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magnitude of consequences of BDBAs has the potential for identifying additional facility features
that could prevent or reduce severe BDBA consequences.
A key issue relates to the severity and associated probability of the accidents that need to
be analyzed. There is no lower limit to a BDBA frequency specified in current DOE documents.
However, it is understood that as frequencies become very low, little or no meaningful insight
can be gained (DOE 1994). In terms of accident severity, the following guidance is applicable.
40 CFR 1502.22 gives some limited guidance on identifying BDBAs. These events have highly
catastrophic consequences, although there is a low frequency of occurrence. BDBAs must be
possible from a scientific viewpoint, not based on conjecture. DOE guidance in DOE Order
5500.3 (DOE 1992c) indicates that scenarios somewhat more severe than that considered in
the design basis should be used. DOE 1994 states that BDBAs can simply be DBA events with
more severe conditions or equipment failures than were in the DBA. For fusion facilities, this is
interpreted as design-basis scenarios in which the loss of active safety systems is assumed.
Another criterion, expressed in terms of the frequency is that internally initiated scenarios with
estimated frequencies of occurrence greater than 107/yr should be considered. Another option
of evaluating BDBAs is to evaluate maximum credible events determined by assuming one
additional system failure beyond the maximum design-basis events. BDBAs are not evaluated
for external events, as stated in DOE 1994. The BDBA analysis is to be performed using realis-
tic best-estimate assumptions.
After the completion of the safety analysis for the postulated events, the results of the
DBA assessment should be compared to the evaluation guidelines established in DOE-STD-
6002. The guidelines should include those associated with the protection of the public and the
environment. As a result of the comparison of the safety analysis results to the guidelines, the
events should be divided into the following groups: those that exceed the public safety evalua-
tion guidelines, those that result in a significant fraction of the public safety function evaluation
guidelines, and those that could affect the worker safety.
For the events that would exceed the public or environmental evaluation guidelines, any
SSC that is required to mitigate the consequences to meet the evaluation guidelines would be
classified as being safety-class. For those events that have a significant contribution but do not
exceed the public or environmental guidelines, any SSCs installed to minimize the conse-
quences or installed to provide defense-in-depth for the public safety functions would be classi-
fied as being safety-significant. For those items that could affect the worker safety, any SSCs
needed to prevent acute worker fatalities and serious injuries from other than standard industrial
hazards (see DOE-STD-3009-94) would also be classified as safety-significant.
5.4.3 Emergency Planning Basis Analysis
The Environmental Protection Agency (EPA) has developed requirements for protection
of the public during events involving a release of significant hazardous material. The require-
ments establish the Protective Action Guide limits under which protective action should be initi-
ated to protect the public. These requirements, established in EPA 1991, the event scenario
severity and assumptions, the method of performing the analysis, and the evaluation guidelines

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