ENS Expert Explains Radiation Protection In Emergency Cases

Prof. Dr. Attila Aszodi (Budapest University of Technology and Economics, Hungary) explains in his blog what to do in the event of a nuclear or radiological accident causing a significant release of radioactive material.

In recent days, following the start of the Russian-Ukrainian conflict, the Russian invasion of the Chernobyl nuclear power plant site and the attack on the Zaporizhzhya nuclear power plant, many people are asking what the impact of a nuclear accident in Ukraine could be on Europe, and what to do in such an event. Prof. Aszodi tries to summarise this briefly in this paper.



As I have written several times before, nuclear power plants are designed for a wide range of malfunctions and initial events of various internal and external origin. These so-called “design basis accidents” can be handled by the safety systems of nuclear power plants without resulting in large radioactive releases. However, as we have seen for example in the Fukushima accident, if a nuclear power plant is left without cooling water and without electricity supply (without functioning emergency diesel generators and a functioning electrical grid connection), this can result in a series of events that can damage the reactor fuel and the so-called engineering barriers that are designed to contain the radioactivity. In such a case, higher levels of radioactive material may be released into the environment, the harmful effects of which can only be mitigated by special measures. These are the so called beyond design basis accidents, possibly leading to a major release of radioactivity.

It is important to stress that the type of accident that caused an immediate large release in the Chernobyl accident in 1986, cannot occur in the water-cooled water-moderated reactors now operating in Ukraine. The Chernobyl reactors were graphite-moderated water-cooled reactors in which a very unfavourable reactor physics property, the so-called positive void coefficient, allowed the reactor to runaway (i.e. to increase its power suddenly and enormously), explode, and then the firing of thousands of tonnes of graphite in the reactor for 10 days increased the release of radioactive material into the environment and its dispersion to the higher atmosphere.

The VVER reactors in operation in Ukraine today do not have this unfavourable reactor physics, nor do they have graphite, which could burn for a long time: the moderator of these current reactors is water. Consequently, the accident at the reactors currently operating in Ukraine cannot cause releases close to those experienced in the 1986 Chernobyl accident. It is therefore inconceivable that an accident at these VVER reactors could result in an environmental release many times higher than the release at Chernobyl.

The other important principle is that radioactivity released into the environment is diluted as it spreads through the environment, its concentration decreases and, over time, radioactive isotopes decay, so the longer the release spreads, the less dangerous it is for the environment. Moreover, it is not primarily the geographical distance that is important here, as the air mass containing radioactivity is spreading in the environment depending on the meteorological conditions. Thus, the contamination of a given area will be determined by air currents and precipitation conditions, since the amount of radioactive material that will be released from a radioactive cloud (which is not really a cloud, but just the common name for the radioactive contamination released) to the surface and fall-out will depend on whether precipitation occurs as the polluted air mass passes, which would wash the pollutant out of the air.

As a consequence, releases from even a severe nuclear power plant accident can typically cause high doses to the population within a radius of 10-30 km of the plant that require public protection emergency measures.

Although the lack of official communication after the Chernobyl accident in 1986 caused a great mistrust among the Hungarian and European public, there would have been no reason to take any real public protection measures in Hungary or in other European countries (except in the immediate vicinity of the Chernobyl nuclear power plant). But what these population protection measures might be, more on that a bit later.

Continue reading more on Prof. Aszodi’s blog “chainREACtion”.