The Chernobyl reactor was scheduled to be shut down for refueling. They only do this once a year and of course it is generating electricity. The city of kyiv needs energy. It needs electricity. We get to the scheduled reactor shutdown and one of the engineers is not a normal reactor operator, but someone who wants to test a safety system. That's so ironic that the Chernobyl reactor
accident
was caused by humans, not mechanical failure, and it was caused. by humans because they wanted to test a security system.You see that a reactor needs electricity to operate, so reactors have many backup electrical systems. There is usually a diesel generator somewhere in Fukushima in case the diesel generator is flooded by a tsunami, that's what happened. It had batteries that would back up the first line of defense and being able to create enough electricity to control things is to use the same generator you've been using now. Maybe you're not going to power the entire city of kyiv, but maybe you can have enough power already. that their turbans are spinning downwards when they turned off the water coolant so they could run the safety systems to run the reactor for a period of time and that's what they wanted to test, they wanted to shut down the reactor and in this shutdown phase they wanted to see how much steam they could still generate and therefore could still run the safety systems in case the diesel generators were not working or perhaps the time when they should be activated etc.
So we get to that night and here's the power level of the reactor and here's the weather and the reactors humming and humming and then in the middle of the night, of course, everything bad happens in the middle of the night and it's the Balban plan. , then they say Well, guys, let's turn off the power. Get your electricity somewhere else. Hopefully, the grid can work well and the plan was to reduce the power to around half, because then from this half power they were going to start. Experiment in which they were going to turn off the cooling water.
They were going to see what would happen if they could slow down. It doesn't sound so good in the experiment, but that was their plan, but an operator pressed the wrong button in the control room. Here is an image. from the control room, too many buttons can be a pretty common mistake and what happened, as you can see, my power level here, instead of going here very quickly, went to a very low power at this point, I should be done the night. You know that we have the power too low, this means that the xenon content is going to increase, xenon is a poison for the reactor, it is something that will not allow you to control the reactor well, they did not do it, we did not know the cardinal rule if any Sometimes you turn off or lower the power and a reactor you can't turn on for three days, so they try to do things they should never do, hey, let's take out some control rods when you do, so nothing happens. it doesn't move remove more control rods a regulation says that you should never remove so many control rods ah who cares so they remove more control rods and more control rods and more control rods and while you do this the reactor becomes unstable The Xenon poisoning is accumulating and increasing, it is becoming more and more in these few hours, quite a brake on the reaction, the reaction simply wants to decay and stop, but they are having to take extraordinary measures to bring the power to its half. level so they can do their experiment and they finally drop so many control rods that their power starts to increase and then when they say, okay, let's try to do the experiment, you know, let's even see the power, they try to put the rums back in, but There's another special thing that happens when they first insert one of their rods, it actually acts as a slightly better moderator than poorly designed control rods, so when they start doing that, they get to this level and hope to turn it off, but instead. an uncontrolled chain reaction shoots through the clouds that releases enormous amounts of heat this is not a nuclear explosion of course this is just generating a lot of heat wherever I said the moderator was carbon like charcoal briquettes, in fact the charcoal became red hot water Hydrogen and oxygen under enough heat decompose into hydrogen and oxygen.
Red hot carbon, that's the recipe for a very large chemical explosion like dynamite and in fact what happens is the reactor heated up uncontrollably and blew off the roof and because there was no containment building. all of these fission products simply flew through the ceiling and into the surrounding field. This is an image of what the core looked like from the air. There are no buildings in the way. Nothing to stop that debris from leaving. It's another aerial image right after the accident. Of course, this was quite a disaster and the world realized that some of the first people to realize that we are scientists.
I think in southern Germany, near Switzerland, they put up some detectors and said, "Oh, it's going to rain," sometimes you can see the consequences of previous atomic emissions. weapons tests and they are very sensitive detectors and when they were watching this experiment and it was raining they said: "My God, this is not from a nuclear experiment from ten years ago, it 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 ringing because of things coming in the air from outside the Union Soviet, at first it was very strict.
There is no free press, there is no good way to try to describe what is happening in the world. They mobilize their own Fire Department, of course, right at the local Chernobyl Fire Department and those were the deaths that the firefighters wanted to put out the fire in the building. maybe they were heroes or maybe they needed more training because in putting out those fires they absorbed huge doses of radiation and over the next few days they contracted radiation sickness and maybe over the next few weeks 30 people died the reactor right next to Chernobyl was a The unit Twin kept going, so it's not a nuclear explosion, it's a chemical explosion, it's like someone put a bunch of dynamite in that reactor and set it off and it blew up that building, it doesn't blow up the building right next to it and that.
The reactor was still generating electricity, so the firefighters came, they were having problems and they had to try to shut it down. Then for a day or two the world started to know what happened because we noticed it from the things in the air and the Russians mobilized a defense to try to stop it, one of the key things of course from the air was placing the shield, there is no containment, so we have to make sure there are no more chain reactions. I don't think there have been any at this point, but there certainly still are.
Fire burns, so you need to throw things on top of it, and in addition to throwing things like sand, you want to throw things that will also absorb some of the remaining neutrons and the product. radioactive, so they dropped heavy lead materials in the next few days, they need to take this structure, the roof blown off, remember it is not a mushroom cloud, but a chemical explosion coming out of the roof of the building and they need to build the containment building after the fact, classic example of after the horses come out, you close the barn. door and they did it, they built a very impressive containment building, they call it a sarcophagus and this had to have very solid sides, a very solid roof, very good containment and, of course, you also had to build underneath, you had to build a building of containment. three foot thick concrete structure under the reactor they mobilized volunteers from their army the volunteers to receive a minimum dose of radiation to Russia and work for a few hours and be able to leave through a tunnel under the building with the miners poured all this concrete to get the basis to tryTo stabilize the reactor it is much easier to first build a containment building.
Chernobyl can't happen anywhere else because we no longer use graphite-moderated water-cooled reactors anywhere, and it can't happen because we have containment buildings. What Chernobyl did was demonstrate that you really need to rely on physics, not humans, to keep reactors safe. I need water there to keep the decay heat of the fission product away and just lose that energy to convection again. Relying on physics makes things even safer. That's what you need to know about Chernobyl.
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