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Page Title: Discussion of Contingencies - Continued
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In other cases, where criticality is possible, the reviewer's task is to assure that no credible single
failure can result in the potential for a criticality accident. To accomplish this, all credible failure
modes known to the reviewer that are applicable to the process should be bounded by the
analysis or adequate barriers to its occurrence must be in place. The reviewer should ask, "How
bad can the situation credibly get? If it gets that bad, will it remain subcritical?" and ensure that
the documented contingency analysis considers the scenario. This contingency discussion must
contain a clear description of the process upsets (i.e., contingencies) considered by the NCS
analyst. The contingencies should be specific enough to provide a definitive boundary to the
process upset. For example, rather than stating a mass contingency as "overbatch", state "double
batching" if this has been determined to be the maximum credible overbatch. The former means
any overbatch no matter how small would have to be an "unlikely" event. The latter concludes
that while a small overbatch would be an "anticipated" event, double-batching would be an
"unlikely" event. The reviewer need not document all conceivable abnormal pathways or
scenarios, only those deemed reasonable and credible.
The second role of the reviewer is to assure that controls (administrative and physical) are
adequate, properly justified and implemented appropriately. The reviewer must be satisfied that
the controls make the identified contingency an "unlikely" event. The controls must be
implementable by operations and there must be an implementing mechanism in place (i.e.,
procedures, configuration control, postings, etc.).
There are several pitfalls to be avoided as a reviewer. First, the reviewer should not be
concerned about quantitative human failure probabilities. Failure rates for human failures exist.
For example, see Alain Swain's handbook. Quantitative equipment failure frequencies are
generally subjective and based on professional judgment because there is no compilation of
failure rates for the types of processes most commonly occurring within DOE facilities.
However, if equipment failure data exists then it should be considered. The reviewer should
simply convince himself that, in his professional judgment, "unlikely events" are unlikely and
"incredible events" are incredible. Second, the format used to document the contingency
analysis is irrelevant - any kind will do as long as it is understandable. Text, charts, fault trees,
event trees and tables are all acceptable formats. Typically, the contingency analysis is
comprised of a simple spreadsheet showing the contingencies, controls, and summary of the
subcriticality evaluations. Third, just because a reviewer conceives a mechanistic scenario that is
a variation on those documented does not necessarily mean the evaluation is incomplete or
inadequate. If the barriers imposed on the operation cover the hypothesized variation, then the
analysis should be deemed adequate. The reviewer must decide when the variation becomes a
distinct scenario that was not adequately considered or, if missing, is indicative of an incomplete
analysis. The reviewer should use discretion when drawing the conclusion that the analysis may
be incomplete in other areas because a specific, bounded and controlled scenario is not explicitly
documented. The reviewer should document his conclusions and file them with the controlled
copy of the evaluation.

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