In a statement released today by the OECD Nuclear Energy Agency (NEA), governmental experts in nuclear safety, members of the NEA Committee on Nuclear Regulatory Activities (CNRA), announce their continued commitment toward prevention and mitigation of severe accidents in nuclear reactors. The statement is also intended to illustrate the common philosophy adopted toward this goal, notably the continuous introduction of improvements in current reactors, and the commitment to make future reactors still safer than those already in operation. The contribution made by international co-operation and research programmes to the resolution of severe accident issues is also recognised. Research will continue to contribute to reactor safety by reducing uncertainties further and maintaining technical expertise.
As a result of safety studies conducted in the 1970's, and the accidents at Three Mile Island and Chernobyl, it was recognised that despite stringent safety requirements, accidents leading to core damage, usually referred to as "severe accidents", could occur. Large national and international research and review programmes were undertaken. Plant modifications and operational changes were and continue to be implemented in current designs to reduce further the risk from these accidents. As a result of these efforts, the safety of operating nuclear reactors is further enhanced.
For future reactors, severe accidents will be addressed in the design stage. Accident management measures will be identified at that stage and will continue to play an important role in the resolution of severe accident issues.
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Current reactors operating in OECD countries incorporate safety systems and features to prevent and mitigate the consequences of a set of design basis accidents (DBA). These safety features are designed and constructed to the highest engineering codes and standards; incorporate redundant and often diverse system configurations; include consideration of environmental qualification and external hazards; and incorporate conservative design margins. Accidents leading to core damage, usually referred to as "severe accidents", were considered as contributors to the residual risk only and were not addressed directly in the design process.
As a result of safety studies conducted in the 1970s, and the accidents at Three Mile Island and Chernobyl, it was recognised that, despite stringent requirements, severe accidents could occur. Large national and international research and review programmes were undertaken. Plant modifications and operational changes were and continue to be implemented in current designs to reduce further the risk from these accidents.
While OECD countries strive both to prevent severe accidents and mitigate their consequences, practical considerations have resulted in greater emphasis being placed on prevention. Moreover, improved operating procedures and accident management measures have been found to be effective and are given large emphasis in most OECD countries. As a result of these measures the safety of operating nuclear reactors is further enhanced.
In general, measures to address severe accidents have been kept separate from design basis accidents and are analysed using best-estimate rather than conservative methods and assumptions. The design features intended to cope with these accidents do not systematically incorporate redundancy or diversity or the same level of qualification as those features intended to cope with design basis accidents. Many countries have considered that severe-accident features are not essential to providing an adequate level of protection to the public. These features provide additional defense-in-depth.
For future reactors, severe accidents will be addressed in the design stage. For instance, plant-specific contributors to the overall risk will be identified through the systematic use of a probabilistic safety assessment. Features should be included to reduce the likelihood and limit the consequences of severe accidents. The performance of the containment under severe- accident conditions should be systematically addressed in the design. Accident-management measures will be identified in the design phase and will continue to play an important role in the resolution of severe-accident issues.
Offsite emergency response should continue to be regarded as an important additional level of public protection. Because future plants will be designed to limit offsite releases further, it may be possible in some cases to reduce the scope of emergency preparedness compared with current requirements.
The understanding of severe-accident phenomena has progressed substantially since the Three Mile Island accident. International co-operation and research programmes have made an important contribution to the resolution of severe-accident issues. Further research will contribute to reducing uncertainties and maintaining technical expertise.