Beyond Design Basis Accidents - doe-std-3009-94_cn3

DOE-STD -3009-94

3 . 4 . 3 Beyond Design Basis Accidents

The Rule requires consideration of the need for analysis of accidents which may

be beyond the design basis of the facility to provide a perspective of the residual

risk associated with the operation of the facility The beyond DBAs serve as bases

for cost-benefit considerations if consequences exceeding the Evaluation

Guideline are identified in the beyond DBA range. However, such cost-benefit

analysis would be performed outside the DSA with the concurrence of DOE.

It is expected that beyond DBAs will not be analyzed to the same level of detail

as DBAs. The requirement is that an evaluation be performed that simply

provides insight into the magnitude of consequences of beyond DBAs (i.e.,

provide perspective on potential facility vulnerabilities). This insight from

beyond DBA analysis has the potential for identifying additional facility features

that could prevent or reduce severe beyond DBA consequences. For nonreactor

nuclear facilities, however, the sharp increase in consequences from DBA to

beyond DBA is not anticipated to approach that found in commercial reactors

where the beyond DBA precedent was generated. No lower limit of frequency

for examination is provided for beyond DBAs whose definition is frequency

dependent. It is understood that as frequencies become very low, little or no

meaningful insight is attained.

Operational beyond DBAs are simply those operational accidents with more

severe conditions or equipment failures than are estimated for the corresponding

DBA. For example, if a deterministic DBA assumed releases were filtered

because accident phenomenology did not damage filters, the same accident with

loss of filtration is a beyond DBA. The same concept holds true for natural

events, but beyond DBAs are defined by the initiating frequency of the natural

event itself (i.e., frequency of occurrence less than DBA frequency of

occurrence). Beyond DBAs are not evaluated for man- made external events.

Page 54