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 anneal ings have been reported to be successful and resulted in no damage. In case s 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 prove n. High thermal stresses may make the thermal treatment troublesome to carr y out. The recovery mechanisms have been difficult to study because of the exceptionally small size of irradiation defects. The degree of recovery can not be yet fully calculated precisely from material and annealing informati on. Especially the recovery processes in restoring elastic and elastic-plas tic fracture toughness properties needs much more work to be carried out in order to be clarified. Recovery annealing at a proper temperature is, howe ver, a very effective method and, in many cases, practically the only alter native for extending the service life of a pressure vessel embrittled by ra diation. There seems to be no restrictions to repeating the thermal treatme nt, but every pressure vessel should be independently studied and assessed for the achievement of safe results. (C) 1999 Commission of the European Co mmunities. Published by Elsevier Science Ltd. All rights reserved.