Since 1997, cracking incidences of pressurizer heaters were encountered in French PWRs.20,21 The heater sheath (outer diameter of 22 mm, 2 mm thick) is made of 316 L stainless steel. The lower part of the heater is attached to electrical connectors, out of the pressurizer, while the upper part, introduced into the pressurizer, consists of a coaxial heating element coiled round a copper mandrel. After the final assembly of the heater, the sheath is cold swaged to reduce the gaps between the heating element and the sheath and therefore, to improve the thermal exchanges. Under operat­ing conditions, the heaters are exposed to hydrogenated and non-polluted primary environment at 345°C. Nevertheless, the temperature at the outer surface of the sheath, in nominal condition, could reach 360°C. Nine cases of SCC were found after 12 destructive examinations out of the 1200 fail­ures observed on heaters. When SCC leads to primary leakage (boron traces on connectors), the heater has to be replaced no later than the next outage. Elemental analyses (energy-dispersive X-ray spectroscopy) did not reveal any trace of pollutant at the surface of the retired heaters. Therefore, it was concluded that SCC occurred in the nominal hydrogenated primary environment. No chromium depletion was present at the grain boundaries of the stainless steel. Such depletion can result in the precipitation of chro­mium carbides at grain boundaries and promotes IGSCC in an oxidizing environment.

A surface annealing heat treatment was developed (induction heating) to counteract the initiation of SCC on the original cold-worked outer layer exposed to the primary water. The goal of the heat treatment is to anneal the surface of the material, decreasing strain-hardening and residual stresses without any damage of the electrical properties of the heater element. As a result, the Vickers micro hardness decreased from 320 HV1 (higher than the threshold necessary to initiate SCC)22 down to 200 HV1 (below the threshold necessary to initiate SCC) at the surface of heat treated heaters, and resid­ual stresses were removed as shown by corrosion tests in MgCl2 medium.

1.3 Conclusion

Despite the original stringent selection of the materials used to manufac­ture the components, uniform and localized corrosion occurs in PWR envi­ronments. Remedies can be of different types: adjusting the water chemistry, reducing superficial strains and stresses, replacing materials or changing microstructures. In particular, experience in the field demonstrated that an increase in chromium content is an efficient strategy: to date nickel alloys containing 30% chromium, used to replace 16% chromium nickel alloys, have exhibited very good resistance to localized corrosion, such as SCC. The history of the degradations shows that for a given type of material, the ten­dency to corrode can largely depend on the manufacturing conditions.