Scenario Development

DOE-STD -3009-94

way, facilities should be analyzed as they e xist when quantifying meaningful

release mechanisms.

Note: The following format is repeated sequentially for each ("X") DBA.

3.4.2.X

[Applicable DBA]

Identify the DBA by individual title, category (i.e., operational, natural, man-

made external) and general type (e.g., fire, explosion, spill, earthquake,

tornado).

3.4.2.X.1

Scenario Development

This subsection describes accident progression linking initiating events with

preventive and mitigative events and other contributing phenomena to formally

define the accidents identified in Section 3.3.2.3.5, "Accident Selection." Note

each response, action, or indication required to initiate action that is relevant to

the scenario progression. Document the rationale used in hazard analysis for

binning the DBA in a broad frequency range.

When summarizing the initiating event for a given natural event DBA, use

DOE 420.1 and its applicable standards (i.e., DOE-STD-1020 through -1024)

to determine the natural event DBAs for the facility. Design basis guidelines

include, among others, load factors, return periods, amplification factors for

the facility, etc. Summarize facility and equipment response (emphasizing

preventive or mitigative equipment) to the loads postulated to be present at the

time the given natural event occurs. Reference the facility documentation of

this evaluation and summarize relevant assumptions. Discuss the degree of

conservatism of the evaluation.

Evaluate secondary events directly caused by natural events, such as

earthquake induced fires, based on their physical possibility for facility

conditions (i.e., the induced accident must already potentially exist in the

absence of the seismic event). For example, seismic induced fires should be

considered DBAs where significant accumulations of flammab le material are

exposed to fire initiators by seismic damage to the facility. If minimal

combustible material is present in a given location, a large seismic induced fire

in that location would not be a DBA as the potential is not physically possible.

Alt hough external events are not typically design bases, this Standard

considers them as DBAs if the frequency of occurrence is estimated to exceed

10-6/yr conservatively calculated, or 10-7/yr realistically calculated. The

specific use of this NRC frequency precedent is limited to external events only

due to their unique nature. External events are presented because frequency

criteria for inclusion are met. Accordingly, the analysis that substantiates

frequency need only be referenced.

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