ML24030A513
| ML24030A513 | |
| Person / Time | |
|---|---|
| Issue date: | 01/31/2024 |
| From: | Christopher Hunter NRC/RES/DRA/PRB |
| To: | |
| References | |
| Download: ML24030A513 (1) | |
Text
Building an IDHEAS-ECA Knowledge Base Christopher Hunter Division of Risk Analysis Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission 2024 EPRI IRT/HRA Annual Meeting
Introduction
- The NRC has begun applying Integrated Human Event Analysis System for Event and Condition Assessment (IDHEAS-ECA) in various risk-informed activities.
- The first step is to build a knowledge base of IDHEAS-ECA application examples.
- Completed six initial evaluations of base SPAR model HFEs identified as being the most risk significant.
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IDHEAS-ECA Examples
- Operators fail to initiate feed and bleed cooling
- Operators fail to initiate high-pressure or low-pressure recirculation (HPR or LPR),
- Operators fail to trip reactor coolant pumps (RCPs)
- Operators fail to emergency depressurize the reactor
- Operators fail to vent containment
- Operators fail to initiate suppression pool cooling (SPC) 3
- The existing HEPs used in the SPAR models are geometric mean of industry average values based on the cutset level reviews performed by INL.
- The same HEPs are used for the same plant type and are typically used in multiple accident sequence contexts.
- NRC analysts determine if the base SPAR HFE(s) needs to reevaluated on case-by-case basis to ensure the appropriate HEP(s) is used in decisionmaking.
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Results Comparison HFE SPAR HEP IDHEAS-ECA HEP Notes Operators fail to initiate feed and bleed cooling 2x10-2 5x10-3
-75%
A larger HEP of 4x10-2 was calculated for scenarios where MFW or condensate is potentially recoverable. In addition, some plants may have a reduced time window, which would result in higher Pt and, therefore, higher overall HEP (as compared to the base case).
Operators fail to initiate HPR or LPR 4x10-3 2x10-2 5x10-3 0.1
+25%
+400%
The base case HEP is not expected to change for scenarios other than medium and large LOCAs. The IDHEAS-ECA HEP for low-pressure recirculation is higher due to relatively short time window and, therefore, it is dominated by the Pt.
Operators fail to trip RCPs 1x10-3 1x10-3
The IDHEA-ECA base case HFE is for loss of service water scenarios that occur prior to the reactor trip. For scenarios where seal injection and cooling are lost after the reactor trip, the time window will be reduced resulting in a more significant influence of Pt and, therefore, a higher overall HEP.
Operators fail to emergency depressurize the reactor 5x10-4 4x10-3
+700%
Bounding case of medium LOCA will likely result in larger HEPs due to reduced time window resulting in dominant Pt contribution to the overall HEP (e.g., 9x10-2). The SPAR HEP is lower than can be calculated using IDHEAS-ECA without recovery credit.
Operators fail to vent containment 1x10-3 6x10-3
+500%
This HEP is not expected to increase significantly for other scenarios due to large Tavail estimates even for bounding scenarios such as a large LOCA. Pc contributions could increase the HEPs for plants with a lot of execution steps outside the MCR.
Operators fail to initiate SPC 5x10-4 4x10-3
+700%
Bounding case of ATWS will likely result in larger HEPs due to reduced time window resulting in dominant Pt contribution to the overall HEP. The SPAR HEP is lower than can be calculated using IDHEAS-ECA without recovery credit.
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Insights
- The HEPs for the same HFE calculated for scenario and technological variabilities show significant increase or decrease from the base case HEP.
- If the Understanding or Decisionmaking cognitive failure modes (CFMs) are selected, IDHEAS-ECA calculations result in a practical minimum HEP of 1x10-3.
- Credit for recovery may reduce HEPs below this minimum level.
- The guidance for crediting recovery is still under development and was not considered as part of this evaluation.
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Method Observations
- The procedure for evaluating Pt can be enhanced.
- Revised guidance, including a discussion of the data that forms its basis, is needed.
An initial data analysis indicates an error factor (EF) of 2 may be appropriate for most MCR actions.
Need to explore potential alternatives for distribution and EF selections for other HFEs.
- The Tavail estimates in the HFE examples are treated as single values with no probabilistic distribution assigned.
The treatment of the potential uncertainties with Tavail estimates in the same manner as Treqd is likely inappropriate.
Therefore, guidance should be developed to address uncertainties associated with Tavail estimates, such as those derived from thermal-hydraulic calculations.
- IDHEAS-ECA software does not currently include the lognormal distribution.
This issue is addressed Version 1.2 (expected to be released in February 2024).
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Method Observations (cont.)
- The guidance for the selection of the PIFs attributes can be enhanced.
- C1, Detection Overload with Multiple Competing Signals The manner of counting how the number of competing signals has a significant effect on how this PIF attribute is evaluated.
- MF2, Time Pressure Due to Perceived Time Urgency Inappropriate selection and the potential for double counting with Pt and other PIF attributes.
- MF8, Emotional Stress (e.g., anxiety, frustration)
Applicability of PIF attribute whose data is collected under different, non-nuclear applications.
- When possible, an explanation of the underlying data for all PIF attributes should be added to the method guidance.
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Pursued Enhancements Continue to explore available data to determine appropriate EFs and technical basis for the Treqd uncertainty distribution.
Revise definition and guidance for multiple competing signals PIF attribute.
- Determine the point at which the signals are no longer competing (i.e., signal decay time).
Provide guidance on how to treat uncertainties associated with Tavail.
- Consideration of scenario variabilities and how Tavail was determined.
Some PIF attributes have sparse data or were drawn from non-nuclear applications.
- At a minimum, the guidance should be updated to include information available to analyst on the underlying data.
Selection of uncertainty distribution for the overall HEP.
- Longstanding issue that is not associated with IDHEAS-ECA alone.
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Next Steps
- The evaluations will leverage the information and previous work from the IDHEAS-ECA FLEX pilot study documented in RIL 2020-13, Volume 2 (ML21032A119).
- Benchmarking activities with the revised HEPs are being performed by INL to determine the overall impact of the HEPs have on baseline CDFs.
- In addition, SAPHIRE calculations associated with some past ECAs will be reproduced to determine the impact of the revised IDHEAS-ECA HEPs.
- Development of an IDHEAS-ECA Desktop Guide.
- CFM Selection
- PIF Selection
- Evaluation of Pt 10