ML20192A002

06 - RG 1.99 Revision Evaluation Effort for Industry / U.S. Nuclear Regulatory Commission Materials Programs Technical Information Exchange Public Meeting on July 14, 2020

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.

• ACRS issued letter to staff on November 27, 2019, supporting revision of RG.

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:

• Licensed P-T limits for plants with high ESDs.

• BWR leak tests with high ESDs.

• 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.

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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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