Technical Advisory Committee of the Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku, Tokyo, 100-8126 JapanMay 27, 2017
Dr. George Apostolakis
Head, Nuclear Risk Research Center
Central Research Institute of Electric Power Industry
1-6-1 Otemachi, Chiyoda-ku
Tokyo, 100-8126 JapanSUBJECT:

IMPLEMENTATION GUIDE ON HUMAN RELIABILITY ANALYSIS(HRA) FOR PRA

Dear Dr. Apostolakis:
During the seventh meeting of the Technical Advisory Committee of the Nuclear Risk
Research Center (NRRC), May 22-26, 2017, we met with representatives of theNRRC staff to discuss the status of guidance for improved methods for humanreliability analysis (HRA) in Japanese nuclear power plant probabilistic risk
assessments (PRAs). This letter report documents our review of the draft"Implementation Guide on Human Reliability Analysis (HRA) for PRA".
CONCLUSION AND RECOMMENDATIONS
1. The draft guidance is an important milestone for the application of modern HRA
methods in Japanese nuclear power plant PRAs.
2. The following recommendations should be addressed before the guidance isissued for trial use:
•To the extent possible, these methods should be used to evaluate personnel
actions in the models that quantify PRA initiating event frequencies, in theLevel 1 plant response models, and in the Level 2 severe accident mitigation
models.

•The guidance should describe how to develop a realistic scenario timeline, thetypes of information that should be documented in the timeline, and how thetimeline is used in the qualitative assessment, operational narrative, andhuman error probability (HEP) quantification.

•The guidance should describe how to perform a "time uncertainty analysis"
and how to account for that analysis during quantification of the scenariospecific HEPs.

-1-•The guidance should be enhanced to place more emphasis on observationsof carefully selected simulator scenarios to supplement the information
obtained from the operator interviews. Simulator runs should also be
conducted for critical operator actions to confirm realistic scenario time
windows (e.g., delay time, time to perform an action, time to get to alocation, etc.).

•Appendix G of the guide should include one or more instructional examples
that show how deviation scenarios are developed and applied in a practicalHRA.

BACKGROUND
In our November 1, 2014 letter report, we recommended that the NRRC staff should
explore possibilities for collaborative research with the U.S. Nuclear Regulatory
Commission (NRC) and the Electric Power Research Institute (EPRI) to supportcompletion and piloting of the Integrated Human Event Analysis System (IDHEAS)
methodology. Those improvements are needed to replace the outdated Techniquefor Human Error Rate Prediction (THERP) methodology, which has been used in thepast by Japanese utilities.
To support near-term needs for improved Japanese nuclear power plant PRAs thatare currently in progress, the NRRC has developed draft interim guidance thatcombines important elements of the IDHEAS methodology with the quantification
tools from the EPRI HRA Calculator.
DISCUSSION
The draft guidance is an important milestone for the application of modern HRA
methods in Japanese nuclear power plant PRAs. Experience from international
benchmark studies has emphasized the importance of a structured qualitative
assessment for consistent understanding of the role of personnel actions during acomplex evolving scenario. Development of a scenario operational narrative is a key
element of the analysis process. In practice, the narrative clarifies understanding of
the scenario, and it provides the necessary context for a systematic evaluation offactors that can have an important influence on cognitive performance. The NRRC
guidance combines the contemporary concepts of a qualitative assessment andoperational narrative from the IDHEAS methodology with the Cause-Based Decision
Tree Method (CBDTM) for quantifying scenario-specific human error probabilities(HEPs) using the EPRI HRA Calculator.
It is very important that the NRRC guide should contain comprehensive guidanceand supporting examples for the qualitative assessment and the operational
narrative, because analysts are less experienced with these elements of the HRA
methods. The following recommendations should be addressed before the guidanceis issued for trial use.

-2-Scope of the Guidance
The draft guide indicates that the scope of this guidance applies only to post-initiator
human failure events (HFEs) in a Level 1 PRA for internal initiating events that occur
during plant power operation. That scope is unnecessarily restrictive. Improved
PRAs that are currently under development in Japan use modern methods toquantify the frequencies of initiating events that are caused by failures of plantspecific support systems. Some of those models evaluate personnel actions thatmay prevent a plant trip from occurring (e.g., start a standby pump, realign aventilation system, etc.). The improved PRAs also include Level 2 models forcontainment performance and offsite releases during a severe accident. Thosemodels typically evaluate personnel actions to alter the accident progression or tomitigate its consequences (e.g., reduce pressure, close containment isolation valves,open a hardened vent path, etc.).
Personnel actions in the initiating event models, the Level 1 models for core damage
prevention, and the Level 2 models for severe accident mitigation have the samefunctional characteristics. They are typically prompted by scenario-specific cues,
they involve the need for cognitive decisions and equipment manipulations, theymust be completed within a time window that is determined by the evolving plantconditions, and they are often supported by written procedures or other structured
guidance. The qualitative and quantitative analysis framework in the NRRC
guidance should be directly applicable to many of these actions throughout the entire
scenario. Use of the same HRA methodology will avoid inadvertent inconsistencies
and possible analyst confusion about the most appropriate methods to apply at theinterfaces between parts of a continuous scenario that are successively evaluatedwithin an initiating event model, a Level 1 model, and a Level 2 model.
Some actions in the Level 2 PRA models may be directed by supervisory personnel
using severe accident management guidelines or other guidance that is lessprescriptive than the emergency operating procedures. The HRA qualitative
assessments and scenario timelines apply for these actions, but other methods may
be needed to quantify the HEPs. In particular, the quantification models in the HRA
Calculator are most readily applied for actions that are directed by well-developed
procedures.
To the extent possible, the NRRC guidance should indicate that these HRA methods
apply for personnel actions throughout the entire event scenario, from the initial
upset condition through mitigation of a severe accident. In cases where alternative
quantification models may be needed, the guidance should caution analysts toensure that evaluations of personnel performance during scenarios that span theLevel 1 and Level 2 models consistently account for the influencing factors that affectcrew dynamics and integrated cognitive decision making.
Guidance for Scenario Timeline
The draft guidance does not describe how to develop a scenario timeline, the typesof information that should be documented in the timeline, or how the timeline is usedin the qualitative assessment, operational narrative, and HEP quantification. It is-3-

crucial for the guidance to include this information, so that analysts fully understandits importance and its relation to the overall HRA methodology.
Development and documentation of the scenario timeline are essential elements of
the qualitative assessment and operational narrative. The timeline is also used todefine some of the input parameters to the HRA Calculator. Experience has shownthat development of a realistic timeline provides a key focal point for the PRA models.
It facilitates consistent understanding of the scenario progression and the operational
context for personnel performance among the analysts who are responsible for theevent trees and fault tree models, the thermal-hydraulic analyses that provide
information about the event timing and functional success criteria, and the humanreliability analyses.
The NRRC guidance should emphasize the use of plant-specific best-estimate
thermal-hydraulic analyses to evaluate the scenario progression. The timeline
should clearly identify the realistic timing of relevant plant conditions and cues that
may influence personnel actions (e.g., pressures, temperatures, levels, alarms, etc.).
The timeline should also clearly identify the time at which plant conditions reach astate that functionally precludes success of the modeled action. The timeline canthen be used to consistently evaluate important parameters such as the total
functional time window, times at which salient cues occur, time available for
personnel response (Tavail), and time required to complete the necessary cognitive
decisions and equipment manipulations (Treqd). A careful description of the event
scenario timing and functional success criteria can also clarify the modeled actions
and illustrate how multiple actions during an integrated scenario interface with eachother (e.g., sequentially, in parallel, partially overlapping in time, etc.).
Time Uncertainty Analysis
The IDHEAS general methodology and the guidance for its first U.S. application
describe how an evaluation of uncertainties in Tavail and Treqd can affect the overall
HEP. In particular, the HEP should account for the probability that Treqd may exceed
Tavail, which simply means that not enough time is available to complete the action
reliably. Experience has shown that this consideration can be very important during
scenarios when the available time window is relatively short, when there are factorsthat contribute to large variations in personnel response times, and when personnel
must perform actions that require substantial time and attention to achieve thedesired end state (e.g., cool down and reduce pressure according to a specified
maximum rate).
In practice, the overall HEP accounts for this "time uncertainty analysis" according to
the following general formula.
HEP = Pt + (1 – Pt) * Pc
Where
HEP = Overall human error probability for the action
Pt= Human error probability due to insufficient timePc
= Human error probability due to cognitive responses and implementation-4-In this formula, Pt accounts for the probability that the amount of time that is required
to complete the action (Treqd) may exceed the amount of time that is available (Tavail).
The probability that Treqd > Tavail is a direct contribution to the HEP. The factor (1 –Pt) accounts for the probability that sufficient time is available to perform the action.
The term Pc is quantified by the analyses that evaluate the cognitive responses andimplementation actions for the HFE.
This framework is directly compatible with the quantification methodology that is
described in the NRRC guidance. In particular, the CBDTM models presume thatsufficient time is always available to complete the action (i.e., Tavail > Treqd). The
available time margin will vary according to the particular action and the scenario
progression. That time margin affects the evaluation of some parameters in theCBDTM quantification models. However, those models and parameters assume thatthe margin is always positive. Thus, implementation of the "time uncertainty
analysis" framework does not require any changes to the draft NRRC guidance foruse of the CBDTM models in the HRA Calculator.
Use of Operator Interviews and Simulator Observations
The draft guidance emphasizes the importance of conducting interviews with plantoperators and training personnel to confirm the analysts' understanding of thescenario, expected personnel actions, procedural guidance, and interactions amongsupervisors and crew members. The guidance contains draft versions of specificquestions and templates to facilitate and document the interviews. It also contains
examples of how to use the interview results to prepare input information for theHRA Calculator.
Experience has shown that observations of personnel performance during simulator
runs of challenging scenarios can be extremely useful. In some cases, the interview
questions may not appropriately challenge the operators to consider how they might
respond during a complex event. Observations of real evolving scenarios can reveal
important aspects of inter-personal communications, ease or complexity of using theprocedures, density and types of alarms and their timing, distractions or potentiallyconflicting priorities, and other factors that affect crew performance. Theseobservations are extremely valuable to support and supplement the informationobtained from the interviews.
The draft guide indicates that analysts should observe crew performance during
simulator runs of PRA scenarios. The example analysis in Appendix C contains
some information that is derived from observations of the base case feed and bleed
scenario. However, the draft guide does not adequately emphasize the importanceof these simulator sessions in comparison to the operator interviews, and it does notprovide guidance for selection of the types of challenging scenarios that are mostuseful to test the crew response. The guidance should be enhanced to place moreemphasis on observations of carefully selected simulator scenarios to supplement
the information obtained from the operator interviews.

-5-Variant (Deviation) Scenario Guidance
The guide uses the terms "variant scenario" and "deviation scenario"
interchangeably to address conditions that can affect personnel performancedifferently from the conditions which are evaluated in the nominal base casescenario.
The discussions about deviation scenarios are conceptuallycomprehensive, but they would benefit from additional clarity. It is important foranalysts to understand these concepts and how they are applied in practice.
Appendix G of the guide extends the general discussions, but it does not contain any
examples to illustrate how deviation scenarios are developed and applied in apractical HRA.
Appendix G should contain one or more instructional examples that show how
analysts can use these concepts. In particular, it would be very beneficial to expand
the feed and bleed scenario that is developed in Appendix C to illustrate how one ormore deviation scenarios can be derived from that base case analysis. Theexamples should show how different conditions regarding the timing of equipment
failures, availability of cues, use of procedures, etc. can alter factors which affect thequantified HEP. The examples should also explain practical options for how analysts
can address these issues in a realistic HRA (e.g., use the most conservative HEP,
use probabilistically-weighted HEPs, modify the event trees or fault trees, etc.).
We look forward to continuing our review of this benchmark NRRC project as theguidance is completed and used in the first pilot applications.
Sincerely,John W. StetkarChairmanREFERENCES
1.

Nuclear Risk Research Center (NRRC), Central Research Institute of ElectricPower Industry (CRIEPI), "Implementation Guide on Human ReliabilityAnalysis (HRA) for PRA (Adoption of Narrative Approach with HRACalculator)," Revision 24.2E, Final Draft, March 2017.

2.

"NRRC HRA Guide Combining the NRC Narrative Approach with the EPRI
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3.

Sandia National Laboratories, "Handbook of Human Reliability Analysis withEmphasis on Nuclear Power Plant Applications", NUREG/CR-1278, SAND
80-0200, prepared for U.S. Nuclear Regulatory Commission, August 1983.

-6-4.

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5.

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8.

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