The Physics of the Disaster: How and why did the Chernobyl nuclear power plant accident occur?

The Chernobyl

nuclear

power

plant

accident

is a terrible tragedy that rightly ranks among the most serious man-made

disaster

s in human history. Contrary to popular belief, it is not the deadliest on this list; For example, the Popal chemical

plant

disaster

in India killed more than 18,000 people and injured 150,000. However, Chernobyl stands out for its social and, if you will, socio-psychological importance, it almost undermined the development of

nuclear

energy as a whole and the specter of Chernobyl will probably forever loom as a dark shadow over this industry, providing its opponents with almost irrefutable arguments that the peaceful atom is not something that a rational person should do despite this or perhaps because of it, it is very important to understand why this terrible

accident

occur

red.

Debates about who is the main culprit for what happened continue to this day. They place primary blame on the station's designers who incorporated highly questionable decisions into its construction. Others place primary responsibility on the personnel of Chernobyl Unit 4 who on that fateful night violated several crucial operating rules. I will not express my opinion on this topic in Instead, in this video I will focus on what is indisputable: the sequence of events from a, if I may say, physical perspective, that is, what was happening with the reactor. night of April 25-26, 1986, what decisions were made and what actions were taken.

Personal station combined with certain technological features of the rbmk 1000 reactor that led to the result, but first a few words about how nuclear

power

plants in general are structured, the rbmk reactor in particular and the fundamental physical principles underlying its operation a nuclear power plant is a device designed to extract energy hidden within unstable atoms of radioactive uranium, a specific isotope known as uranium 235. Uranium 235 is the only naturally

occur

ring isotope capable of sustaining a fishing chain reaction induced when a neutron hits the nucleus of an atom of uranium 235, the nucleus almost instantly splits into two smaller fragments, releasing a significant amount of energy primarily in the form of heat and, more importantly, two or three new neutrons.

Each of which, in theory, can cause additional uranium fission. 235 nuclei, each of these nuclei also releases neutrons, which can cause more nuclei to divide releasing more neutrons, etc., this is how events unfold during a nuclear explosion, while the largest possible part of the radioactive nucleus of the pump must participate in the reaction in the shortest possible time. We don't need this in a nuclear power plant here, on the contrary. They require a relatively constant amount of energy to be released over the same period of time, meaning that the same number of cores must undergo fiction at any given time;

In other words, if at a given stage n neutrons initiated fission reactions in N nuclei, then only n neutrons from the resulting reaction should initiate fission reactions in the next stage. The ratio between the number of neutrons that started the fishing reactions and the number of neutrons that had to be spent to produce these neutrons in the previous fishing reaction is called the effective neutron multiplication factor. In a normally operating reactor, this factor should be equal to one if it is even slightly smaller, fewer cores will be fished at each subsequent time and the chain reaction will disappear if it is larger.

The chain reaction will grow and allowing it to grow uncontrollably carries risks. When destroying the reactor in nuclear power, a quantity associated with this factor known as reactivity is used instead of the effective neutron multiplication factor. The reactivity should be as close to zero as possible in a normally operating reactor. Positive reactivity means that the reactor is accelerating by increasing power while it is negative. Reactivity means that the reactor is slowing down, as we mentioned previously, during a fission event an average of two to three neutrons are released; However, as we just discussed, we only need one of them to participate in the next chain reaction cycle, so it seems logical that we should remove it.

About 2/3 of the neutron flows somewhere, however, not all neutrons released during fishing events will reach their target, in particular, uranium 235 nuclei were found to be quite reluctant to interact with fast neutrons released directly During nuclear fishing, these neutrons travel too fast to be captured effectively by other fishing nuclei, so to start the reaction it is necessary to slow them down. Fortunately, some substances have the property of effectively slowing down neutrons passing through them. Carbon, in particular, graphite was used for this purpose in the Chernobyl nuclear power plant reactor, so it seems simple to take a certain amount of uranium fuel and calculate how much graphite moderator is needed to ensure that two to three neutrons released on average only one starts the next fions event and that's it, you can start operating, actually it's still a little more complex.

The number of neutrons, as well as the degree of their moderation and other parameters depend on the operating mode of the reactor, for example at the beginning, when we first load the reactor with fresh uranium fuel, the neutrons find the targets relatively easily and the reactivity will be relatively high as the concentration increases. The amount of uranium 235 in the fuel decreases during reactor operation. The probability of neutrons finding targets decreases, which means the reactivity decreases. In other words, we need to alter the neutron demographics inside the reactor during its operation. For this purpose, the reactor is equipped with so-called absorbents. of substances capable of effectively absorbing excess neutrons, these can be boron, cadmium, hafnium and similar materials.

Usually these absorbers are converted into special rods if for some reason the reactivity falls below zero, some of these absorber rods are removed from the reactor, causing an increase in the number of neutrons. population and reactivity in reverse, if it is necessary to reduce the reactivity, additional rods are inserted into the active zone, initially a large number of absorbent rods are inserted into the reactor to suppress the high reactivity of a newly loaded reactor and then as As the uranium burns, the rods are gradually removed to maintain stable operation until refueling with new fuel. Reactivity is influenced by other factors and almost all materials absorb neutrons to some extent, so virtually all structural elements of the reactor contribute to reactivity.

Perhaps the biggest impact comes from the water circulating through the active zone of the reactor. To cool the reactor cooling system is a critical element of its operation, special devices known as main circulation pumps continuously circulate water through the reactor maintaining an approximately constant temperature and preventing overheating, on the other hand, the heat transferred from the active zone of the reactor to the water in the cooling system is later used to generate electricity essentially that is why the reactor was built the Chernobyl nuclear power plant used rbmk 1000 reactors in general terms the design of said reactor looks like these fuel rods that contain uranium oxide with a higher content of The fishable isotope uranium 235, raised from its natural 0.8% to about 2%, is found in the active zone and generates heat.

The arrays are placed parallel to each other inside a special chamber called the active zone, each surrounded by a special casing made of graphite blocks. placed between the fuel-containing assemblies into which control and protection rods can be inserted, all this is washed with water passing through the reactor through special pipes from the bottom up, after passing through the active zone, the Water is heated up to 280° C, however, due to the maintained pressure of 70 atmospheres in the pipes of the refrigeration system, the water does not boil although it partially evaporates the mixture of water and steam is directed to the so-called separator drums where it is separated.

In water and steam, water is pumped back to the active zone and the steam is sent to turbines that spin them and generate electricity, the steam is cooled, condensed and returned to the separator drums through the main circulation pumps. of the reactor. This approach solves two main challenges, firstly, to prevent the reactor from overheating, which could lead to the destruction of its structural elements, and secondly, to convert the heat produced into electricity, as we discussed above, the water in the reactor. usually heated to 280°C; However, it is easy to understand that in different operating modes of the reactor its temperature may be lower if the reactor is operating at a low temperature. speeds and higher at higher power levels, consequently, the balance of vapor and liquid in the pipes of the refrigeration system also changes.

Water has an ambiguous influence on neutron fluxes in the reactor, on the one hand, it absorbs neutrons quite well, which leads to a reduction in reactivity, on the other. On the other hand, it slows them down, which leads to increased reactivity. The role that water plays in the reactor, whether it is slowed down or accelerated, depends on the design of the reactor. In the rbmk reactor, South water is used as an additional neutron absorber, which was not a very successful solution. in fact, imagine that for some reason the reactivity is greater than zero, as we know, this will lead to an increase in the speed of the chain reaction, an increase in the thermal power generated and therefore an increase in temperature of both the reactor and the water in the cooling system, the increase in temperature will lead to an increase in the steam content in the pipes, which means a decrease in the density of the steam-water mixture in them.

More steam and less water mean a lower density of the mixture in the pipes and weaker neutron absorption, i.e. Reactivity increases with even greater reactivity, greater energy production, more water converted to steam, weaker absorption, greater reactivity, etc., of course, under normal circumstances, this effect can be compensated by using absorbent rods; However, on the night of April 25-26, 1986, the reactor of the fourth power unit of the Chernobyl nuclear power plant was found in an abnormal state where the coefficient of positive reactivity played a fatal role in the events that followed. . What caused this abnormal mode of operation there were several reasons for this, the main one was an experiment that was decided to be carried out at the station following the idea of ​​its general designer, the Moscow Institute of Hydraulic Projects, the idea of ​​the experiment was the Next: The hydraulic pumps responsible for circulating water in the reactor cooling system are powered by the reactor turbines, in simpler terms the reactor generates electricity for its own operation by the way, this is a common practice used in almost all the nuclear power plants in the world, but what would happen if for some reason the reactor stopped generating electricity?

Clearly, the pumps would stop and the reactor would be left without cooling, which could cause significant problems: in principle, nuclear power plants are equipped with diesel generators for such cases, which were supposed to start automatically about a minute after that the normal power supply would disappear; In theory, during this time the turbogenerators would still produce some electricity simply due to the inertia of the turbine rotor, this is known as rotor free-running energy calculations, indicated that nothing catastrophic should happen within a minute. from when the steam supply was cut off until the emergency diesel generators were activated;

However, the designers wanted to test their estimates in practice and that is why an experiment was conceived that they decided to carry out in the fourth power block of the nuclear power plant. The idea of ​​​​the experiment was as follows: the reactor operators artificially interrupted the steam supply to the turbine, after which, for a short period before the emergency diesel generators were put into operation, they measured the parameters of all reactor systems in coast mode and then shut down the reactor, so it was decided to conduct the experiment immediately before the planned shutdown of the fourth power unit for fuel replacement and maintenance in accordance with the experiment Plan before its implementation of the reactor the thermal power had to be reduced from 3200 to 700 megaw the power reduction was carried out in stages first at nighton April 25 at 347 it was reduced to 1600 megaw then at 14 o'clock on the same day it was planned to lower it to 700 and start the experiment, however, at this moment I do not know why there was a temporary shortage of electricity in the Kergo network and the dispatcher gave the order to postpone further power reduction until it was planned, the ban was lifted only at 23:10 and during all these events some unpleasant phenomena began to develop in the reactor during the uranium 235 fion is produced a certain amount of iodine 135 which is then transformed into Xenon 135 through beta decay with a half-life of approximately 9 and a half hours.

A peculiar feature of xenon 135 is that it absorbs neutrons quite well, thus effectively reducing reactivity. When the reactor is operating normally, the amount of xenon 135 it contains remains approximately constant, as much as is produced during the decay of iodine 135, so so the neutron flux burns much of it, but if the reactor power decreases the combustion rate of xenon 135 decreases an abnormally large amount of xenon begins to accumulate in the reactor, leading to a further reduction in reactivity. That is, the reactor slows down this phenomenon is known as xenon poisoning reactor at 23:10 on April 25, the fourth reactor of the Chernobyl power unit.

The nuclear power plant had been running at half power for approximately 19 hours, causing xenon poisoning, the peak of which occurred shortly before the dispatcher allowed the derating to continue. Remember this fact, at that time there was a significant amount of decelerated xenon inside the reactor. but this amount had already begun to decrease, which led to a gradual increase in reactivity, that is, the acceleration of the reactor, so at 23.10 the dispatcher gave the go-ahead and the reduction in thermal power began to reduce it. to the experimentally designated 700 watts for 00:20 on April 26. Another unfortunate coincidence occurred here during the power reduction process.

The reactor's senior control engineer, Leonid Toptunov, was unable to operate the controls, causing the power to drop unexpectedly to 500 megaw at 00.28 and then, further down, to 30 megaw, the reactor virtually stopping. To do? It may seem simple to remove some of the reactor control rods increase the reactivity speed up the reactor and return to the planned power level however this was absolutely prohibited and the instructions categorically prohibited such actions why it was all because of xenon poisoning imagine this that you have a very slowed down reactor due to the accumulated xenon the reactor begins to heat up by extracting rods reactivity power and increase in the density of the neutron flux the neutrons begin to burn The reactivity of the xenon increases again less xenon means more reactivity neutron flux more dense faster burning of xenon further increases reactivity the more neutrons the less xenon power starts to increase exponentially meaning it is unpredictably fast and there is a huge risk of losing control.

It's like pressing the accelerator pedal in a car going downhill, according to the instructions the reactor was supposed to shut down completely when you inserted all the controls. rods and wait at least 20 hours for the xenon to decay naturally, however, the Chernobyl staff decided otherwise, they were very eager to perform the experiment, instead of turning it off completely, they started against all instructions to accelerate the reactor, who made this dubious decision is not definitely known, the most common version is that the deputy chief engineer of the station, Anat Diof, who was at the reactor control panel to observe the experiment, gave the order.

Deav himself maintained until his death that he gave no such order and that the decision was made by shift supervisor Alexander Akimov. Akimov in turn denied Dav's accusations and since the key figures in this story have already passed away, we will probably never know. the truth. The crucial point is that they decided to increase the power and this decision marked the beginning of the countdown to the catastrophe, the staff managed to remove the reactor from the xenon well by increasing the power first to 160 megaw and then on April 26 to 200 megaw, however this required the removal of almost all the control rods from the active zone, according to various reports of 211 bars provided by the reactor design by 1 to 22 only 8 to two bars remained in the active zone with the allowed minimum of 32 bars CAT instructions orally prohibited operating the reactor with less than 16 bars remaining if this happened operators were instructed to immediately shut down the reactor, however, the station personnel decided to first conduct the experiment and then shut down the facility.

Again we don't know exactly who gave that order. The experiment began at 1 hour 23 minutes and 4 seconds on April 26. It was at this time that the valves regulating the flow of steam from the separator drums to the turbines were closed, the turbogenerators began to coast producing decreasing amounts of electricity, which meant that the cooling system pumps began to work more. Slowly due to the cessation of steam discharge from the system and the decrease in cold water input the temperature of the reactor began to increase, as a result more and more water began to turn into steam. Neutron absorption by the cooling system decreased, leading to an increase in reactivity and a gradual acceleration of the reactor. which already had a tendency to do so due to the gradual overcoming of the effects of xenon poisoning.

Both processes occurred almost unhindered because the reactor was almost completely devoid of control rods. It is also worth noting that the reaction rate increased more rapidly at the bottom of the reactor. This phenomenon is a structural feature of the RBMK 1000 reactor at low power levels and at that time its thermal power was only 200 megaw, 16 times lower. than normal, in addition information about all these processes was not immediately known in the reactor control panel sensor data was transmitted to the panel with a delay of 18 seconds, the backup diesel generators worked excellently and were connected in 12344 only 40 seconds after the start of the experiment;

However, a few seconds earlier at 12340, according to other data, at 12339, someone again exactly unknown pressed the emergency. protection button on the main control board known as button A5, this command was supposed to initiate the insertion of all control rods into the active zone to completely shut down the reaction, in other words, button A5 was a kind of valve emergency stop theoretically capable of quickly stopping the reactor even at full power, however, even in the operation of this final reactor protection mechanism there was one unpleasant nuance in the RBMK reactors, the control and protection rods had an interesting feature As we have mentioned, the main part of the rod is filled with a neutron absorber in this case Boron carbide however at the bottom of the rod there is a displacer made of graphite when the rod is removed from the reactor ID raised to its maximum top position the boron carbide containing part is outside the active zone while the graphite TI remains inside providing additional neutron moderation and increasing reactivity.

That is, the efficiency of the reactor. Now let's see what happens when the rod starts to descend. The top of the rod containing boron carbide enters the active zone, which reduces the number of neutrons and decreases reactivity, but this is At the top, at the bottom of the reactor, the graphite tip of the The rod displaces water from the channel, which leads to moderation of neutrons and increases the probability of their absorption by uranium-235 nuclei. The intensity of the reaction in that part of the reactor increases. This effect is brief. - lasts approximately 3 to 4 seconds only until the absorbent part of the rod completely descends into the channel.

Furthermore, under normal reactor operating conditions, the reaction intensity and neutron flux density at the bottom of the rbmk reactor are low and the described effect is known as The final effect of reactive activity should not have unpleasant consequences. , especially when a small number of bars are introduced into the active zone at a time. However, at 12,340 on April 26, reactor unit 4 of the Chernobyl nuclear power plant was definitely in an abnormal state. Not only was it operating at reduced power, which provided a higher reaction intensity precisely at the bottom of the reactor susceptible to the final effect of reactivity, but the reactor as a whole was unstable due to the consequences of xenon poisoning and the increase of temperature which as we know the withdrawal led to an increase in reactivity, in addition, more than 200 control rods were immediately directed to the active zone through the protective reset command which, due to the final effect, caused an explosive increase in reactivity , especially in the lowest and hottest part of the active zone in 12143, just 3 seconds after the restart order, the thermal power of the reactor increased abruptly to 540 megaw, more than 2.5 times.

At 12:45 it doubled again to 1.20 megawatts and continued to increase, according to estimates, just before the explosion it could have reached 10,000 megawatts, that is, three. times more than what these reactors were originally designed for, the temperature of the water in the active zone of the reactor increased rapidly and the steam pressure in the cooling system soared. The channels along which the control rods moved became deformed making further movement of the rods impossible they stopped having traveled only a third of the planned path they froze in the position which led to an additional acceleration of the reactor due to the final effect of reactivity, the increasing steam pressure ruptured the pipes of the system of the reactor cooling and the superheated steam began to enter directly into the Active Zone where it participated in chemical reactions with various materials, particularly with zirconium that formed the layers of the uranium fuel elements at steam temperatures above 800° C, the so-called reaction zirconium steam begins to produce zirconium oxide and hydrogen and releases a significant amount of heat, further accelerating the process, the hydrogen accumulates in various parts of the reactor under its lid in the separator drums and so on mixing with oxygen from the surrounding air when the concentration of hydrogen reaches a certain level an explosion occurs or rather several powerful explosions in different places which practically led to the complete destruction of the reactor around 012 2351 on April 26, 1986 to be more precise, no there is final clarity regarding the events of the last seconds of reactor unit 4 at the Chernobyl nuclear power plant, in particular, disputes continue over what type of explosions occurred: hydrogen, that is, chemical, as I mentioned above, or simply thermal. because after the pipe broke, the superheated water began to evaporate rapidly forming a large amount of steam.

However, these disputes now have a more academic value. What is most important for us is that the Chernobyl accident was the result of a combination of several design defects of the reactor and a series of errors made by personnel, the first category of factors includes, first of all, a coefficient of positive temperature reactivity, that is, the tendency of the reactor to increase the speed of the chain reaction with increasing power and temperature and, secondly, the notorious final effect of reactivity, in terms of personnel errors, among them we find can list the decision to increase the power in conditions of xenon poisoning the violation of the requirements for the minimum number of control rods in the active zone of the reactor the decision to carry out the experiment at a power significantly lower than permitted by regulations and pressing the az5 button at a critical moment which led to the simultaneous insertion of all control rods, in particular none of these reactor design defects or personnel errors listed alone would have likely led to consequences so catastrophic, unfortunately everything happened the way it did and that is why what happened happened, this is, of course, a considerably simplified explanation.

In the account of the sequence of events I deliberately did not mention some factors that could have influenced its course, for example, the shutdown of the reactor's emergency cooling system before the experiment, which subsequently had to be activated manually under field conditions. monstrous radiation, which cost the lives of several Chernobyl workers, including Akimoff and Toptunov station personnel, are alsocriticized for connecting additional pumps before the experiment, deciding to reduce water consumption in the cooling system during the reactor power-up process, etc. These nuances would greatly complicate the explanation, since it would be necessary to explain why these measures were taken and what negative consequences they could have had and there is no consensus among experts as to whether this had a decisive impact on the course of the accident, in essence, the debate continues even on whether the emergency insertion of the control rods into the reactor was The fact that after the accident the design of the control rods was modified to eliminate the final effect is quite convincing in this regard if the topic is addressed of the physical aspects of the accident.

The Chernobyl accident interests you, we can continue to discuss it in our next videos, for example, talking about the process of response to the accident both in the first hours after it and afterwards, we can also try to analyze in a similar way the development of the second largest nuclear accident , the Fukushima Daichi. nuclear disaster in Japan in 2011 in general write about this in the comments and I will try to consider your requests in the next materials, well, that's all for today, take care and until next time.