ML22140A023
| ML22140A023 | |
| Person / Time | |
|---|---|
| Issue date: | 05/25/2022 |
| From: | David Dijamco, Allen Hiser, David Rudland, On Yee NRC/NRR/DNRL, NRC/NRR/DNRL/NVIB |
| To: | |
| Hiser A, 301-415-5650 | |
| Shared Package | |
| ML22143A408 | List: |
| References | |
| Download: ML22140A023 (16) | |
Text
Proposed Rulemaking Plan for Revision of RPV Embrittlement Monitoring and Prediction in Long-Term Operation Allen Hiser, David Rudland, On Yee and David Dijamco NRC/Industry Materials Exchange Meeting May 25, 2022
Topics
- Discussion of Issues
- Regulatory Guide 1.99 Rev 2 (RG 1.99) and 10 CFR 50.61 embrittlement trend curve
- Appendix H surveillance testing
- Discussion of RPV Embrittlement Rulemaking Plan (SECY-22-0019)
- Refers to Impacts of Embrittlement on Reactor Pressure Vessel Integrity from a Risk-Informed Perspective, Final Report (ADAMS Accession No. ML21314A228)
More information on PDF pages 198-221 of ACRS slides from April 6, 2022 2
Ideal Scenario
- ETC provides conservative predictions of embrittlement
- Surveillance data covers all operating periods 3
RTNDT(u)
Embrittlement (T)
Operating Time (years)/Fluence ART ETC Future Data?
40yr 60yr 80yr RTNDT(u)
Embrittlement (T)
Operating Time (years)/Fluence ART ETC
?
40yr 60yr 80yr IF ETC under-predicts measurements IF Limited Surveillance Data is Available Current Data
?
?
?
Potential Uncertainty Sources
Embrittlement Trend Curve
- May 1988, NRC published RG 1.99, which contained an improved embrittlement trend curve (ETC)
- Fit based on 177 datapoints
- June 1991, NRC updated 10 CFR 50.61 to include the ETC from RG 1.99
- Addressed lower than measured predictions (up to 60°F) of embrittlement in some vessels
- This ETC was re-evaluated for continued adequacy in 2014 (ML13346A003) and in more detail in 2019 (ML19203A089) 4
Issue - ETC 5
-180°F Deviates from mean Statistically significant
+180°F DT41J = T41J is a measurement of embrittlement representing the shift in transition temperature from brittle to ductile fracture at an impact toughness of 41J
Surveillance Capsule Delays Appendix H to 10 CFR Part 50 requires periodic monitoring of changes in fracture toughness caused by neutron embrittlement
- ASTM standard (E185-82) allows final capsule fluence to be 2X RPV design fluence - plants change (intended 40-year) design fluence to current license length (e.g., 60 or 80 years)
- ASTM standard (for 40 years) permits holding last capsule without testing Commission finding (Perry decision, NRC Administrative Letter 97-04) that staff review of requests to change capsule withdrawal schedules is limited to verification of conformance with the ASTM standard (i.e., not based on technical or safety considerations)
- Capsule withdrawal and testing repeatedly delayed in some cases to achieve higher fluence 6
Issue - Appendix H Performance Monitoring 7
Plant Capsule
- of times delayed Turkey Point 5
4 Robinson 5
2 Surry U1 5
2 Surry U2 5
2 North Anna U1 4
2 North Anna U2 4
2 St. Lucie U2 4
1 Point Beach 5
1 Many licensees have delayed capsules (time and/or fluence),
some recent examples:
Not all plants have delayed withdrawal of capsules Capsule withdrawal schedule changes include delays in both time and/or fluence
0 50 100 150 200 250 300 350 400 1.0E+17 2.0E+19 4.0E+19 6.0E+19 8.0E+19 1.0E+20 1.2E+20 1.4E+20 DRTNDT,F Fluence, n/cm^2 Data RG1.99 Fit through Plant data Updated fit with additional data Potential additional data Potential Impact of Issue 8
150 F No correction Fit to original data Embrittlement Fit with potential additional data Follows underprediction from Slide 5
Safety Case Risk of Failure
- Conditional probability of failure during normal operation may increase several orders of magnitude (e.g., 3 orders of magnitude at 50F), but expected transient frequency makes overall risk low.
- Some plants may exceed Pressurized Thermal Shock screening limit in 10 CFR 50.61, but analyses suggest risk is low
- Large uncertainty exists - many plant specific details not evaluated Safety Margins
- Inaccurate embrittlement prediction and increasing uncertainty due to lack of surveillance decreases safety margins to failure - plant specific Performance Monitoring
- Delaying capsule testing does not provide adequate performance monitoring to ensure embrittlement trends are reasonable 9
Pressurized Thermal Shock Considerations
- 10 CFR 50.61 uses ETC from RG 1.99
- RTPTS from 10 CFR 50.61 might be impacted
- Limits of 270 °F for plates, forgings, and axial weld materials, and 300 °F for circumferential weld materials
- However, through-wall crack frequency calculated with corrected embrittlement less than 1x10-6 for all cases investigated 10
Safety Margins
- Uncertainties in risk calculations are high and increasing with time
- Even though the risk appears low, resolving these issues will help maintain the fundamental safety principles that are the basis of plant design and operation
- Safety margins, as provided by regulations and current license bases, provide reasonable assurance against brittle fracture 11
Analysis Summary
- With the current state of knowledge, a generalized analysis suggests the overall risk of brittle fracture is low
- The uncertainty in these results is high and increases with time
- Plant specific details not considered
- Under certain conditions, safety margins are impacted and are decreasing as uncertainty increases
- Delays in testing of high fluence capsules represents a lack of sufficient performance monitoring
- Issues are plants with fluences > 6x1019 n/cm2 12
Who is Impacted?
- Embrittlement Underprediction
- Plant specific details (e.g., limiting material, etc.) may contribute to which plants are impacted
- More work is needed to determine which plants are impacted
- Lack of Surveillance Data
- Impacts any plant that renews its license and delays last capsule testing 13 Percentage of Fleet Surpassing Fluence Levels Percentage of PWRs Surpassing Fluence Levels Year\\Fluence 6 x 1019 n/cm2 8 x 1019 n/cm2 6 x 1019 n/cm2 8 x 1019 n/cm2 60 years 6%
0%
9%
0%
80 years 22%
10%
34%
15%
Staff Alternatives Alternative 1 - Status Quo: Make no changes to Appendix H to 10 CFR Part 50, 10 CFR 50.61, or RG 1.99. Handle issues through plant-specific action and generic communications.
Alternative 2 - Focused Solution: Revise Appendix H to 10 CFR Part 50 to include additional surveillance testing requirements for long-term operation, revise fluence function fit for only impacted RPV materials.
Alternative 3 - Comprehensive Solution: Revise Appendix H to 10 CFR Part 50 to include additional surveillance testing requirements for long-term operation, update the applicable regulations (e.g., 10 CFR 50.61) to require all licensees to use an NRC-approved ETC that properly accounts for radiation effects, update RG 1.99 to contain an ETC with one that appropriately accounts for radiation effects, and update implementing guidance.
14
Staff Recommendation
- Alternative #2 - Focused Solution
- Address issues in a focused and risk-informed manner
- Target those plants with materials that are impacted by the underprediction issue
- Modify current surveillance testing requirements to ensure periodic performance monitoring
- Details of implementation to be worked out during regulatory basis effort 15
Summary
- High confidence that currently operating plants remain safe, and recent licensing actions remain valid
- Issued proposed rulemaking plan for Commission approval (SECY-22-0019) on March 8, 2022
- Status: No Commission response 16