Accident Analysis cont'd

DOE-STD -3009-94

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

the facility).

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.

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