Nuclear: serious accidents involving water reactors for producing electricity. IRSN publication, 12/2008. .pdf 53 pages

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Summary

1 / Introduction
2 / Definition of a serious accident
3 / Physics of heart melting and related phenomena
4 / Containment failure modes
5 / The approach adopted for current PWRs in operation
6 / The approach adopted for the EPR reactor
7 / Conclusions

Introduction

This document outlines the current understanding of severe accidents in Pressurized Water Reactors (PWRs).

First, the document describes the physics of PWR core meltdown and the possible failure modes of the containment in such a case. Then, it presents the measures put in place with regard to such accidents in France, in particular the pragmatic approach which prevails for reactors already built.

Finally, the document addresses the case of the EPR reactor, for which the sizing explicitly takes serious accidents into account: these are design objectives and their respect must be rigorously demonstrated, taking into account the uncertainties.

Definition of a serious accident

A severe accident is an accident in which the reactor fuel is significantly degraded by a more or less complete melting of the core. This melting is the consequence of a significant rise in the temperature of the materials making up the core, itself resulting from a prolonged absence of cooling of the core by the coolant. This failure can only occur following a large number of malfunctions, which makes its probability very low (in order of magnitude, 10-5 per reactor per year).
- For existing power plants, if the degradation of the core cannot be stopped by injecting water before the vessel has pierced (reflooding of the core), the accident may ultimately lead to the loss of the integrity of the containment and significant releases of radioactive products into the environment.
- For the EPR (European Pressurized water Reactor), ambitious safety objectives have been set; they provide for a significant reduction in the radioactive discharges that could result from all conceivable accident situations, including accidents with core meltdown. These objectives are:
- “practical elimination” of accidents that could lead to significant early releases;
- limitation of the consequences of accidents with core meltdown at low pressure.

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conclusions

In 1979, the core melt accident in Unit 2 of the Three Mile Island power plant in the United States revealed that multiple failures were liable to lead to a serious accident.

The releases into the environment caused by this accident were very low thanks to the return of core cooling and the maintenance of the integrity of the vessel. However, for several days, plant officials and local and federal authorities wondered how things were likely to evolve and whether it was necessary to evacuate populations.

This accident marked a turning point in the study of serious accidents.

For PWRs in operation, studies have been carried out, with a concern for realism, by seeking improvements (prevention of core meltdown, limitation of the consequences of a core meltdown, procedures) in a pragmatic manner for installations whose basic design was fixed and defining provisions to ensure the protection of populations in the best possible conditions. This work is constant, taking into account the acquisition of new knowledge resulting from the advances of continuous experimental research in this field.

Regarding the radiological consequences of a severe accident, in France, for the most radiosensitive population, with a source term S 3, the levels of intervention associated with the implementation of actions to protect the population in a situation emergency are reached up to 6 km for evacuation and 18 km for shelter and stable iodine intake, respectively, for average weather conditions.

In addition, discussions are currently underway to lower the level of intervention relating to the intake of stable iodine for the purpose of harmonization with neighboring countries, taking into account discussions at the international level (International Energy Agency Atomic, European Commission).

Finally, the contamination limits for the marketing of food products defined by the European Commission in the event of a new accident are very low.

These findings have led to an attempt to further reduce the potential for releases and their magnitude for operating reactors and to further limit releases to third-generation reactors.
generation. Thus, for the EPR reactor, ambitious safety objectives were set in 1993 providing for a significant reduction in radioactive discharges that could result from all accident situations.
conceivable, including heart-melting accidents. This involves the implementation of specific design provisions, such as the corium recuperator.

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