Time at risk: ALARP trade-offs over time

摘要

Current regulation in the UK and elsewhere specify upper and target risk limits for the operation of nuclear plant in terms of frequencies of various kinds of accidents and accidental releases per annum. 'As low as reasonably practicable' (ALARP) arguments are used to justify the acceptance or rejection of policy changes that lead to risk between these limits. Much of the ALARP methodology has been developed with the focus is on a single timeframe, usually a yearly risk. However, there is growing interest in risk-informed regulation in which policies extend over several timeframes and distribute the risk unevenly over these, or in policies that lead to a nonuniform risk within a single timeframe (such as maintenance policies). Not all regulators are happy to allow such departure from normal operating conditions. We discuss the problems associated with framing such decisions and supporting them with evidence from Probabilistic Risk Analyses (PRA's).