Some eighteen months after discovery of the first indications of stress corrosion cracking (SCC) in stainless steel safety system piping at its 1450-MW-class Civaux 1 (N4 type) reactor, Electricité de France believes it has mastered treatment of the problem, although it will still take several years to fully resolve.
The utility recently confirmed its forecast of 300-330 TWh output from its nuclear reactors this year, compared to 279 TWh in 2022 – despite detection earlier this year of new cracking mechanisms, SCC and thermal fatigue affecting welds that were repaired during original construction. This will require inspections of hundreds of such welds throughout the French PWR fleet and safety authority ASN says it “considers that the discovery of a thermal fatigue defect among the major defects characterised recently, on a weld on which this damage mode was not expected, requires further analyses.”
The original stress corrosion cracking phenomenon (Figure 1) has been found to most affect EDF’s larger reactors, the 1450-MW-class N4 series (four reactor units, including Civaux 1) and the 1300-MW-class P’4 series (12 reactors). It required immediate action because of the safety functions of the affected systems: the safety injection system (used in emergencies to inject borated water into the core) and the residual heat removal system (used to remove decay heat generated by a shutdown reactor). Both are connected directly to the pressure vessel that contains the fuel elements and pressurised water coolant.
Inspection and repair outages stemming from the generic SCC found initially in October 2021 shaved 81.7 TWh off the 2022 output of EDF’s nuclear fleet. To meet demand, EDF was obliged to buy power on the European market during a period of very high prices, costing the group an estimated 29 billion euros and contributing to a record net loss (Ebitda) of 17.9 billion euros for 2022.
The flaw indications were found by ultrasonic inspection (UT) on the inner diameter of pipes within the safety systems, both in the base metal and adjacent to the welds, during mandatory checks carried out every 10 years. Their propagation was intergranular (the phenomenon is known as IGSCC in the US). The UT checks were originally meant to detect the presence of cracking by thermal fatigue, which is a known phenomenon for stainless steel lines in the affected lines; stress corrosion cracking was not expected in these locations.
After the discovery at Civaux 1, EDF ran additional checks on the other three N4 units (one at Civaux and two at Chooz). These revealed flaw indications that had been classified as spurious in UT inspections during previous decennial outages.
The defects in the safety injection system pipework do not affect normal operation, but since the defective zones are located between the reactor vessel and the first isolation valve, rupture of the piping – for example, in the event cold water was injected to cool the core – could lead to loss of coolant. However, both EDF and safety authorities have said that redundant systems would have ensured reactor cooling even had the faulty piping failed.
Massive response
EDF’s response to the findings was massive. After the first flaw was discovered, EDF cut out all four pipe elbows at Civaux 1 and subjected them to destructive examination and lab analysis.
Meanwhile, EDF had found similar flaw indications at Penly 1, a 1300-MW-class P’4 unit that was undergoing its 30th-year outage.
After initial analysis of the phenomenon, EDF identified 12 reactors considered most sensitive to the SCC – the four N4 units, plus Penly 1 and Golfech 1 (another P’4 unit) and six more 1300 MW units – and pulled them from the grid for inspections.
The results of those checks led to the conclusion that the cracking was due to thermal stratification, Hubert Catalette, deputy director of EDF’s nuclear generation division in charge of the SCC recovery project, told a French Nuclear Energy Society conference in Paris on 29 March.
Thermal stratification, a separation of warmer and cooler flows within the lines, created high stresses on the U-shaped piping which led to cracking in pipe elbows adjacent to welds.
Catalette said that the stratification created both constant and variable stresses and was “a consequence of a vortex penetration associated with the geometry”, that is, the layout, of the piping.
EDF’s chosen strategy is “massive preventive replacement” of the sensitive pipe sections on the P’4 series, Catalette said, calling it “much more efficient” than other alternatives. But IGSCC can have multiple causes, and Catalette noted that welding parameters are also being “optimised” in replacement pipe sections to minimise residual stresses.
EDF is, however, studying an “overlay” repair method that was suggested by international experts invited to review its response to the SCC problem, Cedric Lewandowski, EDF executive director in charge of nuclear and thermal generation, told the French parliament’s technology assessment office during a hearing in October 2022.
Lewandowski also praised contractors who stepped up to the plate to supply EDF with piping and skilled manpower, including forges and foundries in Italy and welders from France and North America.
As of late March, treatment of the 16 reactors (four N4 and twelve P’4) most sensitive to the SCC phenomenon (because of their piping layout) was “underway or finished,” Catalette said.
To that date, 450 pipe elbows had been delivered, 340 of them installed, and 320 meters of straight pipe sections had been replaced, he said, and 200 laboratory analyses had been carried out.
EDF’s strategy for inspecting and repairing, if necessary, all 56 PWR units in its fleet was approved by ASN last July. Thanks to record fast development of the new ultrasound technique that can detect cracks through 3 cm of pipe wall, EDF has begun to inspect the incriminated circuits as part of planned maintenance programmes.
Repaired welds under scrutiny
The flaw indications found this year (Figures 2 and 3) pose a different problem: they are on straight pipe sections not subject to thermal stratification. The first, a very deep flaw attributed to SCC, was found on a pipe section that had been cut out of Penly 1 and analysed early this year, EDF said in an information note published on 6 March. The second was detected on SIS piping of Penly 2, with a depth of 12 mm. Both flaws are deeper than the critical defect calculated for these lines.
EDF said the Penly 2 crack was caused by thermal fatigue and that similar flaw indications were found on piping from Cattenom 3.
In an information notice published 14 March ASN reports that the SSC crack at Penly 1 extends over about a quarter of the pipe circumference and its maximum depth is 23 mm, for a pipe wall thickness of 27 mm. “The presence of this crack means that the strength of this pipe is no longer demonstrated,” ASN concluded, adding that however rupture of one of these lines is taken into account in the reactor safety case.
The Institute of Radiological Protection and Nuclear Safety, IRSN, which serves as technical support for ASN, said in a separate information note, dated March 16, that the weld affected by the deep crack at Penly 1 had been repaired twice during initial construction of the reactor, first to correct the alignment of the pipe sections and again to correct a welding defect.
EDF is implementing an inspection programme on the repaired welds of the SIS and RHR systems, said ASN, with more than 150 welds having undergone laboratory assessments.
The inspection programme is to cover all EDF reactors this year, it said.
EDF has already drawn up an “exhaustive inventory” of welds on its reactor fleet that underwent repairs during the construction phase, allowing their categorisation according to susceptibility to the cracking phenomenon, EDF’s Catalette noted.
The inventory found a total of 320 welds on SIS and RHR systems that had been repaired during construction; 69 of them were found to be highly sensitive to SCC, notably those repaired twice, he said.
A revised strategy submitted to ASN on 10 March proposed a schedule for prioritising weld inspections and/or pipe replacements to deal with the repaired-weld 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, he said.
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 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.
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. See Figure 4.
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.
Author: Ann MacLachlan
