scope:

Scope and Applicability for LPSD PRA

This standard establishes requirements for a Level 1 PRA of internal and external hazards for low power and shutdown modes. These modes include operating states ranging from those in which the plant is at power levels substantially below nominal full power (low power) to operating states in which the plant is shutdown with the reactor subcritical and the primary system depressurized and cooled sufficiently to be placed on Residual Heat Removal (RHR) cooling. This standard bases these requirements in large part on ASME/ANS RA-Sa-2009 [1]², which at present is restricted to requirements for plant operating states atpower³.

This standard has been developed to specify the requirements for an evaluation of risk during LPSD conditions. Depending on the application, these evaluations are likely to focus on two end states: (a) a full Level 1 analysis of the core damage frequency (CDF); and (b) a limited Level 2 analysis sufficient to evaluate the large early release frequency (LERF). This is consistent with Reference [1]. The emphasis for the Level 2 analysis during shutdown conditions is more on containment isolation failure than containment structural failure for many applications since there may be LPSD plant operating states where the containment has an equipment hatch removed or other large openings to permit maintenance activities. Also, the dissipated decay heat during shutdown results in a reduced source term when compared to full power (although the radiological impact of the source term decreases at a slower rate than the decay heat does). Thus, while the definition of LERF is the same as for full power states, the determination of "large" releases must include additional considerations to those for full power.

It is recognized that alternative risk metrics to CDF and LERF have been used in many assessments of LPSD conditions. These alternative risk metrics include approaching coolant boiling in the core, depletion of coolant to the point of core uncovery, and radioactivity release. For consistency with the requirements of this standard, the alternate metric(s) are to be computed as a frequency of an accident class, and the selected metric(s) are to be surrogates for both CDF and LERF. Note that one alternative metric may, by itself, be a surrogate for both CDF and LERF. The alternate metric(s) are to be justified in the context of an application, against which the scope of the PRA is to be compared against all requirements. Any deviations from the requirements herein are to be justified for the alternate metric(s). Non-mandatory Appendix 2-B discusses risk metrics further.

Furthermore, the scope of the requirements for LPSD PRA in this standard excludes sources of radioactive material other than nuclear fuel within the reactor vessel; i.e., only "core" damage accidents are considered. Thus, accidents involving nuclear fuel in the spent fuel pool, in dry storage, or in transit are excluded from the scope of this standard. Accidents involving radioactive material sources other than nuclear fuel are also excluded.

This standard's technical requirements are presented in a manner to support a range of applications including the calculation of time-averaged4 CDF or LERF, consistent with Reference [1]. Additionally, alternative risk metrics may be used. This standard may also be used for LPSD PRA applications involving a specific outage. Modifications to the LPSD PRA technical requirements to support timedependent risk metrics of a specific LPSD evolution are noted in Section 1.3.7.

The only initiating event hazards explicitly excluded from the LPSD PRA scope are accidents resulting from purposeful human-induced security breaches (e.g., sabotage) and accidents initiated by internal fires.

This standard applies to PRAs used to support applications of risk-informed decision-making related to operating power plants⁵. They may be used for plants under design or construction, for advanced LWRs, or for other reactor designs, but revised or additional requirements may then be needed.

² Numbers in brackets refer to corresponding numbers in Section 12, "References."

³ Reference [1] is defined to apply broadly to "at-power" plant states. The LPSD PRA Standard includes "low power" states, which could be considered part of the broad "at-power" definition. To avoid confusion, this standard defines (and applies to) "low power" states in contrast to "full power" or "nominal-full-power " states. The "low power" states are defined to include all at-power operations below nominal full power. See the definitions of these terms in Section 1.2.2.

⁴ "Time-averaged" risk metrics refer to the occurrence of low power and outage evolutions averaged over time, in contrast to risk metrics applied to a specific outage (e.g., refueling number 4). All PRAs have other elements averaged over time, such as initiating event frequencies and component failure rates.

⁵ Here, "operating power plants" means plants that have operated and, thus, have spent fuel and decay heat. An operating plant could be "operating" at-power or in shutdown.