Analysis Approach for the Safety Analysis Report

DOE-STD-6003-96

5.4.2 Analysis Approach for the Safety Analysis Report

Because there is no previously identified design basis for large fusion facilities like the

International Thermonuclear Experimental Reactor (ITER), a subset of the event scenarios

identified in Section 5.4.1 needs be selected to form the design basis and to undergo detailed

quantitative analysis as part of the SAR. There is varying information that can be used to

develop an appropriate criteria to be used in this selection. DOE Order 420.1 (DOE 1995a) and

DOE-STD-3009 (DOE 1994) indicate that events down to ~106/yr should be considered. Many

advanced fission plants are also considering similar criteria. It is recommended that for fusion

facilities, internally initiated event sequences down to ~106/yr be used. For external events,

guidance given in DOE Order 420.1 (DOE 1995a) should be consulted.

Two different types of calculations should be performed: best estimate and conservative.

Conservative calculations should be performed for those events identified as part of the design

basis. As part of the calculation, all key assumptions need to be stated and the level of conser-

vatism noted (e.g., 110% nominal power). The results of these conservative calculations are

then compared to the evaluation guidelines (discussed in DOE-STD-6002) to determine classifi-

cation of safety systems (see Section 5.1). The best-estimate calculations are then performed

so that the degree of conservatism or the safety margin in the facility can be established.

The deterministic approach used for evaluating the safety of the facility design basis pro-

vides a conservative approach for assessing the safety of the facility by using bounding esti-

mates of the releases from the postulated off-normal conditions, bounding estimates for the

release fractions, and bounding estimates for the transport through the environment. This

approach is designed to result in a bounding estimate of the safety consequences from the

postulated events.

Guidance on the conservative release assumptions to be used in the design-basis

accident (DBA) analysis is available from several safety analysis reference sources. One of

these sources is a report published in the late 1980s, Elder 1986. This report was published to

provide guidance for assessing the radiological consideration for siting and the design of DOE

nonreactor nuclear facilities. Some of the information may be dated, but in general it provides

useful guidance for the assumptions and release fractions that are appropriate for deterministic

analysis methodology. Guidance for the selection of conservative assumptions to be used in

assessing the transport of the release to the receptor can be found in Nuclear Regulatory

Commission (NRC) 1974 or NRC 1983. Guidance for documenting the analysis methodology

used in assessing the consequences is provided in DOE 1994.

To assess the residual risk associated with the operation of a facility and to provide per-

spective on possible facility vulnerabilities, an evaluation of beyond-design-basis accidents

(BDBAs) should be performed per DOE 1992c. Such BDBAs evaluations are not required to

provide assurance of the public health and safety. These results are to serve as basis for eval-

uating the completeness of the events identified in the DBAs and to ensure that there is no sig-

nificant threshold increase in the facility risk. For a well-designed facility, there should be no

sharp increase in consequences when moving from DBA to BDBA scenarios. It is expected that

the BDBAs would not be analyzed to the same level of detail as the DBAs. The insight into the