Nuclear reactor accidents
3) Chernobyl 1986 Pripyat Russia
That is perhaps the most notorious nuclear accident in history and for this reason I will describe it in more detail. What happened here was the worst case scenario: a steam explosion that blew up the reactor releasing fission products and everything else it had inside 1 km up in the atmosphere.
The kind of reactor involved was the RBMK, a design totally produced in the URSS and very different from western designs. Its most notorious characteristics were the huge size and power and the fact that it uses graphite (moderator) and water (coolant). RBMK´s could have powers of up to 1.5 GW, which was huge particularly in the 50´s, which were the early days of nuclear technology. Fortunately the Chernobyl RBMK reactors had “only” 1 GW of power each. There were 4 of them working and other 2 in construction.
This exclusive reactor design was very handy for soviet objectives: it could produce lots of electricity and also bombs! It was used to produce plutonium for atomic bombs. The most difficult part of building an atomic bomb is to obtain the material , plutonium, which doesn´t exist in nature and must be fabricated (in a nuclear reactor). The RBMK reactor permits online refuelling, which means that the fuel elements (containing uranium oxide enriched at about 2%) could be replaced without shutting down the reactor. Most western reactor designs have offline refuelling so that they must be periodically shut down for refuelling and maintenance. That is an inconvenience because it stops producing electricity and plutonium for a while.
So why is online refuelling better to produce plutonium? The useful plutonium is Pu 239, produced by irradiation of U 238. The problem is that if the produced Pu239 stays in the reactor for very long it may end up absorbing a neutron and becoming Pu 240 which is useless. Because the RBMK reactor can change fuel elements while working, that can be done more often so that more Pu 239 can be recovered.
The RBMK is considered a dangerous design because, having water and graphite, it is vulnerable to two of the worst reactor accidents: a graphite fire and a steam explosion. Even after the safety modifications introduced after the Chernobyl accident, in order to alleviate some of its design flaws (described below) , it still considered dangerous. So it is that the Ignalina Nuclear Power Plant in Lithuania , which had two 1,5 GW RBMK reactors, had to be shut down to satisfy a condition for this country to enter the EU, although it had experienced no extreme accidents up to that moment.
Next I will describe some of the design flaws of RBMK reactors. Some of these were not known even to soviet engineers and technicians that operate these plants at that time. The soviet regime involved a lot of secrecy, specially during a cold war, and such flaws could be embarrassing to be discussed. This is a political element that contributed to the disaster. In the case of Windscale, international cooperation was crucial for the safety of the plant, although an accident happened anyway.
RBMK reactor design flaws
1) The reactor core is too big so that it takes about 20s for the control rods to be fully inserted in order to shut down the reactor, procedure called SCRAM . Other designs like the PWR can perform a SCRAM in about 3s.
2) Control rods have a tip of graphite (which is also a lubricant material) to make easier their way through the holes in the graphite blocks. The problem is that graphite is also a moderator and it increases the reactivity, by slowing down neutrons. That means that as the control rods enter the reactor, its reactivity is increased for a moment, before it gets reduced as intended. This design flaw was first noticed in 1983 at the Ignalina Power Station
3) It has a positive void coefficient. That means that if water boils inside the reactor, due to overheating, reactor power will increase and cause more boiling and increase a gain the power and so on. It is a dangerous positive feedback loop. That happens because water (ordinary or light water) absorbs neutrons and as a result reduces the rate of reaction causing a slight reduction in power. When it is not present because it boiled, power will increase.
In other types of reactor, like PWR´s, the effect is the opposite. Because water is acting as the moderator, an absence of water will slow down or even stop the reaction. IN RBMK´s the moderator is not water but graphite.
4) It has no containment building, like for instance PWR reactors. In the TMI accident, most of the radiation released from the reactor core was contained within the building. The RBMK is very large and it has a huge machine on top of it which is responsible for replacing fuel elements. For this reason it was not practical to build a containment building (there was only a roof to stop the rain from coming in). Instead it had a very heavy lid on top of the reactor (the so-called biological shield), held down by gravity (which went flying after the reactor explosion).
The iodine valley
It is also known as Xenon poisoning . This is not a flaw of RBMK reactors in particular but it is a problem that affects all types of nuclear reactors. All will describe it briefly here because it played a major part in the disaster.
The nuclear fission process produces lots of different products. One of these is iodine-135, The higher the power of the reactor, the more I-135 is produced. I-135 decays into Xe-135 (beta minus decay-9 hours half life). The problem is that Xe-135 is a very strong neutron absorber so that its presence will reduce the power of the reactor or even shut it down. The good news is that Xe-135 can absorb a neutron and become Xe-136 which doesn´t pose a problem.
This situation is not usually a problem because when the reactor is running at full power the neutron flux is very high and the Xe-135 produced is converted into Xe-136 before it can cause problems. The problem happens when the power of a reactor is reduced. In this case there are lots of Xe-135 around because the reactor was running at full power, but because the power has been reduced there are no enough neutrons to convert all the Xe-135 into Xe-136. This can cause the reactor to shut down. The reactor went down the iodine valley. Now it will be necessary to wait for all the Xe-135 to beta minus decay into Cs-135, which can take about 40 hours, before the reactor can function again.
THE ACCIDENT
A security test was scheduled for Chernobyl reactor number 4 that day. It consisted of shutting down a steam turbine (and the electricity generator attached to it) to check if it would spin for a while and keep producing some electricity until the diesel generators were being started. It was important to know that because security systems rely on electricity and engineers wondered if in the event of a reactor (and turbines) shut down things would go smoothly without any power shortages.
The problem was that in order to run the test the power level of the reactor had to be reduced and all the automatic security systems disconnected, so that it had to be controlled manually. When the test was going to start, authorities in Kiev phoned up and explained that power could not be reduced at that time and the test was postponed for a few hours. The test started after midnight, what was a problem because there was a change of shift during the test and less experienced operators replaced those that working during the day.
Big powerful machines are usually projected to run at full power. This is the case of jet engines, for instance, or nuclear reactors. Operators had a hard time trying to decrease the power of reactor number 4 to the level specified on the manuals for the test, especially with the automatic systems disabled. Eventually the power dropped too much and the reactor went into the Iodine Valley. At this point it had to be shut down but operators apparently were not aware of what was going on. Instead of shutting down the reactor, it was decided to remove all control rods in a desperate attempt to get the reactor back. The power started to increase and the turbine was disconnected to initiate the test. Without this heat outlet, and with all the control rods removed, water began to boil inside the reactor, triggering yet another mechanism for increasing power (positive void coefficient). The result was that power rose to 30 GW (that is 30 times higher than full operating power) and a steam explosion blasted nuclear fuel, graphite and fission products high in the sky. That was followed by a second explosion which could have been a steam or a hydrogen explosion. Unit number 4 was completely destroyed. Unit 4 had been running for 3 years so that it was heavily loaded with the dreaded fission products and that is why some people says it released more radioactivity than the atomic bombs dropped in Japan.