Serious nuclear accidents on PWR and EPR

Nuclear: serious accidents of reactors with water of production of electricity. IRSN publication, 12/2008. .pdf 53 pages

Download the document here: serious accidents on PWRs and EPR nuclear safety

Summary

1 / Introduction
2 / Definition of a serious accident
3 / Physics of heart melting and related phenomena
4 / Failure modes of the containment
5 / The approach adopted for the 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 of all, the document describes the physics of the core fusion of a PWR and the possible modes of failure of the containment in such a case. Then, it presents the provisions 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 design takes explicit account of serious accidents: these are design objectives and their respect must be rigorously demonstrated, 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 melting of the core. This fusion is the consequence of a significant rise in the temperature of the materials composing the core, itself resulting from a prolonged absence of cooling of the core by the heat transfer fluid. 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 stations, if the deterioration of the core cannot be stopped by injecting water before the vessel has broken through (drowning 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 European Pressurized water Reactor (EPR), ambitious safety objectives have been set; they provide for a significant reduction in radioactive releases that can result from all conceivable accident situations, including accidents with core meltdown. These are:
- "practical elimination" of accidents that can lead to significant early releases;
- limitation of the consequences of accidents with melting of the core at low pressure.

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conclusions

In 1979, the core meltdown accident at Unit 2 of the Three Mile Island power station in the United States demonstrated that cumulative failures were likely to lead to a serious accident.

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

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

For PWRs in operation, studies have been made, with a concern for realism, looking for improvements (prevention of core fusion, limitation of the consequences of core fusion, 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 from advances in continuous experimental research in this area.

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

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

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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 releases 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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