Cracks in backup circuits of reactors in EDF's nuclear fleet. A historical analysis

The information from EDF to the ASN, on October 21, 2021, of the detection of cracks in an emergency cooling circuit of reactor no. 1 of the Civaux power plant, followed by the shutdown of the three other 1500 MW reactors (N4 level) and of the Penly 1 reactor (1300 MW) for the same reason, reducing the theoretical capacity of the fleet by about 10%, followed by the discovery of the same defects on a significant number of reactors during their scheduled shutdowns, has plunged the world of 'observers' into stupefaction, the loss of production caused by this decision being the very proof of the seriousness of this discovery. The cause of this series of decisions, approved by the French Nuclear Safety Authority (ASN), would be the confirmation by more thorough inspections, after cutting the incriminated pipe sections, of the discovery of cracks in the welds of the cooling circuit pipes: safety injection circuit (RIS) and shutdown cooling circuit (RRA) due to 'stress corrosion cracking (SCC)'. Chapter 1 of this document presents the information available to us at the end of May 2022, from EDF, IRSN and ASN, on the 'discovery' of these cracks in the RIS and RRA circuits of a number of reactors. Chapter 2, 'History', goes back to what happened in May 1998 at Reactor 1 of the Civaux power plant, with a net electrical power of 1500 MW (N4 level), shortly after its start-up: a major water leak occurred on one of the channels of the RRA circuit, caused by a through crack. The examination of the elbow in question revealed a thermal fatigue cracking, due to the unfavorable mixing conditions between the cold and the hot fluid. The same observation was made on the other N4 reactors. Furthermore other cracks were found. Finally, it appears that the situation at Civaux 1 was different from that of the other N4 reactors, essentially because of the welding process. Chapter 3 looks at what happened in the United States in 1975 following the discovery of cracks in nuclear reactor backup circuits. On February 5, 1975, a long hearing was held by the Atomic Energy Committee of the United States Congress. The very complete and detailed report of these hearings tells us that, following the discovery of cracks in a backup circuit of a boiling water enriched uranium reactor (BWR), the NRC ordered the shutdown of the 27 operating reactors of this type. On a certain number of them, the presence of cracks was confirmed and the affected sections of piping were either repaired or replaced. Chapter 4 goes back in time: we realize that there is a vast literature of studies, research and tests on the cracking phenomena observed in 2021 and presented in Chapter 1, under the very general term of stress corrosion cracking (SCC). The most astonishing thing about this 'cracking affair' is the fact that none of the three main protagonists of this crisis, EDF-Framatome, ASN and IRSN, have informed us that a similar situation of cracks in the RRA circuit had already occurred in 1998, attributed to thermal fatigue due to unfavorable mixing conditions between the cold and hot fluids. And that in 1999, 2000 and 2001, the RRA sections on all 58 reactors in the fleet were cut out and replaced. Similarly to what happened twenty years ago, it is therefore likely that in 2022, all 56 reactors in operation will be concerned, to varying degrees, including those of the 900 MW range. Those of the 1500 MW and 1300 MW ranges having shown after inspection, the presence of cracks attributed to stress corrosion. A scientific review shows that the metallurgical phenomenon leading to cracks is generic and affects the whole park and concerns the weld zones of the safety injection circuit (RIS) and the shutdown cooling circuit (RRA), and perhaps other circuits. If the vulnerability of the 900 MW reactors were confirmed, the question of extending the operating life of these reactors beyond 40 years would have to be re-examined