In-service determination of acceptable defect sizes in VVER-1000 reactor vessel metal

Design modification of nuclear reactor vessels is a continuous and indispensable process. The design should provide vessel durability and strength. The subsequent stages of manufacture, assembly and operation also require additional theoretical and experimental study of vessel strength. The results of in-service inspection of the reactor vessel base metal and weld metal may provide relevant information for such appraisals. Some defects may be revealed and it should then be decided whether remedial work is needed or whether further vessel operation without such remedial work is possible. In-service vessel maintenance is a rather complicated technical process. For this reason the assessment of what is an acceptable defect size in the reactor vessel metals is an urgent and important task. The solution of this problem will allow one to make well-grounded decisions about the need for remedial work and when it should be undertaken. This paper gives the results of the design estimation of acceptable defect size in base metal and VVER-1000 reactor vessel weld metal. The problems of resistance of the structure to brittle fracture are thoroughly examined. This type of fracture is one of the most dangerous as it takes place at high speed and releases accumulated energy from the deformed material. Disastrous crack propagation may occur in brittle, quasi-brittle and tough structure materials. The proposed calculation techniques take this aspect into consideration. (author)