Evaluation of Quantification of Margins and Uncertainties Methodology for Assessing and Certifying the Reliability of the Nuclear Stockpile (2009) / Chapter Skim
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Appendix A: A Probabilistic Risk Assessment Perspective of QMU
Appendix A: A Probabilistic Risk Assessment Perspective of QMU
Pages 51-66
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From page 51... ...
The quantification of margins and uncertainties (QMU) methodology refers to the methods and data used by the national security laboratories to predict nuclear weapons performance, including reliability, safety, and security.
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From page 52... ...
constitutes the "Theory of Scenario Structuring." Quantifying the L i and the Xi is based on the available evidence using Bayes' theorem, illustrated later. In accordance with this set of triplets definition of risk, the actual quantification of risk consists of answering the following three questions: 1.
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From page 53... ...
an initiating event and initial condition assessment that quantifies the threats to the system, and (3) a vulnerability assessment that quantifies the resulting risk scenarios and different consequences or damage states of the system, given the possible threats to the system.
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The third part of the process structures the course and Figure A-1 consequence of events (scenarios) that could emanate from specific initiat Bitmapped, low-res ing events or initial conditions.
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This theorem has a long and bitterly controversial history but in recent years has become widely understood and accepted. A central feature of probabilistic risk assessment is making uncer
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A deductive logic model -- that is, a fault tree or master logic diagram -- is developed for each initiating event of a screened set. The structure of the logic model is to deduce from the "top events" -- that is, the selected set of hypothetical IEs or ICs -- the intervening events down to the point of "basic events." A "basic event" can be thought of as the initial input point for a deductive logic model of the failure paths of a system.
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From page 57... ...
The actual quantification of the risk scenarios is done with the aid of event trees similar to the one in Figure A-2. An event tree is a diagram that traces the response of a system to an initiating event, such as a terrorist attack, to different possible end points or outcomes (consequences)
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From page 58... ...
Once the scenarios have been quantified, the results take the form shown in Figure A-4. Each scenario has a probability-of-frequency curve in the form of a probability density function quantifying its likelihood of occurrence.
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Showing different levels of damage, such as the risk of varying numbers of injuries or fatalities, requires a different type of presentation. The most common form is the classical risk curve, also known as the frequency-of-exceedance curve or the even more esoteric label, the complementary cumulative distribution function.
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Often the most important output is the exposure bitmapped art and interior type of the detailed causes of the risks, a critical result needed for effective risk management. The contributors to this risk are buried in the results assembled to generate the curves in Figures A-6 and A-7.
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From page 61... ...
In fact, because of the advanced state of development of the laboratory codes for calculating confidence ratios of performance margins and uncertainties, the most prudent use of the PRA thought process is probably for safety and security issues and elements such as the impact on weapon performance of stockpile storage or other events associated with the stockpile-to-target sequence. In fact, these elements could be the major contributors to the risk of poor weapon performance.
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From page 62... ...
(More information on this topic is included in Note 11 in the classified Annex.) In particular, an initiating event is defined as the frequency per mission hour of an external radiation pulse of sufficient energy to impact weapon performance.
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From page 63... ...
Having a POF distribution for each of these scenarios sets the stage for developing risk curves for a particular initiating event. The outputs of the event tree are calculated as described in the section "The PRA Approach to Quantification." The results from the event tree can be assembled into several different forms.
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From page 64... ...
To be sure, many nuclear explosive safety activities go beyond the nuclear explosive package of nuclear warheads and some data needs of the nuclear explosive package are unique, thus limiting data collection opportunities. Nevertheless, it would appear that opportunities exist for large-scale data collection and processing in the weapons field.
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From page 65... ...
Experience with PRA strongly supports the view that the information and knowledge base created in the course of performing the PRA could contribute to the credibility of the QMU process. Almost every phase of nuclear power plant operation has been favorably affected by PRAs, from maintenance to operating procedures, from outage planning to plant capacity factors, from sound operating practices to recovery and emergency response, and from plant simulation to operator training.
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From page 66... ...
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.
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