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A Figure of Merit: Quantifying the Probability of a Nuclear Reactor Accident

In recent decades, probabilistic risk assessment (PRA) has become an essential tool in risk analysis and management in many industries and government agencies. The origins of PRA date to the 1975 publication of the U.S. Nuclear Regulatory Commission’s (NRC) Reactor Safety Study led by MIT professor Norman Rasmussen. The “Rasmussen Report” inspired considerable political and scholarly disputes over the motives behind it and the value of its methods and numerical estimates of risk. The Report’s controversies have overshadowed the deeper technical origins of risk assessment. Nuclear experts had long sought to express risk in a “figure of merit” to verify the safety of weapons and, later, civilian reactors. By the 1970s, technical advances in PRA gave the methodology the potential to serve political ends, too. The Report, it was hoped, would prove nuclear power’s safety to a growing chorus of critics. Subsequent attacks on the Report’s methods and numerical estimates damaged the NRC’s credibility. PRA’s fortunes revived when the 1979 Three Mile Island accident demonstrated PRA’s potential for improving the safety of nuclear power and other technical systems. Nevertheless, the Report’s controversies endure in mistrust of PRA and its experts.