STRESS CORROSION CRACKING | OUTAGE MANAGEMENT AND O&M
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 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
Welds with cracks
Pipe elbow
Above, Figure 1: Location of SCC Source: ASN
www.neimagazine.com | June 2023 | 21
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49
