On March 11, we commemorate the 15th anniversary of the Fukushima Daiichi nuclear power plant accident. Every event with an impact on nuclear safety is analysed in detail. Experts from all over the world aim to undertake maximum measures to prevent a similar incident from happening again. The same was true of the Chernobyl and other accidents at nuclear facilities. In this respect, nuclear power is remarkably similar to the aviation industry. There were several causes leading to the accident, as is the case in aviation – an aircraft does not crash after the failure of single system, but always a combination of two or more different failures.
The main technical causes of the Fukushima accident were three-fold:
Let us therefore look at what measures were and are being taken to improve the safety of nuclear power plants around the world.
In Japan, focus landed on the construction of higher breakwaters and walls to protect the power plants from tsunami waves, and the ability of hermetically sealing the reactor buildings as a prevention against water penetration into the interior, where it could cause the loss of internal power sources. Everywhere around the world (including Dukovany and Temelín), measures were taken against all conceivable external hazards (extreme rain, snow, wind, seismicity). In Dukovany, for example, fan cooling towers were built as a backup for the possible collapse of large cooling towers due to extreme wind. Great attention was then paid to materials - Accident Tolerant Fuel (ATF) and concrete:
The application of the stress test approach, which identified weak points in individual projects, played a crucial role. Based on this identification, programs increasing safety of individual reactors were implemented, based on the so-called Defense in Depth approach. This approach defines a completely self-reliant protection system for each of the protective barriers, independent in terms of design compared to the protections of other levels, so that multiple systems cannot fail due to a common cause. Great emphasis is placed on the use of passive solutions, i.e. methods where the safety systems do not need energy sources not only for its operation, but also for its activation. This approach is strictly required for new projects of Gen III and Gen III+, but it was also retrofitted to the maximum extent in order to increase the safety of already operating nuclear reactors.
The footprint of CVŘ is very noticeable both in the areas of power plant retrofitting and in the development of new materials. Shortly after the accident, together with Hitachi, we successfully developed methods for separating debris below the Fukushima reactor vessels. For Mitsubishi Heavy Industries we measured the physical properties of corium (a molten mixture of fuel, structural materials and concrete) so that Japanese experts could better determine the distribution of the melt under the reactors and prepare for its removal. In cooperation with Kajima, Mitsubishi Research Institute and Nagoya University, we are implementing a long-term program focused on studying radiation aging of aggregates and concretes used in the construction of Japanese power plants. The data obtained were included in the documents needed to restart Japanese nuclear power plants. Together with world fuel producers and nuclear power plant operators, we are participating in the development and testing of new cladding materials, which will also be introduced at Czech nuclear power plants. CVŘ is also involved in the OECD/NEA FACE (Fukushima Daiichi Nuclear Power Station Accident Information Collection and Evaluation) project, within which CVŘ specialists performed analyses of melt sample simulants from the failed reactors.