The chernobyl nuclear power plant is why explosion?Siu dee teacher speak principle of nuclear reactors

Hello, everyone. I am Mr. Li Yongle.Recently, HBO shot a new TV series.The Chernobyl was quite poplular.So many pupils of mine asked meHow the Chernobyl disaste...

Apr 24,2020 | Ishara

Hello, everyone. I am Mr. Li Yongle.Recently, HBO shot a new TV series.The Chernobyl was quite poplular.So many pupils of mine asked meHow the Chernobyl disaster happened.Today, I will explain it to you.In order to have a better understanding of it,Let’s share some background knowledge first.Let’s share the principle on which a nuclear power plant generates powers.A nuclear power plant generates electricity, so it has to have some sorts of an energy, right?In this case, the source of energy comes from nuclear energy.It is a product of nuclear reaction.It is a chain reaction.Why is it called so?For instance,The material used in nuclear power plants is uranium235.Uranium235 plus neutron.If a neutron hits uranium235, what will happen?We will get barium56-141, krypton36-92 and 3 neutrons.So you can see, when a neutron hits the atomic nucleus of uranium235,What we will get, after the collision, is two fragments and three neutrons.Then what will happen to the three neutrons?

They will again collide with three more uranium235, right?So we will get nine neutrons this time.An avalanche effect takes place, which we call chain reactions.In this process, the quality of reactant is greaterThan the quality of the product, which means the loss of quality.The general quality will decrease, thus leading to the loss of quality.When the loss happens, energy is released.This is what the mass–energy formula of Einstein explains to us.The energy released takes the form of heat.We then drive the generator by the steamer using heatElectricity is then generated.This is the basic theory of nuclear reaction.What the Chernobyl Nuclear Power Plant used is calledRBMK, or graphite-moderated nuclear power reactor.What is it?Let’s draw a picture of it.Ok, let’s take a look at it.It’s not complicated at all.In the middle is what is called the nuclear fuel.It is uranium235.

Remember it. Right?Then the nuclear fuel is placed in a water tank.In it, the water flows like this.It flows past the nuclear fuel, gets heated, then it flows out.It keeps flowing in this direction.Until it reaches this place. We call water the coolant.It cools the reactor, and carries away heat.Then electricity can ben generated. Right?Ok, besides them,We also have a huge container, which is made of graphite.Graphite is used to slow down neutrons.That is the purpose of graphite.Why do neutrons need to be slowed down?The nuclear fuel sends out neutrons at a very high speed.They move so fast, move at a speed so highThat they are hard to be capturedTo create chain reactions.So we allow neutrons to collide against these places in the middle,Where there are graphite.Then the graphite slows down neutrons,The neutrons will then be much easier to be captured by uranium235.This leading to the chain reaction.OK, the basic goes like this,The water gains heat while flowing in certain directions.When hot water circulates to this point,It is so hot that it evaporates.So the steam escapes from this point.We then have steams.The temperature of the steam is so highThat it is able to drive turbines to generate electricity.In this process, certain amount of water is consumed, right?

Then cold water is refilled through this place.The cold water circulates in this direction.It escaped again in the form of steam, right?OK, this is basically how it works.However, how do we control the fission rate of the reaction?It involves a key part which is called the control rod.It means we insert certain rods at this point.These rods have a unique capability.They absorb many neutrons without themselves fissioning, so we call them control rods.They will be placed between graphites.They are to absorb neutrons.The reason why we need to do so is obvious.If the reaction is too severe, we will have too many neutrons. Right?In order to slow it down,We need to insert the rod into the central core, right?So, if we insert it into the central core,It will decrease the neutron flux.Or we can say the temperature of reactions drops.All that is done is to decelerate the reaction.What if I want to have a higher fission rate?Then I will have to remove the rod from the central core, which will absorb fewer neutrons.With more neutrons, the fission rate is high. Right?

So, if we remove the control rod, the fission rate will be higher.This is the capability of the control rods. Right?What if we need a complete emergency shutdown?We need to have all the control rods inserted.With all the neutrons absorbed, the reaction will stop. Right?OK, this is the basic theory of a nuclear reactor.Why is it called graphite-moderated thermal reactor?First, graphite is used to slow down the neutrons.We call it thermal because it boils the water to get steam.Then the steam generates electricity.So, except for the core part,It resembles other power plants.They all drive turbines to generate electricity.This is what they share in theory, so we need not to explain anymore.Then we talk about the Chernobyl disaster.How did it happen?the Chernobyl disasterHappened for a reason.It occurred in a test.What was the test for?It was to test the power failure protection.How did we test that?In a nuclear reactor,Even if all the control rods are properly inserted,And the chain reaction comes to a haltBut, products, like barium and krypton, continue to decay. Right?In the decay process, they continue to release heat.So the waste heat will gather.And it needs to be carried away.So, the water need to continue to circulate.Even if with a power failure, the water has to circulate.

A consistent power supply has to be maintained.But what if the power supply fails completely?The water turbines have no means to keep working?Diesel generators will supply the power.We will use diesel generators for backup.So even with a power failure, the diesel generators will keep the water turbines working.Keeping cold water circulating will prevent the reactor from being damaged from over-heat.But there is an issue with diesel generators.It will take one minute for them to achieve full availability.Within that one minute, the reactor will probably get damaged.Then a solution got brought up.It explained that even there was a power failure.And the reactor stopped working.Yet, the steam from the reactor was still able to drive massive turbines.They could go under rotational energy.So the turbines generating electricity could still go for a while.Could we take advantage of that rotational energy?To generate electricity, keeping it supplied.Which means to make use of its own rotational energy.So they thought of a solution.It was that in this one-minute power gap,Could the rotational energy be employed to generate electricity?Electricity generated from rotational energy.We are talking about the rotational energy of the generator turbines,Not the water turbines.The solution went like this.The test was to see whether it was possible for rotational energy to generate power for one minute.Then the diesel generator would follow up.In this way, the power failure protection was in effect.This test was repeated for four times.An initial test carried out in 1982, then second 1984, the third 1985.The first three tests ended in failure.

Then great anxiety arose.The common opinion was that it had to be dealt with.So one more test was summoned.With the adjusted plan, at the end of April, 1986,The fourth test was ready to go.The fourth test involved No. 4 nuclear reactor.There were four reactors altogether, with two under construction.No. 4 nuclear reactor was ready for one more test.It was on 25th, April,1986.The plan was to carry out the test during the day time.The test started with slowing down the fission rate.Then shut down the reactors, and took advantage of rotational energy to generate power.Diesel generators would then follow up.However, there was a minor issue on 25th ,April.The issue was in Kiev, the capital and most populous city of Ukraine.A regional power station unexpectedly went offline.So there was a shortage of power supply in Kiev.Then the Chernobyl Nuclear Power Plant was requestedNot to cut down the power supply for now.It had to wait until the power supply came back to normal.So they waited.Until the night of 25th, April.The original plan was during the day time of 25th, April.

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Was there a difference between day test and night test?Surely, there was, because in the daytime, qualified engineers were at work.But they were off work in the night.Only people on night shift were present.All they had to do was to keep an eye on the equipment.Without complicated operations.The test was supposed to be carried out by qualified engineers on day shift and evening shift.In order to be off work on time, engineers on the evening shift needed to carry out the test as soon as possible.So, everything was in a rush.At midnight,They decreased the temperature of the reactor to a low degree.So the fission rate dropped sharply.But for some reasons,An issue arose along with the decrease.It was called reactor poisoning.What does it mean?It means that during the reaction, some short-lived fission products will build up.The main isotope responsible is 135Xe135Xe is the strongest known neutron absorberCombined with neutrons, they turn into 136Xe.135Xe is able to absorb neutrons.But, in normal condition when there is a large quantity of neutrons, we do not need to worry about 135Xe.Because there are too many neutrons for 135Xe to absorb.But if the fission rate is lowThe number of 135Xe exceeded that of neutrons by a big margin.In this way, 135Xe will continuously absorb neutrons,Then the nuclear reaction will be interrupted, which means the drop down of the fission rate.

This is called reactor poisoningIn Chernobyl nuclear power plant, we witnessed reactor poisoning.The reactor was now producing 5 percent of the minimum initial power level.The test would be impossible to carry on with such a low power level. What could be done then?There were two options.First, shut down the reactor.And admitted the failure of this test.The second was to raise the fission rate.To get rid of the reactor poisoning.It turned out that two engineers suggested the reactor be shut down.However, Anatoly Dyatlov, deputy chief-engineer was present to supervise and direct the experiment.He was newly promoted.He insisted that the test go on.And two measures be taken.First, we turned off the automatic control rod regulation system.And operated manually. Why?To remove the control rods.As we understand, once control rods removed,

Fission rate will be raised.Maybe it will get rid of the reactor poisoning.But something was done wrong.According to safety regulation, a minimum of 28 rods is supposed to remain fully inserted.But only 18 of the "fail-safe" manually operated rods were left inserted.The reactor configuration was outside the safe operating envelope.If anything pushed it, an explosion would take place.Let’s take about the explosion.How did it actually happen?It happened at 01:23:04 on 26th.The test began.A test on power failure protection, remember?As we just discussed, power supply needed to be shut down for that test.To prove the theory of generating electric power by rotational energy of the turbines.They started to shut down the power supply.To see whether the water turbines could continue to maintainThe supply of cold water with a power failure.We needed a backup electric power to keep the water supplied, which didn’t work yet.The water flow rate decreased, leading to increased formation of bubbles in the core.This happened because of the rise of water temperature.When only water surrounded the nuclear fuel, bubbles would come into being.These bubbles absorb fewer neutrons than water.

We know water absorbs neutrons, but bubbles don’t do that well as water.It resulted in a large quantity of neutrons rushing to nuclear fuel.While nuclear fuel was undertaking severe reactions.Nuclear reactions raised the temperature of water.Which in return increased the number of bubbles.The result was the sharp increase of the reactor's power outputIt exceeded the designed power output by ten times.After the shutdown of power supply, it increased to 10000 MW.Which is ten times of the designed power output.In circumstances like these,The automatic control system was supposed to insert control rods into the reactor core to limit the power rise.But it had been turned off manually.All that could be done was to sound alarms.There was nothing else the automatic control system could do.It came to 01:23:40 on 26th.The staff finally realized the situation was out of control.AZ-5 button of the reactor emergency protection system was pressed.

For an emergency shutting down of the reactor.What would happen when the emergency button was pressed?It engaged the drive mechanism on all control rods to fully insert.But it took time.The control rod insertion mechanism moved the rods at 0.4 meters per second.But the full height of the core is about 7 meters.It had to travel all this distance.So the control rods took 18 to 20 seconds to complete the travel.Besides, there was a bigger problem with the design of the RBMK control rods.The problem was that control rods were supposed to absorb neutrons, right?But to boost reactor output by displacing water when the control rods were fully withdrawn,Each of control rods had a graphite neutron moderator section attached to its end.Graphite is able to slow down neutrons, thus boosting the reaction.So, when control rods are inserted,Injecting a control rod downward into the reactor initially increased the reaction rate in the lower part of the core.To make it worse, the control rods were jammed at one-third insertion.

Yes, it was jammed.It was because of the over-heated rods.It was melt and caused some of the fuel rods to fracture.Thus the control rods were blocked and jammed.So, there was nothing else that could be done.The explosion happened.At 01:23:47 on 26th,Only 43 seconds elapsed from the start of the test to explosion.After the first explosion,the second and the third explosion followed.Until reactor no. 4 was burnt down.The disaster dispersed large quantities of radioactive isotopesinto the atmosphereand caused an open-air fire that increased the emission of radioactive particles carried by the smoke.Shortly after the accident,Firefighters arrived to try to extinguish the fires.They were told it was nothing more than a regular electrical fireInstead of the explosion of a nuclear reactor.Not knowing how dangerously radioactive the smoke and the debris were,Many firefighters lost their lives.More than twenty died on the spot.At first, the Soviets only conceded that a minor accident had occurred.

They sent a committee to Chernobyl for investigation.Yet, the committee was reluctant for a specific answer.So the Soviet government may be confused of the nature of the accident.The nearby public was not evacuated until 36 hours later.But many had already been affected by radioactive materials.A week later, the Swedish government found outradiation levels set off alarms in Sweden.After careful investigations,Finally the Swedish government contacted the Soviet government to inquire about whether there had been a nuclear accident in the Soviet UnionIt was then the Soviets knew how serious the disaster was.After the disaster,Was the Chernobyl Nuclear Power Plant completely shut down?No. because of the shortage of power supply, even with No. 4 reactor exploded,With the construction of the No. 5 and 6 reactors suspended,No. 1, 2, 3 reactors still served their purposes.They generated power until the year of 2000.The key issue is the proper disposal of the nuclear wastes.The remains of the No. 4 reactor building were soon enclosed in the sarcophagus.But it only lasts for 30 years.In the year of 1997,International society set up a commission to deal with this issue.The Chernobyl New Safe Confinement was designed and built.For a proper enclosure.It was not finished until 2016.After Chernobyl disaster, many people over-reacted to nuclear power.But it is indeed an indispensable source of power for our life.If it were not for the nuclear energy as a backup,We would suffer a more severe pollution.If you like my video,Please subscribe to view more of my videos by clicking below. Thank you and see you again.

Sources of article:
https://www.youtube.com/watch?v=Cc-xlILOeEM

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