time would also be available for implementing preventive and mitigative measures.
Therefore, the emphasis on SC SSCs in terms of immediate availability and operation is
not generally necessary for safety SSCs associated with these pathways.
A.3.1 Scenario Definition
The concept of a DBA has historically been applied in the nuclear industry for
deterministic evaluation of potentially high consequence accidents (primarily for
nuclear power plants). The DBA analyses encompass evaluations of the need for
and the adequacy of those important SSCs whose failure could have an adverse
impact on the public (i.e., SC SSCs). For may DOE facilities, due largely to their
age and the absence of safety documentation, the original design bases for their
SSCs, including safety-related features, are severely lacking or nonexistent. In
recognition of this deficiency, the standard requires the development and analysis
of derivative DBAs (which for simplicity were also referred to as DBAs in the
body of the standard) for the existing facilities in lieu of actual DBAs. The
primary purpose of the DBA analysis is to identify SSCs that warrant SC
designation. In doing so, the concept of "unmitigated release" was developed to
conservatively estimate the consequence potential from the candidate DBAs that
are selected from the hazard analysis without taking credit for any safety features.
Note that the standard already requires that unmitigated consequences be estimated
as part of a hazard analysis, though largely in a qualitative manner. Thus, the
unmitigated release calculation is a critical step in the DBA formulation process
calculation is compared to the EG to (1) determine if any SC SSC is required and
(2) provide insight for selecting the appropriate SC SSC(s) for each DBA scenario.
For existing DOE non-reactor nuclear facilities, some safety systems may already
be known and designated a s such (e.g., fire protection systems and confinement
systems, which include HEPA filtration). Some SC designations for such safety
system may also be self-evident. Nevertheless, it is necessary to provide the basis
for such designation, and this Appendix provides the guidance for the analysis and
documentation required. In some cases it has been found that these analyses
provide useful insight into subtle safety issues.
U N M I T I G A T E D R E L E A S E C A L C U L A T I O N . The unmitigated
release calculation represents a t heoretical limit to scenario consequences
assuming that all safety features have failed, so that the physical release
potential of a given process or operation is conservatively estimated. The
unmitigated release should characterize both the energies driving the release,
and the release fractions in accordance with the physical realities of the
accident phenomena at a given facility or process. As a result, there may be
assumptions that are necessary to make in order to define a meaningful
scenario, but which also impact the magnitude of the resultant consequences.
In order to clearly capture these assumptions, and their resulting potential
impact on safety SSC designation and/or Technical Safety Requirements
(TSR) protection, the unmitigated calculation should:
(1) Take no credit for active safety features such as ventilation filtration