ML22033A006
| ML22033A006 | |
| Person / Time | |
|---|---|
| Issue date: | 03/10/2022 |
| From: | NRC/NRR/DNRL |
| To: | |
| References | |
| Download: ML22033A006 (8) | |
Text
Office of Nuclear Reactor Regulation Reactor Pressure Vessel Embrittlement Monitoring and Prediction in Long-Term Operation The embrittlement trend curve (ETC) provides estimates of change in Together, they are used to determine pressure-temperature (PT) fracture toughness (T,RTNDT) as a function of fluence. limits for normal operation and to demonstrate sufficient toughness for protection from pressurized thermal shock events.
Surveillance capsule testing generates monitoring data to ensure the ETC predicts plant-specific behavior properly.
ART = Adjusted Reference Temperature
Embrittlement Uncertainty Ideal capsule withdrawal Expected Uncertainty Uncertainty Impact schedule and accurate ETC With an ideal surveillance withdrawal and testing schedule and an accurate ETC, the level of uncertainty is known and manageable.
Delayed capsule withdrawal Large Uncertainty If surveillance testing is delayed and schedule and inaccurate ETC the ETC is inaccurate, the embrittlement uncertainties can be large and unquantified. These uncertainties may impact confidence in the integrity of the reactor pressure vessel for long-term operation.
One Source of Uncertainty ETC Underprediction
+180oF T41J = a measurement of Deviates Statistically embrittlement representing the shift in from mean significant transition temperature from brittle to ductile fracture at an impact toughness of 41J or 30 ft-lb.
ETC underprediction starts at a fluence of about 3x10 n/cm .
19 2 At a fluence of about 6x1019 n/cm2, -180oF underprediction becomes relevant.
ETC shown in figure found in:
Equation 2 in Regulatory Guide (RG) 1.99, Revision 2, Radiation Embrittlement of Reactor Vessel Materials Equation 3 in 10 CFR 50.61, Fracture toughness requirements for protection against pressurized thermal shock events
Another Source of Uncertainty Lack of Surveillance Data 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 reactor pressure vessel design fluenceplants change (intended 40-year) design fluence to current license length (e.g., 60 or 80 years).
The ASTM standard permits holding the last capsule without testing.
Capsule withdrawal and testing are repeatedly delayed in some cases to achieve higher fluence.
Impact of Underprediction and Lack of Data 400 PT Limits Fit with potential additional data 350 With a lack of data at high fluence 75°F 150°F values, the current ETC adjusted with 300 No correction only low fluence data can lead to Embrittlement underprediction of up to 150°F (blue 250 curve).
200 Even if the current ETC (orange curve) 150 is adjusted with additional data (yellow Data Potential additional data curve), underprediction could still be 100 RG1.99 up to 75°F due to the functional form Fit to original data Fit through Plant data of the ETC. 50 Updated fit with additional data Improved prediction would require an 0 1.0E+17 2.0E+19 4.0E+19 6.0E+19 8.0E+19 1.0E+20 1.2E+20 1.4E+20 update to the functional form of the ETC and high fluence surveillance Fluence, n/cm2 data for validation of the ETC.
Risk-Informed Analysis Risk: Low risk of brittle failure, but uncertainty increases with time.
Safety Margins: Regulations and current licensing basis provide safety margin against brittle failure, but the safety margin decreases as embrittlement uncertainty increases. Safety Margins Performance Monitoring: Monitoring is needed to ensure analysis results remain valid with time (i.e., no unexpected adverse safety issue occurs).
Risk Performance Monitoring
Assessment and Staff Goals NRC Staff Goals Assessment
- With the current state of Who Is Impacted?
- Currently, regulations are sufficient for knowledge, a generalized analysis reasonable assurance of adequate protection suggests the overall risk of brittle Percentage of Fleet Surpassing Fluence Levels Percentage of PWRs Surpassing against brittle fracture of the reactor vessel.
fracture is low. Fluence Levels Year/Fluence 6x1019 n/cm2 8x1019 n/cm2 6x1019 n/cm2 8x1019 n/cm2
- The staff wants to ensure continued reasonable
- The uncertainty in these results is 60 years 6% 0% 9% 0% assurance in long-term operation.
high and increases with time.
80 years 22% 10% 34% 15%
- Remedies for the identified issues with Plant-specific details are not reactor pressure vessel surveillance considered.
requirements and embrittlement
- For plants with fluences Embrittlement Underprediction predictions, on a risk-informed,
>6x1019 n/cm2, safety margins are
- Plant-specific details (e.g., limiting material) may contribute to performance-based basis impacted and are decreasing as which plants are impacted.
uncertainty increases.
- More work is needed to determine which plants are
- The staff does not want to impact those plants
- High fluence surveillance data are impacted. that are not adversely affected by the issues.
needed to decrease embrittlement - Plant-specific surveillance data that cover uncertainty and validate Lack of Surveillance Data the end-of-license fluence level embrittlement trend predictions. - Projected fluence at end of license
- Any plant renewing a license that chooses to delay the last
- Plants with fluences <~3x1019 n/cm2 capsule may lack proper surveillance data.
>6x1019 n/cm2 present issues.
Contact Information David RudlandSenior Technical AdvisorDavid.Rudland@nrc.gov David DijamcoMaterials EngineerDavid.Dijamco@nrc.gov