How many RBMK reactors are there

The RBMK - an unusual type of reactor

The reactor design contributed significantly to the Chernobyl accident. That is why the other reactors of the RBMK type were quickly improved. However, some important retrofits are still pending.

On December 15, 2000, the last reactor block in Chernobyl was shut down. In Russia, however, a total of 11 Chernobyl-type reactors, so-called RBMK, are still in operation in Sowosni Bor near St. Petersburg, Smolensk and Kursk, as is one reactor in Lithuania. Because the RBMK construction method made a significant contribution to the accident in Chernobyl, its most important safety deficiencies were quickly remedied - an accident like that of April 26, 1986 can therefore be practically ruled out today, the experts agree on this. However, other design errors have not yet been satisfactorily improved everywhere.

Specific weaknesses

The reactor type RBMK (Russian «Reaktor Bolshoi Moschnosty Kanalny», large capacity canal reactor) was developed in the USSR. It differs significantly from western light water reactors. The fuel elements with weakly enriched uranium are not located in a single large pressure vessel, but in over 1,600 separate water-cooled pressure tubes. Neutrons split the uranium nuclei, producing more neutrons and heat. As a result of the heat, the water flowing in from below is partially converted into steam in the pressure pipe, which is taken from the top and fed to two turbo-generators - in the case of Chernobyl with a maximum of 500 megawatts of electrical power each.

In order for a chain reaction to be maintained in the reactor, the neutrons must be slowed down (moderated). At the same time, the moderator (the cooling water in western light water reactors) also absorbs a certain part of the neutrons. If more vapor bubbles form in the water than usual - for example due to overheating - fewer neutrons are absorbed because of the lower density of vapor compared to water. But the moderation effect of water also decreases: Because fewer neutrons are moderated, fewer fission takes place. In RBMK, however, primarily graphite blocks between the pressure tubes serve as moderators. When bubbles form, fewer neutrons are absorbed in the water, but the graphite moderates them unchanged. The number of neutrons for fission processes increases; the reactor can become very unstable.

This major weakness of RBMK was quickly brought under control after 1986 by taking suitable measures. The same applies to the “positive shutdown effect”: the control rods with which the reactor is to be shut down in an emergency first locally increased the number of fission neutrons instead of reducing it due to their design when entering the core. The bars were therefore designed differently; in some cases their number has also been increased and the running-in time has been shortened. The emergency cooling systems, fire protection and the reactor protection system were also greatly improved in the RBMK after 1986.

These successes are not least due to intensive international studies. In 1999 the International Atomic Energy Agency (IAEA) published a report on what had been done about the safety deficiencies of RBMK and what still had to be done. A second, completely independent shutdown system was also required. Such is essential, says Aybars Gürpinar from the IAEA. The agency does not keep an official list of the retrofits that have already taken place; It is therefore difficult to assess their current status. According to Jochen-Peter Weber from the German Society for Plant and Reactor Safety, several of the Russian RBMK are not yet equipped with a second shutdown system. In some cases, retrofitting is only planned in 4 to 6 years. The situation is different for the Ignalina 2 reactor in Lithuania; extensive upgrades were a prerequisite for joining the EU.

The necessary improvements are made more difficult in many places by a lack of money, but also by the fact that each of the RBMK reactors is different, even if they are roughly divided into three generations according to age. Changes therefore often have to be carried out very specifically, says Maciej Jankowski from the IAEA. The same applies to the simulator programs.

Containment cannot be retrofitted

One accident for which the RBMK is not designed is the simultaneous rupture of a large number of pressure pipes, which, however, is extremely unlikely. The conditions under which the breakage of one pipe can lead to the breakage of others is still being investigated. For the time being, the reactor shaft in some plants was modified in such a way that it should withstand the breakage of around 10 pipes instead of just 1 to 5 pipes.

However, certain deficiencies in the RBMK cannot be remedied by retrofitting. This includes full containment, says nuclear expert Ernst Knoglinger (formerly at the Paul Scherrer Institute). Such a cover, which safely encloses the radioactivity in the event of an accident, is completely absent from the first generation. With the second and third, a special safety system only ensures partial confinement in the lower area of ​​the water-steam cycle. However, whether this is sufficient is a matter of dispute among experts. In the meantime, the Russians are also relying primarily on pressurized water reactors from their VVER construction lines. RBMK are no longer planned. Only Block 5 of the Kursk power plant is still under construction. However, it is doubtful whether this reactor will ever go online - the maintenance of the discontinued model would be very expensive.

Hanna Wick