Nuclear: severe accident power generation water reactors. Publication of the IRSN, 12 / 2008. .pdf 53 pages
1 / Introduction
2 / Definition of a serious accident
3 / Physics of the meltdown and associated phenomena
4 / Modes of failure of the containment
5 / The approach for current operating REP
6 / The approach adopted for the EPR reactor
7 / Conclusions
This document provides an overview of current knowledge on severe accidents Reactor Pressurized Water (EPR).
First, the document sets out the physics of the core meltdown of a PWR and the possible failure modes of the containment in such a case. Then it presents the provisions established in respect of such accidents in France, especially the pragmatic approach prevails for reactors already built.
Finally, the paper discusses the case of the EPR, for which the design takes explicit account of serious accidents then it is design objectives and compliance should be a rigorous demonstration, taking into account the uncertainties.
Definition of a serious accident
A serious accident is an accident in which the reactor fuel is significantly degraded by a more or less complete fusion of the heart. This melting is a consequence of a significant temperature rise component of the core material itself resulting from prolonged lack of cooling of the core by the coolant. This failure can occur only after many failures, making its very low probability (in order of magnitude, 10-5 per reactor per year).
- For existing plants, if the deterioration of the heart can not be stopped by injecting water before the breakthrough of the tank (reflooding of the heart), the accident may lead eventually to the loss of integrity containment and significant releases of radioactive materials into the environment.
- For the EPR (European Pressurized water Reactor), ambitious safety objectives have been set; they provide a significant reduction of radioactive discharges may result from any conceivable accident situations, including accidents with core meltdown. These objectives are:
- "Practical elimination" of accidents that could lead to large early releases;
- Mitigation of accident with fusion of the low pressure center.
In 1979, the meltdown of the core of the 2 installment of the Three Mile Island in the United States revealed that failures accumulations were likely to lead to a serious accident.
Releases to the environment caused by the accident were very low due to the return of core cooling and maintaining the integrity of the tank. Yet for several days, officials of the central and local and federal authorities have asked how things were likely to change and whether to evacuate people.
This accident marked a turning point in the study of severe accidents.
For PWRs in operation, studies have been made, with a concern for realism, seeking improvements (prevention of core melt, limiting the consequences of a core meltdown, procedures) pragmatically for installations whose basic design was frozen and defining arrangements for ensuring the protection of people in the best possible conditions. This work is constant, taking into account the acquisition of new knowledge from advances in experimental research continues in this area.
Concerning the radiological consequences of a serious accident in France for the most radiosensitive population with a source term S 3, intervention levels associated with the implementation of actions to protect the population living in radiological emergency are achieved respectively until 6 km for evacuation and 18 km for sheltering and stable iodine to average weather conditions.
In addition, discussions are underway to lower the level of intervention on the stable iodine in order to harmonize with neighboring countries, taking into account the discussions at international level (International Energy Agency atomic, European Commission).
Finally, the contamination limits for the marketing of food products set by the European Commission in the case of a new accident are very low.
These findings have led to try to further reduce the possibilities of rejection and amplitude for reactors in operation and a further restriction releases for the third reactor
generation. Thus, for the EPR reactor, ambitious safety objectives were fixed from 1993 providing a significant reduction of radioactive discharges may result from all accident situations
conceivable, including accidents with core meltdown. This requires the implementation of specific design provisions, such as the core catcher.