OUTAGE MANAGEMENT AND O&M | STRESS CORROSION CRACKING French nuclear fleet
Dealing with cracking in the
EDF is confident about fixes it is implementing to address the problem of defects in safety injection and residual heat removal system pipework in French pressurised water reactors. The problem is mainly due to stress corrosion
cracking, but in recent months thermal fatigue defects have also been encountered By Ann MacLachlan
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 (See
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
Above: The first indications of stress corrosion cracking (SCC) in stainless steel safety
system piping appeared at the Civaux nuclear power plant site in southwestern France Source: EDF
20 | June 2023 |
www.neimagazine.com
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 EUR29bn and contributing to a record net loss (Ebitda) of EUR 17.9bn 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.
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