Fire PRA Component Selection (Task 2)

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Task Overview

Objective

The selection of components that are to be credited for plant shutdown following a fire is a critical step in any Fire PRA. Components selected would generally include components credited in the 10 CFR 50 Appendix R post-fire safe shutdown (SSD) analysis. Additional components will likely be selected, potentially including components credited in the plant's internal events PRA. Also, the proposed methodology would likely introduce components beyond either the 10 CFR 50 Appendix R list or the internal events PRA model. Such components are often of interest due to considerations of combined spurious operations that may threaten the credited functions and components.

Purpose

This section provides the procedure for creating the Fire PRA Component List. This list serves as the basis for those components modeled in the Fire PRA, and it is the key source of information for which corresponding cables need to be identified and located for the Fire PRA. As such, the Fire PRA Component List, Fire PRA Model, and corresponding cable identification are iterated upon to ensure an appropriate correspondence among these three items. The product of this task is a list of the equipment to be included in the Fire PRA and for which corresponding cables need to be identified and located for the nuclear power plant under analysis.

Scope

This procedure addresses creating the Fire PRA Component List, which needs to span (a) equipment that, if affected by a fire, will cause an initiating event such that the appropriate fire-induced initiators can be defined; (b) all equipment necessary to support those mitigating functions and operator actions that are credited in the analysis in response to any initiator, as well as (c) that equipment which can be a source of undesirable responses adverse to safety during a fire-induced accident sequence, such as a component that can spuriously operate . The terms “equipment” or “components” as used in this procedure are considered synonymous and meant to include plant components such as valves, fans, pumps, etc.; structures; barriers; indicators; alarms; and other devices as appropriate. It is recommended that all the equipment credited in the Internal Events PRA (especially equipment in electrically diverse systems) be included in the Fire PRA Component List. More specifically, the scope of the Fire PRA Component List should include the following major categories of equipment:

  • Consideration of equipment whose fire-induced failure will cause an initiating event to be modeled in the Fire PRA Model (in this case, the appropriate initiator for a compartment needs to be defined, not that the equipment itself has to be modeled);
  • Equipment to support the success of mitigating safety functions credited in the Fire PRA, including equipment implicitly included in Internal Events PRA recovery models;
  • Equipment to support the success of operator actions credited in the Fire PRA;
  • Equipment whose spurious operation or other fire-induced failure modes could have an adverse effect on the success of the mitigating safety functions credited in the Fire PRA; and
  • Equipment whose spurious operation or other fire-induced failure modes could likely induce inappropriate or otherwise unsafe actions by the plant operators during a fire damage sequence.

In many cases, the same equipment might be in several of the five major categories. For example, the reactor vessel safety relief valves (SRVs) in a boiling water reactor (BWR) may be credited to open in the analysis for emergency depressurization purposes. However, should the SRVs subsequently close due to a fire-induced failure mode or open when not desirable, they can be a source of an undesirable response during a fire and may cause a plant trip. For these reasons, the SRVs would be on the Fire PRA Component List. Sometimes the equipment may only relate to one or two categories. For example, the residual heat removal (RHR) high-low pressure interfacing valves in a pressurized water reactor (PWR) may not be needed to perform a safety function if the shutdown cooling mode is not credited in the fire analysis. However, fire-induced opening of these valves, if they remain powered, could cause an interfacing-systems LOCA (initiator) and possibly cause environmental-related failure of other systems that are credited in the analysis. For this reason, and because such a failure could potentially lead to a high consequence event involving both core damage and containment bypass, the RHR interfacing valves would be on the Fire PRA Component List.

Similarly, a limited set of mitigating equipment, as well as instrumentation and diagnostic equipment such as indicators, lights, alarms, and similar devices considered necessary to support successful operator actions (e.g., such as carrying out the Emergency Operating Procedures (EOPs), following specific Fire Emergency Procedures (FEPs), or to credit certain recovery actions), or the failure of which could cause inappropriate operator actions, should also be added to the Fire PRA Component List. Examples could be remote shutdown panel (or areas) equipment and controls, pump room high temperature alarms, or certain plant parameter indications with no or little redundancy in the indication, among others.

Because a key emphasis of the Fire PRA Component List is to identify and track relevant cables in Task 3 that could be affected by fires in the plant, the list need not contain passive/mechanical equipment (i.e., non-electrical components) deemed by the analyst to be unaffected by fires. Such equipment may be manual valves, check valves, filters, heat exchangers, tanks, etc. (However, note that temperature, level, or other indications associated with this equipment may need to be on the list for operator action purposes). It is recommended that as part of this procedure, the analyst has identified those types of passive/mechanical equipment that do not need to be on the Fire PRA Component List, even though the equipment may be in the Fire PRA Model with regard to other mechanical failures, such as random plugging. The plant’s existing fire analyses or the internal flooding PRA will typically have a similar list of component types not considered affected by fires or flooding, and should be good starting points for creating a list of components not vulnerable to fire. In considering components that should not be affected by a fire, any potential damage to valve packing and other valve internals, filter materials, etc., should not be possible or at least not prevent the equipment’s operation, should it be necessary. As part of identifying whether non-electrical equipment is or is not vulnerable to fire effects, the analyst should also be sensitive to identifying such situations as instrument air piping/tubing that is copper or has soldered joints that may fail under high heat conditions and thus fail the instrument air function. In such cases, the PRA model needs to reflect these possible non-electrical equipment failures for applicable compartment fires.

Related Element of ASME/ANS PRA Standard

Equipment Selection (ES)

Related EPRI 1011989 NUREG/CR-6850 Appendices

Appendix A, Appendix for Chapter 2, Technical Bases

Supplemental Guidance

There is no supplemental guidance associated with Fire PRA Component Selection (Task 2). The analyst however should review the following documents to ensure that the component selection is complete:

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