ML22097A130
| ML22097A130 | |
| Person / Time | |
|---|---|
| Issue date: | 03/22/2022 |
| From: | David Rudland NRC/NRR/DNRL |
| To: | |
| J. Lynch, NMSS/MSST | |
| Shared Package | |
| ML22097A145 | List: |
| References | |
| Download: ML22097A130 (15) | |
Text
RPV Embrittlement - Monitoring and Prediction Remarks by David L. Rudland, Ph.D.
Senior Technical Advisor for Materials Division of New and Renewed Licenses Office of Nuclear Reactor Regulations 2022 National State Liaison Officer Conference March 22, 2022
Reactor Vessel Embrittlement unirradiated irradiated Reactor Shell CORE Near the Core Surveillance Capsule 2
RPV Integrity Regulations Appendix A to Part 50 Criterion 31 - Fracture Prevention of Reactor Coolant Boundary 10 CFR 50.60 - Acceptance 10 CFR 50.61 - Fracture criteria for fracture prevention Regulatory Guide 1.99 toughness requirements Rev 2 - Radiation measures for normal operation for protection against embrittlement of
- Must meet Appendix G and reactor vessel materials pressurized thermal shock H events
- ASME Sec XI App G
- Surveillance per ASTM
- PT limits: HU/CD (normal) E185-82 10 CFR 50.61a - Alternate
- Leak test
- Periodic monitoring PTS rule
- Flange limits based on 40-year life
- Different Prescriptive
- Upper shelf limits 10 CFR 50.66 - Requirements Embrittlement equation
- must account for the for thermal annealing of the effects of neutron radiation reactor pressure vessel Embrittlement Trend Curve, May 1988, Fit based on 177 datapoints 3
- Regulations are unchanged; surveillance program addressed in guidance
- Aging Management Program XI.M31, Reactor Vessel Material Surveillance
- Continues reliance on Appendix H program using ASTM E185-82
- GALL Report (NUREG-1801, Rev. 1) for license renewal (40 to 60 years)
- shall have at least one capsule with a projected neutron fluence equal to or exceeding the 60-year peak reactor vessel wall neutron fluence prior to the end of the period of extended operation
- Describes use of reconstituted specimens and use of operating restrictions (neutron flux, spectrum, irradiation temperature, etc.)
- GALL-SLR Report (NUREG-2191) for subsequent license renewal (60 to 80 years)
- withdrawal and testing of at least one capsule . . . with a neutron fluence of the capsule between one and two times the peak neutron fluence of interest at the end of the subsequent period of extended operation - or data from a prior tested capsule
- Specifies - it is not acceptable to redirect or postpone the withdrawal and testing of that capsule to achieve a higher neutron fluence that meets the neutron fluence criterion for the subsequent period of extended operation 4
Monitoring and Prediction of Embrittlement
- Embrittlement Trend Curve (ETC) provides estimates of change in fracture toughness (T or RTNDT) as a function of fluence
- Surveillance capsule testing provides monitoring to ensure ETC predicts plant specific behavior properly
- Together they are used to determine pressure-temperature (PT) limits for normal operation 40yr 60yr 80yr Heatup & Cooldown Embrittlement (T)
ART 40yr 60yr 80yr Max Pressure per ETC Data 10 CFR 50 App. G RTNDT(u)
Operating Time (years)/Fluence Coolant Temperature ART = Adjusted Reference Temperature 5
Underprediction of Embrittlement
- Embrittlement Trend Curve in RG 1.99 Rev. 2 and 10 CFR 50.61 does not provide appropriate embrittlement estimates at high RPV fluence levels
- Embrittlement Trend Curve, May 1988, Fit based on 177 datapoints
- Fluence factor in the trend curve is not accurate at high fluence, because it provides non-physical flattening trend above 4-6 x 1019 n/cm2
- Surveillance data are fit to the fluence factor and can not provide good predictions of embrittlement at high fluence levels Fluence function begins to flatten 6
Issue - ETC
+180°F Deviates Statistically from mean 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 7
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 withdraw and testing repeatedly delayed in some cases to achieve higher fluence 8
Issue - Appendix H Performance Monitoring Many licensees have delayed capsules (time and/or fluence),
some recent examples:
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 Capsule withdrawal schedule changes include Not all plants have delayed delays in both time and/or fluence withdrawal of capsules 9
Who is Impacted?
- Embrittlement underprediction 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%
100 years 30% 20% 48% 31%
- Lack of Surveillance Data
- Any plant renewing license that chooses to delay last capsule 10
Risk of Failure ESD represents the underprediction of RTNDT Large Uncertainties:
- Unknown frequency of transient
- Actual plant fluence variations
- Are these analyses bounding?
Unknown plant-specific considerations
- How much protection do administrative and other operational limits provide against violating the PT limit?
RG 1.99 Revision 2 Update FAVOR Scoping Study, May 6, 2021, TLR RES/DE/CIB-2020-09, Rev. 1, ML21126A326 11
Safety Margins Illustration Structural limit Reduced Margin Adequate PT-curve using Margin RG 1.99 Pressure, psi Accurate Operating PT-curve Margin Margin Uncertainty Temp, F Uncertainties increasing due to lack of surveillance, but margin is less due to embrittlement underprediction 12
Staff Goals
- Currently, regulations are sufficient for reasonable assurance of adequate protection against brittle fracture of vessel
- Staff wants to ensure continued reasonable assurance in long-term operation
- Provide remedies for the identified issues with RPV surveillance requirements and embrittlement predictions, on a risk-informed, performance basis
- Do not impact those plants that are not adversely affected by the issues
- Plant-specific surveillance data that covers end of license fluence level
- Projected fluence at end of license < ~3 x 1019 n/cm2 13
Next Steps
- Staff developed a rulemaking plan to ask the Commission for permission to initiate a rulemaking effort
- Once approved, staff will develop the technical basis for how to change to rule to correct these long-term issues 14
Questions?
15