OUTAGE MANAGEMENT AND O&M | STRESS CORROSION CRACKING


welds with flaw indications “as is” – ie, not repairing or removing the lines – for more than one operating cycle. It is clear that the regulators – who did acknowledge


EDF’s major effort in response to the cracking issues, notably the quick development of an effective non- destructive examination (UT) tool – will continue to keep a close eye on the utility’s repair and replacement strategy as well as its justification for keeping reactors with flaw indications online for any lengthy period.


Above: SCC indications were also found at Penly units 1 and 2, 1300 MW-class P’4 units issue. As of late March, a total of 124 welds either had


already been checked or were already destined to be scrapped because they are on P’4 pipe circuits slated for preventive replacement, Catalette said. Under the proposed strategy, 92% of the “most sensitive” welds and 58% of the other repaired welds will have been checked by the end of 2023, he said. Next year, 100% of the most sensitive welds and 87% of the others will be checked. By the end of 2025, all the repaired welds on EDF’s fleet will have been inspected and the piping replaced if necessary. ASN said in an information note published 25 April that


it had agreed to EDF’s proposed strategy, including the priority for inspecting the lines considered most vulnerable to cracking. The technical dialogue that preceded that decision, ASN said, had focused among other issues on reactors with repaired welds on two different auxiliary piping lines, namely Nogent 1 (a 1300 MW unit) and Cruas 2 (a 900 MW unit). At ASN’s behest, EDF had conducted more detailed investigations of the potential flaws on the lines and analysed the safety consequences of simultaneous ruptures of two auxiliary lines on the same reactor, the regulator said.


ASN added that EDF had implemented “supplementary


operating measures” aiming to avoid operational situations that would put large strains on the welds in question, as well as measures to rapidly detect leaks if they should occur. The measures, ASN said, would be in place until the next planned outages of Nogent 1 and Cruas 2, in September 2023. In a letter to EDF dated 30 March, ASN noted that the utility had committed to submit to regulators a further inspection programme covering austenitic steel lines – other than those in the safety injection and residual heat removal systems – that contain repaired welds. Among other details in the letter, ASN said its advisory


group for nuclear pressure equipment will meet on 25 and 26 May to consider EDF’s proposed strategy for maintaining


22 | June 2023 | www.neimagazine.com


Usefulness of periodic inspections The discovery of these unexpected phenomena has posed questions about changes made to reactor designs licensed to France by the US supplier Westinghouse in the 1970s and 1980s. No IGSCC has been detected in analogous welds on US PWRs, although thermal fatigue cracking has been found by UT on ten of those units, according to a 2022 presentation by Carol Moyer of the US Nuclear Regulatory Commission’s Nuclear Reactor Regulation division. But it also underlines the usefulness of obligatory periodic in-depth inspections as practiced on the French reactor fleet. The crack indications had gone unnoticed in the previous inspections 10 years ago because inspection techniques were not adapted to detecting SCC on the stainless-steel lines. The initial SCC indications were discovered first at the 20th-year outage of Civaux-1 and then during the 30th-year outages of the P’4 series units. EDF has determined that the layout of safety systems piping on the 32 reactor units of its 900-MW-class reactor series and the eight units of the P4 series (Paluel, Flamanville and Saint-Alban) makes them much less vulnerable to SCC than the more recent series. The P4 units were the first built to the 1300-MW-class design under Westinghouse licence, followed by the P’4 series with slight design changes (Belleville, Cattenom, Golfech, Nogent-sur-Seine and Penly). According to two French nuclear industry veterans, French designers had been asked to make the lines more compact in an effort to trim construction costs. Combined with already planned long decennial outages,


the SCC-related downtime shaved EDF’s generation in 2022 to the lowest output since 1992, 279 TWh (as already noted), compared to 360.7 TWh in 2021. At one point, only 25 of the 56 nuclear generating units were online, and the utility had trouble meeting its targets for returning units to the grid as reactors undergoing SCC-related outages missed restart dates. According to grid company RTE, the SCC issue was the


second largest factor contributing to reactor unavailability in 2022, with a capacity loss equivalent to 10.2 GW. Planned maintenance accounted for 10.5 GW capacity loss, including the major programme supporting long-term operation of older units, called the “Grand Carénage” or “major overhaul” which involves long outages. Maintenance shifted in time due to Covid took out 2.8 GW, and 3.9 GW was lost for various other reasons (including around 1% due to cooling water issues in dry weather). Altogether, these factors meant that EDF had only 34.1 GW available on average in 2022, compared to a nominal capacity of 61.4 GW. The nuclear fleet’s availability continued to be impacted


in first-quarter 2023, with available capacity down 7.4% as of 31 March compared to the same period in 2022, according to EDF. However, as of 10 May, all N4 units were back on line. ■


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