ML20192A002
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| ML20192A002 | |
| Person / Time | |
|---|---|
| Issue date: | 07/14/2020 |
| From: | Allen Hiser, Ali Rezai NRC/NRR/DNRL, NRC/NRR/DNRL/NPHP |
| To: | |
| Rezai A, 301-415-5650 | |
| References | |
| Download: ML20192A002 (10) | |
Text
RG 1.99 Revision Evaluation Effort -
Status Update Materials Information Exchange Meeting July 14, 2020
Events to Date
- Technical letter report1 (TLR) identified several deficiencies in RG 1.99, Rev. 2.
- Most significant is non-conservatism of RTNDT at high fluence 6x10 19 n/cm2 (some PWRs reach during SLR)
- TLR reviewed by ACRS Subcommittee on August 22, 2019 (ML19260E007) and Full Committee November 6, 2019.
1 - Assessment of the Continued Adequacy of Revision 2 of Regulatory Guide 1.99 -
Technical Letter Report, July 31, 2019, ADAMS Accession 2Number ML19203A089
NRC Staff Actions to Date
- Formed working group and oversight group to address TLR recommendations and evaluate a potential revision or alternative to RG 1.99, Rev. 2.
- Held public meeting on May 19, 2020 to present results of the evaluation effort, including:
- Motivation for revision evaluation effort
- Framework of alternative RG
- Fleet impact smart sample results
- Safety/risk analysis results
- Implementation aspects
- Meeting summary is at ML20168A009.
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Key Messages - Public Meeting
- Conducted risk analysis based on 80-year operating period (RG 1.99, Rev.
2 and ASTM E900-15).
- Results: Fleetwide implementation of a revised RG may not be necessary.
- Questions for certain transients (PWR cooldowns on licensed P-T limits and BWR leak tests with higher cooldown rates) - industry input could help.
- Framework of a potential alternative to RG 1.99 has been developed.
- Potential burden reduction for some plants - could benefit from industry and licensee input.
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Framework of Potential Alternative RG
- Staff presented a framework of alternative RG based on the ASTM E900-15 embrittlement trend correlation (ETC)
- The framework also addresses aspects not fully addressed by the E900-15 standard including:
- Use of plant-specific surveillance data.
- Margins
- Default values
- Limitations 5
Fleet Impact Study
- Smart sample of 21 plants.
- Used licensing basis materials inputs.
- Determined embrittlement shift delta (ESD), the change in adjusted reference temperatures that would result from switching from RG 1.99 ETC to E900-15 ETC (implementing all elements of alternative RG framework).
- ESDs tend to increase, particularly for base materials.
- Only a few ESDs for limiting materials > 50 °F, and these tend to be at fluences ~ 6x1019 n/cm2
- A few (~20%) of plants had a change in limiting material.
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Safety/Risk Analysis Study
- Determine safety impact of a potentially nonconservative material reference temperature (ART or RTPTS) associated with normal cooldown and leak test transients and PTS transients.
- Evaluated potential ETC non-conservatism by calculating the change in the conditional probability of failure (CPF) as a function of the ESD.
- Range of ESDs informed by fleet impact study.
- Both 1/4T and shallow flaws were modeled.
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Safety/Risk Analysis Results
- Based on an 80-year analysis of a smart sample of plants, potentially nonconservative reference temperatures do not represent a significant safety issue in most cases for normal cooldowns, leak tests and PTS transients.
- Higher CPF calculated for the following transients:
- Lower actual cooling rates are expected to result in lower CPF values.
- Additional information desired to help confirm that the risk (TWCF) is low for the high ESD plants.
- Safety analysis results (to date) do not justify generic implementation of a revised RG based on ASTM E900-15.
8
Public Meeting Feedback
- Staff received feedback on the following topics:
- Reduced margins for well-behaved surveillance data.
- Use of sister plant data
- Difficulty of obtaining Mn and P values needed for E900-15 ETC.
- Consider increasing margin to allow use of ETC below minimum temperature limits.
- Event frequencies for exceeding licensed P-T limits are extremely low. System and operational constraints described in technical report MRP-437/BWRVIP-328.
- Assumption of 1/4T flaw in safety/risk analyses
- Not clear the potential alternative RG would increase safety or reduce cost, e.g.,
probably no reduction in burden
- The staff appreciates the feedback.
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Next Steps (NRC staff)
- Based on the results of the staffs evaluation and considering public meeting feedback, NRC has decided not to pursue an alternative to RG 1.99 Rev. 2 at this time.
- Formally document technical work completed under the RG evaluation effort, in two reports.
- Technical basis for proposed alternative
- Safety/Risk Analysis
- Complete a holistic evaluation of RPV integrity, considering both the RG evaluation and RPV surveillance programs, using the principles of risk-informed decision making from RG 1.174.
- Continue engagement with industry to get more information - to confirm risks are low for plants with high ESDs.
- Review the MRP-437/BWRVIP-328 report - insights on event frequencies.
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