ML22143A834

15 - France Edf SCC Ope 20220525-cem

ML22143A834
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Issue date:	05/25/2022
From:	Carol Moyer NRC/NRR/DNRL/NVIB
To:
Kalikian R, 301-415-5590
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EDF Stress Corrosion Cracking Operating Experience Discussion

Carol Moyer NRR/DNRL/NVIB May 25, 2022 French Operating Fleet

• 56 reactors in operation, all Pressurized Water Reactors (PWRs)

• Built during 1970s -1990s

• 3 main styles

- 32 are 900 MW (CP0 and CPY types)

- 20 are 1300 MW (P4 and P4 types)

- 4 are 1450+MW (N4 type)

• Based on Westinghouse design, modified for French grid

2 Stress Corrosion Cracking Detected in French NPPs

• In Oct. 2021, during the 2nd 10-year inspection, flaw indications were detected in safety injection (ECCS) lines at Civaux-1 (1561 MW).

• Indications were at pipe inner diameter, circumferential, located at elbows. Stagnant flow is expected in affected lines.

• Cracks were confirmed, attributed to stress corrosion cracking (SCC).

- In the base metal (AISI 316L)

- In the heat affected zone (HAZ) and mechanical affected zone (MAZ) adjacent to welds

• Additional indications were found at:

- Civaux-2 (1561 MW) and Chooz-1 and -2 (1560 MW)

- Penly-1 (1382 MW)

3 Additional Indications of SCC in Spring 2022

• EDF accelerated plans to inspect safety injection piping for similar degradation in Spring 2022

• Indications have been detected at 4 reactors:

- Chinon-3 (954 MW)

- Cattenom-3 (1362 MW)

- Flamanville-2 (1382 MW)

- Golfech-1 (1363 MW)

• SCC indications were mostly in 1300 MW and 1450 MW type reactors, not in 900 MW (older) plants

• Indications found in safety injection (SI) lines and residual heat removal (RHR) lines

• The regulator (ASN) requested additional information from EDF to assess the degradation and its extent. Is this a generic issue?

4 Non-Destructive Examination (NDE)

• Cracks were detected by ultrasonic test (UT)

• The UT procedure was designed to detect thermal fatigue (TF) cracks. It was not optimized to detect or size SCC cracks.

• Re-inspection of plant ECCS piping will use a revised procedure, accelerated schedule.

• Re-analysis of prior NDE data will look for missed calls, non-relevant indications.

• Destructive exams are needed to confirm and depth-size SCC cracks.

5 Destructive Examination

• Several elbows were removed and sent to EDF hot laboratory for assessment

• Intergranular cracks consistent with stress corrosion cracking (IGSCC) confirmed

• High hardness detected in the vicinity of the root weld pass

• Unusual height of the root pass at Civaux 1 weld A12

• No evidence of contamination

6 Further Root Cause Analysis

• IGSCC has been confirmed by destructive examination

• Degradation not expected, not in accordance with the international operating experience. No SCC on the French 900 MW plant series after 30 years.

• Weld repairs, deviations from normal weld procedures, and thermal stratification in stagnant lines may have influenced cracking.

• EDF has initiated a welding simulation program to estimate hardening and residual stresses in the areas where IGSCC is observed. The early results show that:

- An area of limited depth on the inner side of the weld is subject to tensile stress.

- A compression zone exists within the bulk of the weld. This compression zone could significantly slow down the propagation of the cracks.

7 Similar Operating Experience

• A similar crack was found in Japanese OhiNuclear Power Station Unit 3 pressurizer spray weld in August 2020, which was attributed to hardening from cold work on the inside diameter (ID) surface.

-Unusual heat input at the weld

-Restriction of weld deformation (constraint)

• Operating experience in U.S. PWRs has shown that stress corrosion cracking of 316 stainless steel is unlikely without significant abnormal conditions, e.g., cold working, grinding, contamination.

8 USA Experience

• The use of ASME Code Section XI is mandated by 10 CFR 50.55a with most U.S. plants using an NRC-approved risk-informed inservice inspection (RI-ISI) plan as an alternative to Section XI.

• U.S. plants examine 10-15% of the ASME Class 1 SI and RHR piping welds under their RI-ISI programs.

• No SCC has been found in analogous welds in U.S. PWRs.

• Welds are susceptible to thermal fatigue cracking.

• There have been ten incidents of thermal fatigue cracking since 2013, with seven found through UT examination and three by leakage.

• EPRI updated MRP-146 in 2018 to enhance the owners voluntary programs of inspections to detect thermal fatigue cracking.

9 More on U.S. NDE Examinations

• Class 1 pipes are examined using multiple ultrasonic angles from four directions.

• The personnel, procedures, and equipment used on piping welds must pass rigorous performance demonstration testing under ASME Code Section XI, Appendix VIII.

• The examinations in the U.S. are optimized for thermal fatigue flaws but are capable of detecting stress corrosion cracking.

• The UT examinations have a current ability to detect cracks of 5-15%

through wall and a good probability of detecting larger cracks.

• Challenges include the metal grain structure and geometric features of the pipes and welds.

10 Mitigation Plans -France

• Cracked piping sections have been removed and replaced.

• From September 2022, EDF intends to carry out a complete examination program, concerning all its reactors in operation, on the areas that might be affected by IGSCC.

-Re-evaluation of prior NDE data

-UT procedure optimized for IGSCC detection (not only thermal fatigue)

• A periodic inspection program will be defined, with a periodicity based on the sensitivity of the NDE, the growth rate of IGSCC, and mechanical elastic-plastic fracture mechanics analyses.

• EDF plans to define flaw evaluation criteria (based on length and depth of the crack) for continued operation without repair.

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