frequency and consequence into such broad categories is more of a qualitative
than a quantitative exercise. This effort does not constitute the need for, or
expectation of, a probabilistic / quantitative risk assessment.
An important factor in estimating binning thresholds for public consequences
is to tie the thresholds to the Evaluation Guideline so that accidents that could
challenge the guideline are correctly identified for formal accident analysis.
The binning requirement of this subsection does not preclude the use of other
sorting mechanisms in addition to risk sorting if an analyst finds such
This accident selection activity identifies the process and criteria used to select
the unique and representative potential accidents (i.e., DBAs) to be included in
accident analysis. Unique accidents are those with sufficiently high-risk
estimates that individual examination is needed (e.g., a single fire whose
specific parameters result in approaching the Evaluation Guideline, situations
of major concern from Figure 3-2). Representative acc idents bound a number
of similar accidents of lesser risk (e.g., the worst fire for a number of similar
fires, situations of concern in Figure 3-2). Representative accidents are
examined to the extent they are not bounded by unique accidents. In any case,
at least one bounding accident from each of the major types determined from
the hazard analysis (e.g., fire, explosion, spill, etc.) should be selected unless
the bounding consequences are "Low" (See Figure 3-2). Accidents are
identified and listed by accident category (i.e., internally and externally
initiated) and type (e.g., fire, explosion, spill, etc.).
Since the hazard analysis activity is considered sufficient for Hazard Category
3 facilities, DSAs for these facilities need simply summarize the ma ximum
consequences expected from facility operation and state that detailed accident
quantification is not necessary because potential consequences are well below
the Evaluation Guideline. A possible exception to this case, as previously
noted, is a facility with Hazard Category 3 quantities of radionuclides but
possessing large amounts of toxic chemicals. Such facilities need to
summarize the maximum radiological consequences expected and identify the
chemical accidents selected for accident analysis.
This section presents the formal development of the potential accidents identified
in Section 220.127.116.11.5, "Accident Selection," beginning with a formal sequence of
developing connecting initiating events to preventive feature and mitigative feature
responses. A basic flowsheet for accident analysis is presented in Figure 3-4. The
principal purpose of the accident analysis is to identify any safety-class SSCs,
SACs and TSRs needed for protection of the public.
Each accident sequence needs to be analyzed through the use of a documented,
deterministic, DBA. Whenever possible, DBAs are analyzed using the simplest
applicable deterministic, phenomenological calculations (e.g. pressure estimates
from a simple ideal gas law calculation, hand calculated Gaussian plume