ASME/ANS RA-S-1.4

Description / Abstract: This standard establishes requirements for a PRA for advanced non-LWR NPPs. The requirements in this standard were developed for a broad range of PRA scopes that may include the following:

(a) Different sources of radioactive material both within and outside the reactor core but within the boundaries of the plant whose risks are to be determined in the PRA scope selected by the user. The technical requirements in this trial-use version of the standard are limited to sources of radioactive material within the reactor coolant system (RCS) pressure boundary (RCPB).¹ Technical requirements for other sources of radioactive material such as the spent fuel system are deferred to future editions of this standard;

(b) Different plant operating states (POSs) including various levels of power operation and shutdown modes;

(c) Initiating events caused by internal hazards, such as internal events, internal fires, and internal floods, and external hazards such as seismic events, high winds, and external flooding;

(d) Different event sequence end states, including core or plant damage states (PDSs), and release categories that are sufficient to characterize mechanistic source terms, including releases from event sequences involving two or more reactor units or modules for PRAs on multireactor or multiunit plants;

(e) Evaluation of different risk metrics including the frequencies of modeled core and PDSs, release categories, risks of off-site radiological exposures and health effects, and the integrated risk of the multiunit plant if that is within the selected PRA scope. The risk metrics supported by this standard are established metrics used in existing light water reactor (LWR) Level 3 PRAs such as frequency of radiological consequences (e.g., dose, health effects) that are inherently technology neutral. Surrogate risk metrics used in LWR PRAs such as core damage frequency and large early release frequency are not used as they may not be applicable to non-LWR PRAs;

(f) Quantification of the event sequence frequencies, mechanistic source terms, off-site radiological consequences, risk metrics, and associated uncertainties, and using this information in a manner consistent with the scope and applications PRA.

It is recognized that for some PRA applications, a full-scope PRA (i.e., a PRA that addresses the full set of requirements covered in this standard) may not be required. In addition, for PRAs performed in various stages of design and licensing, especially those PRAs performed prior to selection of a specific site, the level of detail and completeness of the PRA with respect to the PRA scope, coverage of events and event sequences, POSs, hazards, risk metrics, and operational characteristics, may be limited in relation to that for typical PRAs for an operating plant. Hence, the scope of the PRA is established by the user in accordance with the intended PRA applications and the availability of information to support each PRA element. In addition, the requirements in this standard for the level of detail, completeness, and model to plant or design fidelity vary according to the scope and level of detail of design and operational information that is available to support, and is referenced by, the PRA with additional requirements to address assumptions in lieu of as-operated and as-built details.

The only hazards explicitly excluded from the scope of this standard are accidents resulting from purposeful human-induced security threats (e.g., sabotage, terrorism) and risks associated with accidental radiological exposures to on-site personnel. This standard applies to PRAs used to support applications of risk-informed decision making related to design, licensing, procurement, construction, operation, and maintenance of advanced non-LWR NPPs.

This standard includes requirements for addressing the off-site radiological consequences of accidents involving release of radioactive material from the plant. The consequence metrics are common to those found in LWR Level 2 and Level 3 PRAs. The scope of these requirements excludes the consideration of radiological or other consequences to on-site personnel.

Treatment of Hazard Groups

This PRA standard provides specific requirements for the following hazard groups:

(a) internal events;

(b) internal floods;

(c) internal fires;

(d) seismic events;

(e) high winds;

(f) external floods;

(g) other internal and external hazards.

In addition to providing technical requirements for PRAs whose scope includes a range of hazards, this standard provides requirements for screening and conservative analyses of internal and external hazards.

Many of the technical requirements for the internal events hazard group are fundamental requirements for performing a PRA for any hazard group and are therefore relevant to the remaining hazard groups in this standard. They are incorporated by reference to the supporting internal events requirements. This is a reflection of the fact that a fundamental understanding of the plant response to a reasonably complete set of initiating events as defined in the internal events PRA model provides the foundation for modeling the impact of various hazards on the plant. Hence, requirements defined for the internal events hazard group are applicable to all the hazard groups within the scope of the PRA.

Hazards and Initiating Events

The relationship among hazard, hazard event, hazard group, and initiating event, which are defined in Sec. 2.2, is often misunderstood, in part because PRA guidelines and standards issued by different entities use these terms differently. “Hazard group” refers to collections of similar hazards that are assessed in the PRA using a common approach, common methods, and common data. “Hazard” is the specific conceptual phenomenon that puts the plant at risk. So, a hazard group may consist of only a single hazard (e.g., internal fires or seismic events) or multiple hazards (e.g., internal events group, which includes transients and RCPB breaches; high winds, which include hurricanes, tornadoes, and straight winds). In this context, hazard is the phenomenon; hazard event is an occurrence of the phenomenon that can result in a plant trip and possibly other damage when the plant is at power or result in the loss of a key safety function during nonpower operations, and the initiating event is the specific plant perturbation that challenges plant control and safety systems.

In general, there is a range of hazard events associated with any given hazard, and for analysis purposes, the range can be divided into bins characterized by their severity. Hazard events of different severity can result in different initiating events. An example of the overall concept is as follows:

• Earthquakes are a hazard;

• 0.1g, 0.3g, 0.5g earthquakes and their associated spectral shapes and time histories may be defined as hazard events (g = gravitational acceleration);

• A manual plant trip is typically the initiating event for the 0.1g earthquake, and a loss of off-site power is typically assumed as the initiating event for the 0.3g and 0.5g earthquakes.

As another example, for internal events,

• RCPB breach may be identified as a generic type of hazard;

• The specific hazard events would be RCPB breaches of different sizes and locations, etc.;

• Small RCPB breach leading to plant trip on low RCS pressure would be the initiating event for the small RCPB breach hazard event.

¹ For pool-type reactors with no RCPB, the scope includes sources within the RCS.