Research into severe accidents broaches a wide range of scientific and technical subjects. To ensure a global approach to all these issues, IRSN - in collaboration with CEA, with EDF backing - has written a report that aims to present, for each subject, the state of current knowledge and research activities conducted in France or abroad. On 12 October 2006, the report was submitted to the Permanent of Group Experts for Nuclear Reactors that reports to the Director General for Nuclear Safety and Radiological Protection.
A severe accident is defined as an accident in which the fuel in a nuclear power reactor is significantly degraded through complete meltdown of fuel assemblies. Given the preventive measures that are in place, this type of accident is of course unlikely. However, in view of the extensive environmental consequences associated with this type of accident, due to the release of radioactive products, much effort has been - and still is - put into studies to improve preventive measures and to limit the consequences as far as possible. The physical phenomena set in motion during a severe accident are extremely complex. Research aims to better understand these phenomena and reduce the associated uncertainties.
The report produced by IRSN and CEA starts by presenting severe accident scenarios that are conceivable in Pressurized Water Reactors operated in France. Details are then provided concerning the different physical phenomena that could occur in the reactor tank and the containment during such an accident, as well as the ensuing chain of events and measures to limit the effects. For each of the phenomena, the latest principal experiments (either in progress or planned for a future date) and the principal models and computer codes used to simulate the phenomena are described and a state of acquired knowledge is established. IRSN and CEA are thus striving to clarify - as part of a global approach - the state of acquired knowledge for the different physical phenomena so they can then assess the extent to which it can be used to make reliable predictions.
Research is of benefit for both reactors currently in operation and for future reactors: the phenomena incurred during a severe accident are the same. Nevertheless, if severe accidents were not taken into account in the initial design of French nuclear power plants currently in operation, improvements have been made in terms of prevention and limitation of consequences. For the EPR reactor, an important objective was set in 1993 to obtain both reduced probability of core meltdown and a significant reduction in radioactive release resulting from an eventual accident with core meltdown: in particular, all possible measures must be taken to "all but eliminate" accidents of this type that could entail significant early release.
Research in the field of severe accidents requires massive human and financial resources; collaborations with different nuclear players (operators, industrials, and research and safety bodies) are widespread at a national and international level. In France IRSN, CEA, EDF and AREVA have funded or carried out research, often as a group, on a large number of subjects and take part in international programmes - particularly research actions supported by the European Commission as part of the framework programmes for research and technological development (FP) or activities conducted under the aegis of the OECD. As part of FP6, SARNET (Severe Accident Research NETwork of excellence) was set up with a view to make optimum use of the research means available in the field of severe accidents. SARNET is coordianted by IRSN. One of its objectives is to ensure that ASTEC (Accident Source Term Evaluation Code) becomes the European reference code in its field. ASTEC is a code used to simulate an entire accident from the initiating event to the possible radionuclide release outside the containment. It was jointly developed by IRSN and its German counterpart GRS.
The report produced by IRSN and CEA shows that significant uncertainties remain concerning certain phenomena. An example would be kinetic erosion of the containment foundation raft caused by a core that has melted and passed through the tank (for EPR specific devices are implemented to avoid this kind of erosion). Similarly, with regards to fission products, other uncertainties remain - particularly on the behaviour of iodine in the reactor coolant system and in the containment. Experimental programmes are in progress on these subjects, notably as part of ISTP (International Source Term Program), conducted under the Aegis of IRSN in partnership with CEA and EDF and in association with foreign partners: their completion date is set for 2010.
> Download the report "Research and development with regards to severe accidents in pressurized water reactors: summary and future prospects".