Post-Fire Human Reliability Analysis (Task 12)

Task Overview

Background

This task considers operator actions for manipulation of plant components. The analysis task procedure provides structured instructions for identification and inclusion of these actions in the Fire PRA. The procedure also provides instructions for estimating screening human error probabilities (HEPs) before detailed fire modeling results (e.g., fire growth and damage behaviors) have been developed. Estimating HEP values with high confidence is critical to the effectiveness of screening in a Fire PRA. This report does not develop a detailed fire HRA methodology. There are a number of human reliability analysis (HRA) methods that can be adopted for fire with appropriate additional instructions that superimpose fire effects on any of the existing HRA methods, such as Systematic Human Action Reliability Procedure (SHARP), A Technique for Human Error Analysis (ATHEANA), etc. This would improve consistency across analyses i.e., fire and internal events PRA.

Purpose

This task describes the procedure for evaluating the impact of fire scenarios on the human actions addressed in the base PRA study (i.e., the Internal Events PRA or original Fire IPEEE analysis) used to create the Fire PRA Model, as well as how to identify and quantify new actions to be performed as part of the plant fire mitigation plans and procedures. Evaluating the reliability for these human actions supports the Fire PRA Model for calculating such metrics as CDF, CCDP, LERF, and CLERP for fire-induced initiating events. The initial quantification of these metrics makes use of screening probabilities for human failure events (HFEs) where appropriate. As necessary, more detailed best estimate analyses of some human actions will be needed to obtain more realistic assessments of fire risk.

Scope

Task 12 addresses a process for performing both screening and detailed analysis of post-fire human actions identified in accident sequences initiated by a fire. The main focus is to foster the process for assessing the impact of location-specific fires on the human actions taken in response to a fire-induced initiating event, thus preventing core damage and mitigating releases. This task procedure covers three essential elements of most human reliability analysis (HRA) studies:

• Identification of the HFEs to be included in the Fire PRA.
• The assignment of screening human error probabilities for the identified HFEs to assist in focusing the modeling and fire risk analysis to those scenarios and human actions most important to the overall risk results.
• Considerations for the detailed best-estimate quantification of the more important HFEs to properly consider the fire effects on human performance.

In covering the above scope, it is important to stress that this procedure focuses on those unique fire considerations that need to be included in performing a HRA for the Fire PRA using whatever method (e.g., ASEP, etc.) is chosen by the analyst. It is therefore equally important to stress what this procedure does not do. This procedure is not a handbook or a similar stand-alone manual for doing a Fire HRA, in that it does not attempt to duplicate all the typical activities in carrying out a HRA like that specified by the ASME PRA Standard. Nor does this procedure attempt to provide a new or particularly prescriptive method for assessing the HEPs in a Fire PRA, since introducing such a method would be a research project far beyond the intended boundaries and resources for producing these fire procedures. Use of this procedure and the unique fire-related considerations that it covers is expected to be used in concert with already-available HRA techniques and calculation tools by an experienced HRA analyst(s) to perform a defensible and realistic HRA for a Fire PRA.

Notably, the scope of this procedure does not include pre-initiator human failure events specifically related to fire systems, barriers, or programs. Undetected pre-initiator human failures such as improperly restoring fire suppression equipment after test, compromising a fire barrier, or incorrectly storing a transient combustible can all affect the fire risk. Tasks 6, 8, and 11 make use of industry-wide data that within it contains contributions from such human failures. Hence to that extent, these pre-initiator failures are treated within the Fire PRA. Nevertheless, no specific steps are provided here for performing a plant-specific review of the potential for such human failures and thus influencing the use of the industry-wide data. This does not preclude the expectation that pre-initiator human failure events from the Internal Events PRA (i.e., not specifically related to fires) should remain in the Fire PRA Model covering their contribution to component unavailability for safe shutdown systems within the PRA model structure.

Related Element of ASME/ANS PRA Standard

Human Reliability Analysis (HRA)

Related EPRI 1011989 NUREG/CR-6850 Appendices

None

Supplemental Guidance

EPRI 12023001 / NUREG-1921 provides a method for developing best-estimate human error probabilities (HEPs) that account for PSFs and fire-related effects. NUREG-1921 provides a process for identification and definition of human-failure events (HFEs), qualitative analysis, quantification, recovery, dependency, and uncertainty. Three approaches to quantification are offered:

• Screening (based on guidance in NUREG/CR-6850, with additional guidance for long time windows)
• Scoping (intended to provide less conservative HEPs than screening, but requires fewer resources than a detailed HRA)
• Detailed HRA (guidance on how to apply existing methods to assess fire HEPs)

Main control room abandonment (MCRA) is a special case of fire HRA. While NUREG-1921 briefly touched upon this subject, additional research was undertaken to better address a number of factors important to the success of performing shutdown from outside the MCR, including the plant strategy and procedure, capabilities of the remote shutdown panel, and the number of local operator actions. This research is divided into two phases:

• Qualitative Guidance (NUREG-1921 Supplement 1 / EPRI 3002009215) provides additional guidance for MCRA scenarios including: modeling considerations, feasibility assessment, identification and definition, timing, performance shaping factors, and walk-through and talk-through guidance.
• Quantification Guidance (NUREG-1921 Supplement 2 / EPRI 3002013023) provides detailed HRA quantification guidance for fire PRA scenarios resulting in MCRA.

EPRI/NRC-RES Fire Human Reliability Analysis Guidelines (NUREG-1921 / EPRI 1023001)

NUREG-1921 / EPRI 1023001 supersedes the information provided in NUREG-CR/6850 by providing detailed guidance on performing both qualitative and detailed quantitative HRA to support best-estimate HEPs for fire PRAs. In particular, NUREG-1921 provides tools for performing fire HRA such as:

• Tables and diagrams for better understanding the interfaces between fire HRA and other fire PRA tasks with respect to information sharing and the treatment of fire-induced cable failures or electrical faults,
• Specific steps for identifying and defining HFEs to be included in fire PRAs, including operator responses to spurious operations of equipment and indications,
• Criteria for assessing the feasibility of operator actions, especially those taken outside of the MCR,
• Techniques for obtaining or developing more realistic inputs to a structured HFE timeline,
• Identification of factors that can influence ex-control room operator actions (e.g., security and keys for locked doors, communication equipment, and its reliability).

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

The authors of NUREG/CR-6850 recognized that further definition of appropriate methods (especially for developing best-estimate HEPs for HFEs in fire PRAs) and additional guidance for employing these methods were needed. NUREG-1921 was developed to meet those needs in the following key areas:

• Screening is based on the guidance in NUREG/CR-6850, with some additional guidance for scenarios with long time windows.
• Scoping is a new approach to quantification developed specifically to support the iterative nature of fire PRA quantification. Scoping is intended to provide less conservative HEPs than screening but requires less time and effort than a detailed HRA analysis.
• For detailed HRA quantification, guidance has been developed on how to apply existing methods to assess fire HEPs.

EPRI/NRC-RES Fire Human Reliability Analysis Guidelines: Qualitative Guidance for Main Control Room Abandonment Scenarios (NUREG-1921 Supplement 1 / EPRI 3002009215)

Fire PRAs analyze a wide variety of fire-induced scenarios, one of which is fire damage rendering the main control room (MCR) either inhabitable or ineffective. As a result of this fire damage, operators cannot stay in the MCR and the command and control of the plant is transferred from the MCR to another location. This is commonly referred to a main control room abandonment (MCRA). MCRA is analyzed as a special case of fire HRA. While NUREG-1921/EPRI 1023001 briefly addressed abandonment, additional guidance and inputs are needed to properly address the unique contexts of abandonment scenarios.

This report addresses the qualitative HRA and PRA considerations for MCRA including:

• Modeling considerations for MCRA, scenario-specific success criteria, and incorporation of human failure events (HFEs) and equipment failures into the plant response model.
• MCRA scenario development, including consideration of the decision to abandon
• Feasibility assessment and HFE definition and identification
• Timing and timeline guidance
• Qualitative HRA specific to MCRA scenario context, including consideration of performance shaping factors and other influences on operator performance.

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This report provides supplementary guidance for developing the HRA qualitative foundation for MCRA scenarios that was not provided in NUREG/CR-6850. Supplement 1 provides additional guidance beyond NUREG-1921 in several areas, including: modeling considerations, feasibility assessment, identification and definition, timing, performance shaping factors (including a preliminary assessment of command and control), and walk-through and talk-through guidance.

EPRI/NRC-RES Fire Human Reliability Analysis Guidelines: Quantification Guidance for Main Control Room Abandonment Scenarios (NUREG-1921 Supplement 2 / EPRI 3002013023)

This report provides quantification guidance for:

• HRA quantification guidance for the decision to abandon (i.e. Phase II per definitions provided in Supplement 1).
• HRA quantification guidance for the actions to implement the MCRA procedure following the decision to abandon (i.e., Phase III per Supplement 1), including considerations for communications and command and control (C&C).

Impact on EPRI 1011989 NUREG/CR-6850 Guidance

This report provides supplementary guidance for quantifying HFEs related to MCRA.

Alternative Method for Quantification of Decision Making for Main Control Room Abandonment (EPRI 3002016004)

As plants improve their response to MCRA scenarios beyond the current state of practice, a more refined approach may be needed to evaluate the HEP for the decision to abandon the MCR when there is loss of control (LOC) due to fire. This document provides one proposed, alternative quantification method to assess the HEP associated with the decision to abandon the MCR when there is a LOC due to fire.

References

1. EPRI/NRC-RES Fire Human Reliability Analysis Guidelines (NUREG-1921 / EPRI 1023001)

2. EPRI/NRC-RES Fire Human Reliability Analysis Guidelines: Qualitative Guidance for Main Control Room Abandonment Scenarios (NUREG-1921 Supplement 1 / EPRI 3002009215)

3. EPRI/NRC-RES Fire Human Reliability Analysis Guidelines: Quantification Guidance for Main Control Room Abandonment Scenarios (NUREG-1921 Supplement 2 / EPRI 3002013023)