On Saturday 26 April, 1986 the reactor No.4 at the Chernobyl Nuclear power plant, near the city of Pripyat in Ukrainian SSR underwent a catastrophic hydrogen explosion and the world witnessed the worst ever nuclear accident till date. The accident took place while the plant operators were simulating the tests for backup power supply as a part of the safety systems. Ironically, it was caused by humans not by mechanical failures because they wanted to test the safety system.
This post only focuses on the physics and technical side of the accident and has no information about the aftermath.
Fig. 1 :- Schematic representation of cooling loop and power generation system in a RBMK power plant. (Image source)
The Chernobyl Nuclear reactor was a soviet designed RBMK type of reactor. RBMK stands for "reaktor bolshoy moshchnosty kanalny", which means high-power channel reactor, also known as the Light water graphite reactor (LWGR). It is a water cooled reactor with separate fuel channels and was graphite moderated.
Distinguishing characteristics:-
- Use of computerized control system.
- High positive void coefficient under most operating condition.
- Poor reactivity control, the control rods move slowly and induce positive reactivity upon initial entry.
- Low coolant to fuel ratio.
Fig. 2:- Schematic diagram of the pressurised heavy water cooled version of a CANDU (Canada Deuterium-Uranium) nuclear reactor. (Image source)
Let's discuss about some differences between the RBMK reactor and other Light water and heavy water reactors mostly in use around the world.
Figure 2 shows the CANDU reactor which has similar kind of reactor geometry as in a RBMK reactor (fig 1 & 4). The main difference is that the Chernobyl Nuclear reactor did not have a containment building (No. 13 in fig. 2 and fig. 3) like all other nuclear power plants, which is a safety barrier that is put to prevent the harmful radioactive materials from getting into the environment in case of an accident situation. If all other safety and backup systems somehow fails this reinforced concrete (about 1 meter in thickness) containment building will act as a final safety barrier. These structures are built so strong that it even can withstand an airplane crash. But in the Chernobyl Reactor there was no such safety containment structures to hold the radioactive debris from escaping, it only had a building as a radiation shield. This is the first reason why the accident was so catastrophic.
Fig. 3:- NRC Generic drawing of Containment Building and Basic Internals
This was not the only difference, there is another very important factor which was the actual cause of the accident, that has to do with moderation of neutrons. Basically, a nuclear fission reactor depends on thermal or slow neutrons for fission of heavy unstable nuclei, which has a higher chance of causing fission in Uranium fuel than the fast energetic neutrons that are produced in each subsequent reaction. Hence, the reactor requires a moderator which will slow down the fast neutrons to thermal range. For more information about moderation visit - How does a nuclear reactor works ??
Fig. 4:- Fission Cross-section vs Energy
The primary moderator used in most of the commercial nuclear reactors is light water. As in any boiling water reactor (BWR), in the RBMK water boils in the fuel channels (at about 6.9 MPa) and steam is separated above them in a single circuit. But in BWRs the moderator and coolant are both water which is itself boiled. In any accident situation if the reactor somehow loses this water there is a negative feedback set up. Since, there is no sufficient amount of moderator available inside the core the fission reaction stops. The fast neutrons are very bad at causing fission as shown in figure 4. But if the moderator and the coolant are different like in the Chernobyl reactor, where they used graphite as moderator. During any accident situation even if the reactor loses the coolant the fission reaction will keep on producing heat because graphite will still be present there to moderate the fast neutrons. So, Chernobyl reactor did not have this fail safe, that if you lose your coolant the reaction stops.
And this reason ensures that another incident like Chernobyl cannot happen.
What really went wrong that Night??
Fig. 5:- Diagram of RBMK nuclear reactor.
If the RBMK design had the earlier mentioned shortcomings then why was it used at first place?
As mentioned earlier the Chernobyl reactor was a RBMK type of reactor which used graphite blocks surrounding the fuel channels as moderators. The water used as a coolant was driven by two electrically powered pumps from the bottom, this pressurised water flows through the channels absorbing heat from the reactor core and then the steam is separated which eventually is fed to a turbine to generate electricity (fig. 1 & 5).
The design of this reactor was such that it made possible for the reactor to sustain fission reaction with unenriched uranium without resorting to heavy water for moderation, which made it very cost-effective.
Now let's come to the happenings of that day.
That month in 1986 Chernobyl was schedule to shut down for refueling, which is done like once in a year to replace the used fuel with new fuel elements. And the operators wanted to Test a safety system during this power down phase of the reactor. Any nuclear reactor obviously requires some external electricity to operate. Mainly the coolant pumps need to be operated continuously because even after shutdown the core generates a lot of heat, termed as decay heat. Because the fuel is itself radioactive, they generate heat even when not undergoing fission. Usually power plants have diesel generators for backup power supply in an emergency. However, this generators would take a minute or so to come up to power. Hence, another power source was required for the first critical minute. As the first line of safety the backup power was supposed to come from power generated by the plant turbines itself. Even after shutdown the turbines would be spinning down under its own inertia-the flywheel effect and they wanted to test whether or not this power can be used to drive the coolant pumps for decay heat removal. This had never been demonstrated at Chernobyl. They wanted to turn off the reactor and they wanted to analyze the amount of steam that it would be producing which could be used for safe operation of the plant in case diesel generators fail.
Xenon Poisoning
Neutron absorption is critical in reactivity control in any nuclear reactor. Control rods are made up of suitable materials so that they can absorb neutrons properly. One of the fission products that builds up in the core is Xenon-135, which is exceptionally good at absorbing neutrons. Xenon does not appears instantly, about 95% comes from iodine-135 (half-life of 6.5 hours). The amount of Xenon grows until it reaches an equilibrium. When amount of Xenon increases, absorption of neutrons by Xenon means less absorption needed from the control rods. Xenon-135 is referred to as a “Neutron poison” in terms of nuclear engineering.
Fig. 6
Fig. 7
As it is evident that Xenon will form under normal operation, the operators adjust the control rod in such a way to keep the reactor at a constant rate. Xenon is actually burned by getting converted into Xenon-136 after absorbing a neutron (Fig. 8) during active conditions, which is stable and not a good neutron absorber. And if the reactor is not operating it will just decay into ceasium-135.
It is important to notice that at very low power levels the formation of Xenon-135 will continue at the rate of six and half an hour ago, but the burn rate is reduced now.
Hence, at such states the Xenon poisoning continues and it gets build up in the core slowing down the reaction.
The reactor was designed to operate at about 3200 MWth and their plan was to get the power output down to 700 MWth for conducting the simulation, while for the preceding few days before the accident the reactor was operating at 1600MWth.
The reduction of power began at around 11:10 PM from 1600MWth to 700 MWth. But in this process they inserted more control rods than required bringing down the power levels to 30 MWth instead of 700 MWth which was too low to run the test. At that point it was really difficult to bring up the power levels. [ MWth - Megawatts of thermal energy ]
The Xenon-135 was building up as there were no sufficient amount of neutrons being produced at low power levels to burn it up. But this was not the only effect that was eating away the neutrons in the core. Under normal operation water is boiled inside the reactor and boiling results in low density voids. This reduces the density effect and therefore neutron absorption. But during the low heat generation the water was not being boiled which means it was absorbing more neutrons than normal because water is a more effective absorber than steam.
However that day the power demands were higher than other days, that means reactor no.4 was not supposed run at such low power levels. Instead the reactor operated at a relatively higher power levels late into the evening.
In order to raise the power to such a level where they can conduct the test, the operators pulled a lot of control rods out of the reactor. For more effectiveness the design was made such that when the control rods were pulled out the process simultaneously pulled in graphite rods, or else the space would have been filled with water which is also a neutron absorber. And this process enhanced the reactivity as graphite acts a moderator. Normally there were 150-200 rods used in the reactor but as the Xenon was stealing all the neutrons coming under pressure they pulled out almost all control rods leaving behind only 7 to 8.
By doing this they were able to bring up the power to 200 MWth. This was well below the test protocol limits but high enough for generating steam to spin the turbines upto the operating speed. And hence the test began turbines were isolated and they began to spin down.
At this stage the things went wrong, at low power levels and and low coolant flow rate water has become a significant contributor of neutron absorption. The reactor became very sensitive to the boiling of water, as the water boils fewer neutron gets absorbed and they were moderated by the graphite rods sitting there in the core. This led to increase in the reactivity and in turn water boils more and more.
This positive feedback mechanism is described by the positive void coefficient. In a nutshell, it means that if the power (temperature) of the reactor goes up the reactivity tends to increase. As the power goes up the positive reactivity goes up, such designs of nuclear reactor is autocatalytic. Water is both a more efficient coolant and a more effective neutron absorber than steam, a change in the proportion of steam bubbles, or 'voids', in the coolant will result in a change in core reactivity. The ratio of these changes is termed the void coefficient of reactivity. When the void coefficient is negative, an increase in steam will lead to a decrease in reactivity. This is what makes the light water reactors safer.
The void co-efficient is one of the many such co-efficient that describes the reactivity under different conditions. And all reactors are designed in such a way that all these effects together result in a self-stabilizing reaction.
With the reactor in a low power and low flow state small changes in pressure, flow or temperature resulted in a power feedback loop and the power started to rise rapidly. So the water began to boil more in the lower sections of the reactor. At higher flow state the water pressure would push the voids up and the reactor would self-stabilize. As the operations were at low power and flow levels the reactor de-stabilized.
The only way before the operators was to quench the power to zero, for that they had to insert all the control rods at a time. But, unfortunately this was not a very fast process because the rods had to push down water in the channels out of their way.
Fig. 9:- RBMK-Core of Chernobyl-4: Positions of control rods (insertion depth in centimeters) approximately 1min30s before the explosion on Saturday, 26. April 1986, last signal of SKALA control system at 1:22:30 h.
If we consider the reactor geometry, the rods moved at about 40 centimeters per second and the core was about 7 meters long. So, it would take about 18 seconds to fully insert the rods. The control rods were also supposed to push down the reaction enhancing graphite rods, which were about 4.5 meters long. This means there was about 1.25 meters of gap at the bottom and at the top, this ensures that graphite rods are placed centrally for better reactivity control. Initially when the graphite rods were pushed down they replaced the water at the bottom. This rapidly enhanced the reactions at the bottom of the core as soon as they were trying to shut the reactor down.
What actually happened is in order to conduct the test the operators put the reactor into such a condition where they started getting the power but it became highly unstable. The process of shutting it down actually temporarily enhanced the reactivity for few moments and in these few seconds the power skyrocketed and went beyond design limits. Followed by this there were two subsequent explosions occurred, the second one a larger explosion blew off the roof of the building. The final power readings were found to be about 33 gigawatts but further studies suggest that the peak may have been reached as high as 300 gigawatts.
Some investigations shows that the second larger explosion may have been due to hydrogen explosion, at the power levels mentioned the water would have dissociated into hydrogen and oxygen by the heat and subsequently collecting and combusting.
Well I hope you got a good understanding on why and how the Nuclear reactor caused a mishap. We learn from mistakes and the present reactors are thus made as automatic as possible with enhanced control systems. The modern reactors are also well contained and designed with passive safety systems, which do not require any human intervention. The passive systems entirely depend on natural forces of convection and gravitation. In present generation reactors an explosion would be controlled more efficiently. For more information you may try these :-
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