Update on stress corrosion cracking phenomenon: causes, evolutions, reparations…

The discovery of a stress corrosion cracking phenomenon at an unexpected location in the primary circuit of several nuclear reactors has mobilized EDF’s experts to identify the causes of this anomaly and optimize its detection and repair.

At the end of 2021 and the beginning of 2022, EDF was confronted with a rare anomaly in the French and the world’s fleets of pressurized water nuclear reactors (PWR). This anomaly is a stress corrosion cracking phenomenon (SCC) observed on stainless steel piping in the safety injection system (SIS) of several reactors, where it was unexpected. It results in micro-cracks in the material, close to the welded areas, in components with a thickness of several tens of millimeters. Previously known cases on PWR auxiliary circuits were due to conditions where pollution was accidentally present in non-circulating portions of specific circuits. The current issues are related to cracking in the nominal primary medium, which is still much rarer in the world’s fleet. After an initial series of investigations, on May 13th, the electricity company submitted a study of the state of knowledge on this phenomenon to the French Nuclear Safety Authority (ASN) and proposed monitoring and repair scenarios. A few days later, on May 17th, ASN President Bernard Doroszczuk was questioned on the phenomenon of stress corrosion cracking (SCC) by the Parliamentary Office for the Evaluation of Scientific and Technological Options (OPECST).

Twelve French nuclear reactors are undergoing “in-depth assessment” of the SCC phenomenon: four units in the N4 range, five in the 1300 MW range, and three in the 900 MW range. Régis Clément, Deputy Director of EDF’s Nuclear Generation Division, explains that “today, beyond the 12 shutdown reactors, there is no need to anticipate new one for additional controls”. Checks dedicated to the search for SCCs will, of course, be carried out for all the reactors in the fleet, with specific attention to the safety injection system (SIS), the coolant system of the shutdown reactor (RCS), the Chemical and Volume Control System

of the reactors (CVCS) and the pressurizer expansion lines. They will be carried out during the partial, ten-yearly, or refueling visits in 2022 and 2023, possibly at the beginning of 2024.

Defects that are difficult to characterize

Non-intrusive ultrasonic inspections by EDF have made it possible to discover these anomalies in the Civaux 1, Chooz B1, and Penly 1 reactors. The limit with this method is that it cannot easily give the exact nature of the defects, nor can it characterize their dimensions, especially their depth. In the case of SCC, the crack propagates between the grains of the material. It is called intergranular Stress Corrosion Cracking (or IGSCC). It is therefore not straight, is very closed, and has many changes of direction (or successive branches), a bit ‘like lightning,’ says the EDF expert. Ultrasonic inspection techniques, developed until now to characterize thermal fatigue cracks (a phenomenon initially feared in RIS piping), are not very well suited to detecting and characterizing indications of SCC. Therefore, it was necessary to carry out additional, much more intrusive investigations: parts of the piping were cut out for laboratory examination. Thus, 35 lines were removed before the report was submitted to the ASN, and 105 additional cuts are planned to refine the knowledge and adapt the inspection procedures.

Only four reactors are known to be affected by the SCC phenomenon. On the one hand, Civaux 1, Chooz 1, and Penly 1 on the RIS and RRA circuits. On the other hand, the case of Chinon B3 is somewhat specific: the indication found on the RRA circuit (the SIS circuit is intact) is linked to a technological welding defect, explains Régis Clément of EDF, which is a different problem from the other three.

On the SIS circuits, the pipes have a diameter of 25 to 30 centimeters and a thickness of 30 millimeters. Cracks due to SCC reached 5 mm on Civaux 1 but are smaller than 2 mm in most other cases.

The design factor

The stress corrosion cracking occurs under three concomitant factors: a sensitive material, a favorable chemical environment, and tensile stress in the material. Therefore, this corrosion phenomenon is a multi-parameter mechanism, making it even more challenging to assess the situation.

The first factor is the sensitivity of the material to stress corrosion cracking. In this case, the material is a standard stainless-steel grade, commonly used in industry for its good corrosion resistance. It is not known to be susceptible to SCC in the primary PWR environment unless the material is worked hardened. Work hardening is a deformation of the material during the various stages of manufacture, which increases its hardness and sensitivity to stress corrosion cracking. To avoid this hardening, a high-temperature solution heat treatment is carried out at the end of the manufacturing process to restore the material’s properties. However, post-fabrication operations, particularly welding pipe sections on-site, can lead to material deformation near the welded area.

Concerning the second factor, chemistry, the operator monitors the water quality in the primary circuit very precisely (pH, oxygen level, pollutants, etc.) and thus has data on all the years of operation. To date, it has not detected any anomaly that could explain a significant increase in the risk of SCC in the reactors concerned without sensitizing the material. Therefore, this absence of “contaminants” significantly differs from previously known SCC cases on auxiliary systems. Cracking of stainless steel in the nominal environment, although identified in the PWR community, remains significantly rarer.

Material stresses (third factor of SCC) can be generated by welding (residual stresses), line assembly, or loading in service (pressure and temperature). The design of the pipes can play a role. “Today, we are convinced that the complexity of the configuration and the geometry of the lines (…) are a major cause,” explains Régis Clément. On the oldest reactors, the lines are not very sinuous. They have become increasingly so with the design of the new 1300 MW and N4 stages. This observation is corroborated a priori by the ASN. During his hearing, the chairman of the Authority, Bernard Doroszczuk, explained that “this is not a phenomenon linked to aging. The geometry of the lines favors the thermal stratification of the fluids. This generates thermomechanical stresses. The making of the welds seems to be a second-order cause today”.

Simulations and calculations

According to the ASN, all the circuits affected by SCC will have to be repaired or replaced. For its part, EDF has carried out simulations of the SCC phenomenon to model the propagation of a crack. “We are facing a rather slow phenomenon,” assures Régis Clément. In addition to understanding, other calculations are intended to define the maximum permissible defects in terms of the mechanical strength of the lines. These calculations and measurements are currently being deployed and should be completed by September for all the lines and welds on the series. As things stand, the company is convinced that the extent of the cracks observed is within the margins imposed by the safety rules.

In addition to the a priori limited extent of the indications, another element supports the safety of the reactors. Indeed, initial studies show that the core cooling would still be ensured even in the event of a rupture of two SIS lines. The reactors have two safety trains, including the SIS circuits, subdivided into four lines connected to the primary circuit on the 1300 MWe and N4 levels and three on the 900 MWe level. “We are capable in all circumstances of shutting down and returning to a safe state a reactor that would need this safety injection circuit,” assures Régis Clément, specifying that this conclusion will be submitted to the ASN for analysis.

Italian suppliers for the steel

EDF must now plan repairs to the lines for which sections have been or will be removed for expertise (with or without indication of a crack). Although the company had some stocks, it had to order these steel parts on a massive scale. Two suppliers have been identified in Italy. They should be able to meet all the needs of the fleet. The industry is mobilizing its skills of pipe fitters, welders, and component makers on a massive scale. Several hundred employees are currently being trained on models to prepare for the work in situ. The work is even more complex because it is carried out in a radiological environment, and the company assures special attention will pay to the employees carrying out the repairs.

Some repairs concern pipes removed following the ultrasonic inspections, for which the destructive expertise later revealed no indication. In the future, EDF intends to be able to limit or avoid the removal of parts of undamaged circuits. To this end, the company is developing non-destructive testing methods that are better able to characterize the indications. This involves various automated control methods (ultrasound, dye penetrant testing, television inspection, etc.), some of which will be able to enter the interior of the piping uncut at the level of the device valve. These tools should be industrially usable by January 2023, the industry hopes.

Defining a “zero point.”

The increasingly detailed understanding of this SCC phenomenon in the French fleet will not lead to the “redesign” of the loops connected to the primary circuit according to the areas most affected by these constraints. “We are going to rebuild the lines identically. On the other hand, their monitoring will likely be particularly different,” explains Régis Clément. The idea is to define a “point 0” of the situation of the entire fleet during the shutdowns scheduled for 2022 and 2023. Then, under the supervision of the ASN, a frequency of inspections will be set up to anticipate the occurrence of the SCC phenomenon.

The challenge for EDF is now to begin repairs to shut-down reactors as quickly as possible to restore fleet availability. The ASN must approve the file on the return to service of EDF’s reactors. The electricity company hopes to obtain the authorization to start repair work in July. Then, in the autumn, it will define the new maintenance doctrine, i.e., the commissioning of new means of non-destructive testing able to measure the depth of the defects. Although EDF intends to optimize the availability of the fleet as much as possible over the current year, the company has nevertheless had to lower its production forecasts, now between 280 TWh and 300 TWh for its 56 reactors, compared with approximately 360 TWh in 2021.