dispersions). The nondeterministic aspects of DBA analysis are simplified by
estimating overall sequence frequencies in broad frequency ranges in hazard
analysis. This process is considered sufficient for DSA purposes and accident
analysis need only document the basis for the binning performed in hazard
analysis. Detailed probabilistic calculations are neither expected nor required.
Natural events and man-made external events are special cases. Natural event
DBAs are those events with a phenomenon initiating frequency as specified in
DOE 420.1 and its applicable standards. External events are not typically design
bases for facilities. However, they will be referred to as DBAs and analyzed as
such if frequency of occurrence is estimated to exceed 10-6/yr conservatively
calculated, or 10-7/yr realistically calculated.
Accident analysis typically starts with formal descriptions of accident scenarios.
Basic event trees may support such descriptions. All major assumptions in
scenarios must be identified. The next step is determination of accident source
terms. Source terms for accidents are obtained through phenomenological and
system response calculations. Once a source term has been determined,
consequences due to atmospheric d ispersion or other relevant pathways of
concern are determined. As with every phase of the analysis, the effort expended
is a function of the estimated consequence. If the source term is small, a simple,
dispersion hand calculation for consequences would be sufficient. If source
terms are large, computer modeling to determine consequences may be required.
The consequences finally determined are compared to the Evaluation Guideline
(see Appendix A). From this activity, it is determined if safety-class SSC
designation is needed. The need for accident specific TSRs to meet the
Evaluation Guideline will also be determined. Detailed description of safety-
class SSCs, SACs and TSRs are presented in Chapter 4, "Safety Structures,
Systems, and Components," and Chapter 5, "Derivation of Technical Safety
Requirements." The nature of the accidents to be analyzed will vary depending
upon the facility and processes considered. However, it is anticipated that for
most facilities or processes, the number of accident s requiring formal analysis
will not be large. The categories of DBAs examined are:
Operational accidents (caused by initiators internal to the facility).
Natural events (e.g., earthquakes, tornadoes).
Man- made external events (caused by man- made initiators external to
All assumptions made in the accident analysis (i.e., defining points in scenario
progression) are to be validated as part of the accident analysis activity.