Evaluation of Design Robustness of Nuclear Installations Against External Hazards
Among the lessons learned from the accident at the Fukushima Daiichi nuclear power plant were the importance of (a) nuclear installation design margins against external natural hazards exceeding those selected for the design basis, (b) the adequacy of design margins to avoid cliff edge effects and (c) independence of different levels of defence in depth. IAEA Safety Standards Series No. SSR-2/1 (Rev. 1), Safety of Nuclear Power Plants: Design, established specific safety requirements for a robust nuclear power plant design based on those lessons. The present publication provides information to support compliance with the requirements established in SSR-2/1 (Rev. 1) and IAEA Safety Standards Series Nos SSR-3, Safety of Research Reactors, and SSR-4, Safety of Nuclear Fuel Cycle Facilities.
Design robustness against external hazards can be expressed in terms of adequacy of design margins against external events exceeding the design basis. The margin assessment is performed at the nuclear installation level to determine its capability to perform the intended fundamental safety functions. As the margin assessment needs to be performed within the defence in depth framework, the margin adequacy requirement may be different at each level of defence. Failure events that involve cliff edge effects require special consideration in assessing their seismic margin.
This publication provides information on evaluating the adequacy of design safety margins against external events on the basis of the performance goals applicable to a nuclear installation. Performance goals are classified into scenario based and annual frequency based objectives. An approach is first developed for characterizing the design margin of a nuclear power plant against seismic hazard and determining the adequate margin above the design basis seismic hazard. This approach is then generalized to other external hazards. An approach is provided to identify cliff edge failure modes, which are classified into classic and non-classic cliff edge failures. The development of criteria to assess the adequacy of safety margins for seismically induced cliff edge effects is presented. This approach is generalized to other hazards. Information is provided on considerations concerning decisions on design improvements, multi-unit sites and application to nuclear installations other than plants through a graded approach.
Publisher
International Atomic Energy Agency