way, facilities should be analyzed as they e xist when quantifying meaningful
Note: The following format is repeated sequentially for each ("X") DBA.
Identify the DBA by individual title, category (i.e., operational, natural, man-
made external) and general type (e.g., fire, explosion, spill, earthquake,
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 220.127.116.11.5, "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.