Need for Consideration of Beyond Design Basis Accidents

DOE-STD-3007-2007

Appendix A of DOE-STD-3009-94, Change Notice 3 provides guidance on unmitigated accident analysis.

Unmitigated analysis means that controls intended to prevent or mitigate an accident are assumed not to

function. In the case of a criticality event, this means, in part, that the accident is assumed to happen with

no mitigative features taken into account. An exception to this is that passive safety features that can be

shown to survive the initiating event may be considered in the analysis. For example, if a seismic event

causes a criticality accident in a shielded area, and the shield can be shown to survive the event, then the

effectiveness of the shield in mitigating worker accident doses can be accounted for in the analysis.

For accident analysis of criticality accidents that have the potential for lasting longer than an initial pulse,

the accident duration should be limited to two (2) hours, except for scenarios that are slow to develop and

complete. In those cases, the accident duration should be limited to eight (8) hours (based on ample time

for emergency response and the relative historical rarity of such extended accidents). These analyses

should be based on bounding scenarios. The dose integration time should be based on the specific power

history of the accident, limited as discussed above.

For the criticality accident analysis, DOE-HDBK-3010-94, Change Notice 1 provides estimates of fission

yields, and is a recognized source for accident analysis parameters. In addition, ANSI/ANS-8.23-1997

requires evaluation of bounding operational-specific accidents, including locations, yields, and dose

determinations. As with any accident parameter, sources for fission yields other than DOE-HDBK-3010-

94, Change Notice 1 may be used, providing they can be justified and shown to be applicable to the

accident situation.

B. NEED FOR CONSIDERATION OF BEYOND DESIGN BASIS ACCIDENTS

To satisfy the requirement in 10 CFR 830 Subpart B for consideration of the need for beyond design basis

criticality accidents, an acceptable approach is to examine the contingencies that were discussed in the

applicable CSEs but were not carried further based on their being not-credible. Collectively, the results

may indicate that the overall risk of a criticality accident in a facility needs further evaluation. The

purpose of the beyond design basis section is to inform the DOE that there may be accidents that were not

considered either collectively or individually in the normal CSE process. An example is that of a very

large earthquake that may result in a criticality accident with a larger maximum total number of fissions

than expected from a design basis earthquake. The individual CSEs would not address such an event in

the normal course of process analysis. If such a review shows that there are good and sufficient bases for