On thermal annealing of irradiated PWR pressure vessels

Radiation embrittlement in some pressurised water reactors has been so fast that, in spite of other applied mitigation methods, thermal annealing has been practically the only solution permitting further operation. The annealings have been reported to be successful and resulted in no damage. In cases where the whole fuel core zone area of the reactor pressure vessel has to be annealed, a fully successful annealing has yet to be convincingly proven. High thermal stresses may make the thermal treatment troublesome to carry out. The recovery mechanisms have been difficult to study because of the exceptionally small size of irradiation defects. The degree of recovery cannot be yet fully calculated precisely from material and annealing information. Especially the recovery processes in restoring elastic and elastic-plastic fracture toughness properties needs much more work to be carried out in order to be clarified. Recovery annealing at a proper temperature is, however, a very effective method and, in many cases, practically the only alternative for extending the service life of a pressure vessel embrittled by radiation. There seems to be no restrictions to repeating the thermal treatment, but every pressure vessel should be independently studied and assessed for the achievement of safe results.