In order to fully appreciate the historic catastrophic consequences of the Chernobyl nuclear accident, it is important to first discuss the earlier models of nuclear reactors, which were mostly built during the Cold War Era. During the Cold War, reactors were primarily built for two purposes: plutonium for military use and for the generation of electricity. Such reactors used natural uranium, NU, where the content of U-235 is 0.7%, and that means for every 1000 uranium isotopes, 993 of them are U-238, but only 7 are U-235 isotopes. Due to very small amount of U-235 in NU, nuclear fuel was made to spend relatively short time, a couple of months at the most, in the core of the On-Load Reactors. With such reactors, it is possible to load/unload fuel while they are operating. That makes it possible to produce weapon-grade plutonium, relatively more readily than enriching NU with very low concentration of U-235 from 0.7% to over 90%.
The Choice of Moderator and Coolant
A good moderator or coolant should not unduly absorb neutrons. Such materials are limited. For instance, graphite and carbon dioxide gas were used as moderator and coolant in some reactors such as Magnox reactors in the UK, and French reactor UNGG (Uranium Naturel Graphite Gas) respectively. On the other hand, CANDU reactors, used mostly in Canada, are moderated and cooled by heavy water, D2O, where D (deuterium), thus H-2 is a heavy isotope of hydrogen. Note that hydrogen has 3 isotopes: H-1, H-2 and H-3 with different characteristics. H-1, protium, the main hydrogen, is made up of one proton, the only isotope without a neutron. The deuterium, H-2, is made up of one proton and one neutron, hence H-2 is heavier than H-1. That explains why D2O is known as “heavy water”, and H2O as light water, which has high affinity for neutrons. Therefore, light water can never serve as moderator and/or coolant in reactors where NU is used.
As a matter of interest, let’s note that the third hydrogen isotope, H-3, known as tritium, is made up of one proton and two neutrons. It is not a staple isotope, thus a radio-isotope, produced by inter-actions between cosmic rays and nitrogen in the atmosphere. H-3 has a half-life of 12.3 years, and it is used for many purposes, including its use in exit signs that glow in the dark.
A Short History on the RBMK Reactors
The first commercial reactor, developed in the science city of Obninsk in Russia, was a prototype for RBMK reactors and was commissioned in in 1954 and decommissioned in 2002. The acronym of RBMK (in the Russian language) means a highly powered reactor based on channels or pressure tubes, since RBMK is an On-Load Reactor, as described above. The prototype, which was moderated by graphite and cooled by water, was fueled by 5% enriched uranium, but the enrichment of uranium fuel for the subsequent RBMK reactors were reduced.
In 1986 there were about 14 RBMK reactors in operation, including that of Chernobyl Unit-4, the fourth operating power reactor at the Chernobyl site, with the 5th and 6th in advanced stages of their construction. RBMK-1000, with thermal power of 3200MWth and generating capacity of 1000MWe, was then the largest operating nuclear power plant. Certainly, these were being constructed purposefully for electricity production only.
In this reactor, there were over 1600 fuel channels and another 200 channels for control rods and measuring instruments, drilled in the solid graphite moderator. The inner diameter of the reactor vessel is 16.6m, with active height of 19m, while the inner diameter of its core is 11.8m, with active height of 7m for the core. The core is covered by a disc cylinder of 3m thick, with a diameter of 17m. These given dimensions for this RBMK-1000 reactor show that it is more bulky than the Light Water Reactors (LWRs), such as PWRs, WWERs and BWRs.
From the above given dimensions, it is quite clear why RBMK reactors do not have containment buildings, the main major physical barrier to prevent any inadvertent release of radiation into the environment. If, the infamous Chernobyl nuclear power had had a robust containment building, the radiation spewed into the environment would have been greatly limited.
Negative Void Coefficient
In LWRs, water, which serves as the moderator and coolant can invariably be in two phases: as liquid or void of steam from local boiling areas. The greater the void, the less the water’s ability to function as the moderator. Hence fewer neutrons would be moderated to sustain the fission process, leading to a decrease in power production. Simply put, the greater the void, or the higher the temperature of the water, the lower will be its efficiency to function as the moderator. This serves as an inherent safety-control that keeps LWRs operating safely at their given power level, and that underscores that the LWRs have a negative void coefficient.
Positive Void Coefficient
In RBMK reactors, the graphite moderator readily slows down the fast-moving neutrons emitted from fission with great speed, while light water serves as a coolant. When the light water coolant starts to become hotter and hotter, voids appear, and the water starts to absorb less and less neutrons. Hence more and more neutrons would be slowed down to increase the rate of the fission process exponentially in a very short time, and that was exactly the cause of the first explosion of Chernobyl accident.
The Experiment That Went Wrong
In 1986, Chernobyl Unit-4, the newest power plant at the site, had been in operation for 2 year4s non-stop without any hindrance. It was scheduled for its first routine maintenance from 25th of April in that year.
It must be said that the principal objective of the experiment was about how to obtain power promptly in the case of power loss in the Loss of Coolant Accident, LOCA. When the national grid which would ordinarily give prompt power supply is not available during a LOCA, the only other solution is to use a diesel generator, which takes about 40-65 seconds to reach its maximum operation level. That is not good enough to handle the decay heat in the core fuel.
The above-mentioned experiment had been done perhaps twice before, and the experts were looking forward with great expectation for the planned experiment for 25th of April, and the research team was ready for the experiment. From the early hours of the 25th of April, the operators started the process of stopping operating nuclear power plant, and around 8:00am, the reactor’s thermal power of 3200MWth had been reduced to 1500MWth. It was required to go down further before the experimental.
But at the same moment that morning, a call came from the state authorities requesting the Unit-4 power plant to maintain its power level at that time until further notice. The state authorities informed the operators before mid-night that the Unit-4 could go on with its shutting down program.
Two operators and their supervisor, who started their shift from the early hours of 26th of April, decided to carry out the experiment. When they had problems setting the power of the reactor to the prescribed low level, they manually switched-off the emergency control systems. Soon later, there was a huge steam explosion due to the fact that RBMK reactors have positive void coefficient as explained above. Some seconds later, when the enormous amount of the over-heated graphite moderator in the core was exposed to the air, led to the second explosion, and the graphite burst into flames, burning and spewing out radioactive debris into the air.
Ultimately, the most destructive nuclear disaster in human history, as seen in the attached picture, was caused by human error. Switching off the emergency safety control systems, was the immediate cause of the Chernobyl nuclear accident. Sadly, none of the operators survived to tell the full story.
