In the Nuclear Power Plants currently in operation in Europe, the defence- in-depth concept is applied to protect the public against hazards of a possible release of radioactive material for a series of postulated accidents. Consideration of beyond design basis accidents is an essential component in this safety concept. After the severe accidents in the Three Mile Island Unit 2 in 1979 and Chernobyl in 1986, different severe accident management measures have been developed and installed during the 80s and 90s of the last century. However, the severe accidents in the Japanese nuclear power plants Fukushima at Daiichi on March 11, 2011 showed that a continuous improvement of severe accident management measures is necessary in order to mitigate the consequences to the environment in such an emergency case. It is fundamental to understand the course of the Fukushima accidents to draw conclusions for the improvement of severe accident management. While the nuclear power plants seem to have successfully withstood the seismic event, the adopted defence against the seism-generated tsunami proved to be inadequate.
The Fukushima accidents highlighted a number of issues and weaknesses covering a very wide spectrum of technical fields and responsibilities. The availability of state-of-the-art validated numerical simulation tools is important to improve the understanding of the phenomena relevant during the Fukushima accidents. Integral codes simulating the whole accident are able to model the interaction of different physical phenomena in different parts of the reactor. This broad approach makes such integral codes an ideal tool to estimate the overall plant behaviour and the source term into the environment. Fast running codes are desirable in emergency centres to perform sets of different accident scenarios to investigate, which scenarios are most probable. The coupling of such a fast running tool with environmental dispersion codes could be used to calculate the potential source term to the environment and the atmospheric dispersion of radionuclides. On that basis, decisions on the evacuation of the population can be supported and analyses can be performed to improve severe accident management.
The ASTEC (Accident Source Term Evaluation Code) code, jointly developed by “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN, France) and “Gesellschaft für Anlagen- und Reaktorsicherheit mbH” (GRS, Germany), is now considered as the European reference code. The main objective of this project concerns the improvement of ASTEC with respect to severe accident management analysis capabilities, considering in particular relevant phenomena observed during the Fukushima accidents. Furthermore, ASTEC will be extended to become an analysis tool for usage in emergency centres to predict the most probable scenarios in accident situations in a short time and to support decision-making for emergency teams. The project includes R&D activities for the processes relevant to the Fukushima accidents, i.e. the behaviour of the fuel in the reactor core and to a smaller extent, the behaviour of the fuel in the spent fuel pool of the nuclear power plants.