Why Chernobyl Exploded - The Real Physics Behind The Reactor

Hi, I'm Scott Manley. And today I want to do a bonus episode of Go Nuclear where we talk about why

reactor

number four at Chernobyl

exploded

in 1986. Now obviously the catalyst for this is the HBO and Sky TV show Chernobyl, and I'm going to say about I immediately love the program. It's fantastic. I watched it from start to finish and I think some of the details in it are great. It's not perfectly accurate in every way, but I think the courtroom scene in the final episode is an exceptionally good explanation of what happens. the layman? However, I think they overlooked a number of important mechanisms, which I would love to talk about.

So, I mean, the TV show is still superior to pretty much any documentary. I have seen on the subject. So, you know, there are some

real

ly terrible documentaries out there. I would watch this TV show before those. I'm not going to talk about the people involved either. I'm not going to name names, I'm not going to talk about the aftermath, which is also full of amazing stories of, you know, human heroism and tragedy, because of course the TV show does it exceptionally well. So let's try to delve into how nuclear

reactor

s work. Many of the details I'm going to talk about here have already been discussed in my "Going Nuclear" series, so that's a good place to be, but if you haven't.

I have observed that it will be suitable for everyone. A nuclear reactor works as a self-sustaining chain reaction where uranium atoms. Our plutonium splits into smaller, lighter atoms and, in doing so, releases energy. This is called fission now, since atoms split. They also spit out high-energy neutrons that bounce around inside the nucleus and some of them end up hitting other uranium or plutonium atoms and splitting them. Now the reactors are balanced so that the average of one neutron per reaction triggers another reaction if the average is slightly higher then the reaction rate will actually increase and if it is slightly lower the reaction rate will decrease over time.

Since each catch produces two to three high-energy neutrons, they need to get rid of the excess neutrons somehow. There are three things that What can happen to these, first, can trigger another reaction. They could escape the nucleus completely and be lost, or they could be lost by being absorbed by another type of atom that does not undergo fission. Reactor designs include materials inside that are designed to absorb the actual extra neutrons. These are our materials, such as boron or cadmium, so that they can keep the reaction rate constant. Some of these absorbers are in the form of movable control rods that can move in and out of the core to dynamically change the neutron absorption rate and keep the number of excess neutrons constant.

The nuclear fuel used in the Chernobyl reactor is uranium. Natural uranium is made of two primary isotopes. There is uranium 238, which is the most common and instead of splitting when hit with neutrons. Instead, it tends to absorb them. Uranium-235 is much rarer, but when it is hit by uranium, that is what splits and releases the energy. So, of natural uranium only about 0.7 percent is fissile uranium-235. However, in the first nuclear bomb the boy dropped, his uranium was enriched to something like 80% uranium-235 due to complex nuclear quantum

physics

. Completely ignoring the chances of a neutron being absorbed or causing fission changes with the energy of neutrons in uranium-235, the chance of a neutron causing fission is about 1,000 times greater for low-energy neutrons. than for high energy neutrons.

The neutrons that come out of each fission event are very high energy. They move at a good fraction of the speed of light. Therefore, reactors are designed to be able to reduce the speed of these neutrons to a speed closer to the speed of sound and they do so by having the Neutrons bounce off the nuclei of atoms and each time they bounce they slow down a little. The best atoms for this job are those that rarely absorb neutrons and are very light. So that each rebound transfers the greatest amount of energy possible, it is most common to see reactors that use carbon in the form of graphite, hydrogen and oxygen in the form of water.

These neutron-slowing materials are called moderators and make it possible to run a nuclear reactor without spending a lot of time and effort enriching its fuel to weapons-grade levels. It's worth noting that regular hydrogen still has a reasonable chance of absorbing neutrons, which offsets its usefulness as a moderator, but works if the fuel enrichment is high enough. Deuterium is much less likely to do this, which is Why is heavy water used in some reactor designs at Chernobyl? The reactors were of the RBMK design, which is a Russian acronym for reaktor Bolshoy Moshchnosti Kanalny, which I believe roughly translates to high-power channel-type reactor.

Canals are a series of pipes that run vertically. the reactor core that carries cooling water and contains things like fuel rods, control rods, neutron source instrumentation depending on the reactor configuration, while many water-cooled reactors use water as a moderator in RBMK reactors. The neutron moderator is mainly graphite blocks. that are placed around the channels. This was a design decision that made it possible for the reactor to sustain a reaction using unenriched natural uranium as fuel without resorting to heavy water as a coolant and this made it a very cost-effective design. Electric pumps would drive water through the core.

Pressurized water enters from the bottom of the core. It absorbs the heat of the reaction and, after leaving the core, the steam is separated and used to drive the turbines. And so we come to the test which was a series of procedures that were being carried out the night of the accident in case of an emergency The reactor would be shut down but the cooling process had to continue the reactor continues to produce heat after the reaction stops because The split atoms are radioactive and the Y They are slowly decaying and releasing energy and that meant that the reactor cooling water could not be immediately shut off.

So those pumps had to keep running and to make sure this happened there were diesel generators that could start up and provide power to the pumps in an emergency, however, those generators would take about a minute to ramp up. Therefore, another source of power was needed for that first critical minute and that power was supposed to come from the turbines and generators that were already spinning when steam generation stopped the turbines. they would start spinning but would still have kinetic energy and therefore could continue to generate electrical energy as they converted their kinetic energy into electricity and that would keep the pumps running long enough for the generators to activate.

This had never been successfully achieved. showed for a reactor at Chernobyl that the voltage regulators had always let the power drop too quickly. So this test was supposed to be the one where they finally proved that this security system could work. The reactor was going offline for maintenance anyway, so it was scheduled for the time they were shutting it down. However, that day the power grid had required more power than expected, which meant that reactor 4 was not allowed to go offline when planned and instead the reactor continued to operate at a relatively high power level. Late into the night, I remember explaining that the balance of neutrons being absorbed is important and how neutron-absorbing isotopes are used to control it.

One of the most critical fission products that accumulates in the nucleus is xenon-135 and it is exceptionally good at absorbing neutrons that would otherwise sustain its reaction, but xenon-135 does not appear instantly. About 95% comes from iodine 135, which has a half-life of about six and a half hours. So if a reactor has been operating, the xenon reactions only appear about six and a half hours later. When a reactor operates, the amount of xenon 135 in the core grows until it reaches equilibrium. And the additional neutron absorption resulting from this means that less neutron absorption from control rods is needed in nuclear engineering. 135 is known as neutron poison due to its ability to kill the reaction by robbing it of the neutrons it needs.

Now this is expected and under normal operating conditions the reactor control system will adjust the control rods to keep the reactor speed constant in an active state. In the reactor, xenon-135 is burned away when it absorbs a neutron. It is converted to xenon 136, which is much less likely to absorb a neutron and is very stable or if the reactor is idle at the time. Does not generate neutrons. It will therefore decay to cesium-135 with a half-life of approximately nine and a half hours, but it is important to note that if the reactor power is reduced, xenon production will continue at a rate of six and a half hours. hours ago, but the combustion rate now occurs at the lowest power levels.

Therefore, shutting down a reactor puts it in a situation where neutron poison builds up and slows things down even more. Normally, the reactor was designed to operate at about 3,200 megawatts, but for most of the day before the accident it had been operating at about 1,600 megawatts for testing. The power was supposed to be reduced to about 700 megawatts and, to be clear, when I talk about reactor powers here, this is the thermal energy that is generated inside. the reactor. This is not the electrical energy that comes out of it, so the reduction of power from 1600 megawatts to 700 began shortly after ten past eleven and a little more than an hour later the operators who had just changed shifts managed to stop the reactor and power levels. plummeted to about 30 megawatts, too low to test.

At this point the reactor was in a state where it was going to be very difficult to get it back online because the xenon 135 was still building up and not burning. Because there weren't enough neutrons flying around, the neutrons that were being generated were being used to burn the xenon. This is colloquially known as being trapped in the xenon well. But that was not the only effect of suffocating the reactor with its neutrons. Remember that water is a weak neutron absorber; now during normal operation the cooling water is boiled and that creates low density voids in the water and this reduces the effect of water density and therefore the amount of neutron absorption, so when the reactor is turned on. crashed all the way, the washing machine wasn't boiling and that means it was absorbing even more neutrons than normal.

So, under pressure from the chief engineer, the controllers attempted to restore the reactor to a power level where the test could occur, which meant they had to reduce the neutron absorption. And of course they did it by moving the control rods further and further out of the core. The control rods in the RBMK reactors used boron carbide as a neutron absorber. But if they were just taken out of the core, then space. What is left would contain water and that is also a neutron absorber, so to improve the effectiveness of the rod, they would instead extract a piece of graphite that would act as a moderator and therefore improve the reactivity of the system, for So in theory this made the Control Rods controls much more powerful than the reactor.

Normally, more than two hundred rods were used to control the core, but with all the xenon stealing their neutrons, the operators took out almost all of them. There were less than eight rods actively controlling the reactor. core and yes, that sounds dangerous, but I imagine the operators certainly felt comforted and encouraged to do this to push the limits because they knew that if things got out of control there was always the emergency kill switch. They could reinsert all of the control rods as quickly as possible. By manually removing so many rods from the core, they managed to get the reactor back up and running at about 200 megawatts, well below the power they should have been under the test protocol, but high enough. that by manually managing the water flow through the reactor.

They were generating enough steam to spin the turbines up to their operating speed and so the test began, the turbines became isolated and began to spin downwards and it was at this point that things went largely wrong with so many control rods. With water removed it had become a significant contributor to neutron absorption and with energy levels so low and the systemcooling system running at a correspondingly low speed for the energy. The reactor becomes very, very sensitive to water boiling when the water starts to boil and create a vacuum. Fewer neutrons are absorbed, which in turn means the reaction speeds up and heats the water even more.

This positive feedback mechanism is summarized in the phrase positive. void coefficient and this term appears regularly when describing the Chernobyl accident in reactors where water is a coolant and moderator, so an increase in voids reduces the moderating effect and therefore slows down the reactor. This is called a negative vacuum coefficient and tends to make the reactor more stable. But the vacuum coefficient is just one of several reactivity coefficients that describe how the reactor core responds to changes in reactor conditions. Another

real

ly good example is the fuel temperature coefficient, which tells us how the reactivity of the reactor changes as the fuel heats up.

This is also usually negative. A very good example of this fuel temperature coefficient is triger reactors, which are research reactors that could supposedly be operated by high school students. They have extremely negative fuel temperature coefficients. They generate great shorts. long duration energy pulses that stop very quickly as the fuel heats up, the mechanism behind this is actually quite complicated. But generally speaking, as the fuel heats the atoms in the solid, they vibrate more and more. This motion must be added to the speed of the neutrons. Changing the effective speed of neutrons flying through the nucleus, which in turn affects the absorption and scattering parameters, so the fission cross section effectively drops and is sometimes called the Doppler coefficient because it is a result of the shift Doppler of the neutrons that find the atom.

But see, what I mean here is that the reactors are designed to operate in regimes for all these coefficients, all these factors result in a self-stabilizing reaction and then the RBMK reactors generate the positive destabilizing effect. The voic efficiency of water was normally compensated by this and other stabilizing mechanisms. With the reactor in this low flow, low power state some changes in pressure or temperature flow began, a power feedback loop began and the power of the reactor began to increase rapidly and as the power began to rise the washer began to boil at increasingly lower points in the channels, which allowed the reaction to effectively move down the course.

Typically at higher water flow rates. If this were to happen, the pressure from the washer would push the gaps upwards and they would self-heat. stabilize. But since the water flow was low due to the low power which contributed to the instability of the reactor at that time, the reactor shutdown control was activated. This was switch az5 or control five of the EPS emergency protection system. It is actually a group of six buttons with plastic covers. and wax seals. Therefore, you couldn't activate them casually. Each of the buttons initiates different security procedures. For example, I think one would reduce it to 50% slowly and then with one that would reduce it to 50% quickly, but a Zed 5 It turned off the power to zero as quickly as possible And of course the way it turned off the power to zero was inserting those control rods as quickly as possible.

Unfortunately, this was not a particularly quick process because the bars had to push down these channels. and push the water out of the way and it would only move at about 40 centimeters per second. The core was about 7 meters long. So it took us 18 seconds to fully push the rod down through the core. The control rods also had to push the graphite rods that had taken their place out of the way. Now those reaction enhancing rods were about four and a half meters long and there were positions where they were central to the core, which meant they had about 1.25 meters of water above and below them.

This had the effect that when pushed through the core, the bottom two meters initially had displaced water. and replaced by graphite and that meant that the reaction at the bottom of the core improved as soon as you started shutting down the reactor, so in an effort to get power for the test, the controllers had pushed the reactor to a configuration where it didn't. They had started receiving power, but it was no longer stable. And when that energy started coming, they tried to turn it off, but the process of turning it off actually temporarily enhanced the reaction for a few seconds and in those few seconds, the energy spiked. it shot way past the design limit and that's what happened seconds after the fight started.

There was an explosion followed a few seconds later by an even larger explosion that blew off the top of the reactor and left it at an angle. The final power reading recorded in the control system was 33 gigawatts, approximately 10 times greater than design. station power, but most models from this excursion suggest it may have peaked at more than 300 gigawatts. The actual mechanics of the explosion are still somewhat open to debate. Most people believe that the increase in power boiled the water into steam in the canals. and then caused the pipes to burst and the second largest explosion may have been due to the water dissociating into oxygen hydrogen by the heat and then building up and burning elsewhere.

But there are those who argue that this was literally a small nuclear explosion where the reaction went critical and leaked so quickly that it only stopped when the fuel vaporized and

exploded

beyond the critical point. In one scenario, the vaporized material literally shoots out of the channel as a jet of plasma directed toward the sky anyway, once the power is exhausted. There were many mechanisms that could have led to the destruction of the reactor and its containment. It was important to realize that at the time most of the world's experts thought that the idea of ​​a nuclear reactor having a runaway excursion and explosion like this was impossible and, of course, the reason this was It was so far-fetched because reactors are designed to prevent these circumstances and even the deficiencies of the RBMK reactor were remedied in later years.

There are still several RBMK reactors operating around the world. Even though Chernobyl reactors one through three were only closed a few years ago, but that's my cold analytical

physics

side. The events that happen after the explosion are where the true essence of the drama lives and the TV show does a fabulous job of putting viewers in some of these situations. and yes, there are some horrific scenes of victims suffering from extreme radiation poisoning. And that's the reality that some of the first responders had to experience, but having seen that, it's important that you keep in mind that scientific study of power generation and the human cost of power generation. shows that nuclear energy is actually one of the safest options out there.

So I don't want you to come away from this video thinking that every nuclear reactor is a potential Chernobyl waiting to happen. That is simply not the case. But I hope you come out of this with a little more understanding of physics and perhaps a little more respect for power in general. I'm Scott Manley. Fly safe.