Scenario Definition

DOE-STD -3009-94

Appendix A

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

that estimates the potential magnitude of the radiological release. The result of the

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

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