conducting weapon performance assessments, and for carrying out the nuclear explosives safety process.

Some of the characteristics of PRA that might enhance the QMU process are (1) explicitness of event sequences (scenarios) leading to degraded performance, (2) ranking of contributors to nonperformance, (3) the probability of frequency concept for presenting results (see earlier discussion), (4) increased emphasis on evidence-based distribution functions (as opposed to assumed distributions such as Gaussian), and (5) the actual quantification of the risk of degraded performance.

As suggested above, the PRA thought process could very well be the primary vehicle for quantifying the safety and security risk of nuclear weapon systems and of other steps in the nuclear weapon functional life cycle such as the stockpile-to-target sequence and the issue of the aging stockpile and its effect on performance. The PRA framework is compatible with tracking multiple performance measures including safety, military compatibility, and logistics.

One final thought about how PRA might enhance the QMU process has to do with the changing of management mindsets about performance metrics. PRA has altered the thinking of nuclear power plant management about the importance of having multiple metrics for measuring risk and performance of complex systems. Maybe the weapons community has to do the same thing with its leadership. A single number for weapons reliability is not a confidence builder in understanding the performance characteristics of something as complicated as a nuclear weapon, where there is a need to expose the uncertainties in the reliability predictions.


Garrick, B.J. 2008. Quantifying and Controlling Catastrophic Risks, Elsevier Press.

Jaynes, E.T. 2003. Probability Theory; The Logic of Science, Cambridge University Press.

Kaplan, S., and B.J. Garrick. 1981. On the Quantitative Definition of Risk, Risk Analysis 1(1): 11-27.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement