The Chernobyl and Fukushima accidents are comparable

While these two accidents originated in nuclear power plants, they differ markedly in their consequences and the sequence of events that caused them.

Two nuclear accidents with very different causes

The Chernobyl (1986) and Fukushima (2011) accidents are similar only in that they both involved civilian nuclear reactors and resulted in the dispersion of radioactive elements into the environment. Neither the technology of the reactors involved, nor the nature of the accidents were similar.

The case of Chernobyl

The RBMK technology (with fuel channels, cooled by boiling water, inserted into a graphite core block) of the reactor involved in the Chernobyl disaster had design flaws making it difficult to control. For example, the positive void coefficient of this type of reactor meant that an increase in core power led to an increase in reactivity, which in turn caused a further increase in power [1]. During a poorly prepared test, a series of errors led to a power excursion, i.e. a sudden and uncontrolled increase in reactor power [2].

The case of Fukushima

At Fukushima-Daiichi, the reactors involved all used American boiling water reactor (BWR) technology. During the initial earthquake, the reactors automatically shut down (insertion of control rods) and the emergency diesel generators activated. However, these generators and their fuel reserves, placed at sea level, were drowned by the tsunami that followed the earthquake. This resulted in a loss of cooling of the reactor cores, causing them to melt and produce hydrogen, which then caused several explosions, some of which compromised the containment of radioactive material inside the reactor enclosure [3].

Very different levels of radioactive releases

We then come to the question of the amount of radioactive releases caused by these two accidents. In 2017, the Japanese Ministry of Environment published a table comparing the releases of the two accidents:

Still, the health consequences of the releases are not directly proportional to the amount of radioactivity emitted. They depend greatly on the type of radionuclide - Xenon-133 being much less dangerous than Iodine-131 - and on the protective measures taken by public authorities. The management of the accident in Japan proved to be much more effective than that of the Soviet authorities at Chernobyl [4].