Structural integrity of nuclear reactor pressure vessels

Abstract

The paper starts from concerns expressed by Sir Alan Cottrell, in the early 1970s, related to the safety of the pressurized water reactor (PWR) proposed at that time for the next phase of electrical power generation. It proceeds to describe the design and operation of nuclear generation plant and gives details of the manufacture of PWR reactor pressure vessels (RPVs). Attention is paid to stress-relief cracking and under-clad cracking, experienced with early RPVs, explaining the mechanisms for these forms of cracking and the means taken to avoid them. Particular note is made of the contribution of non-destructive inspection to structural integrity. Factors affecting brittle fracture in RPV steels are described: in particular, effects of neutron irradiation. The use of fracture mechanics to assess defect tolerance is explained, together with the failure assessment diagram embodied in the R6 procedure. There is discussion of the Master Curve and how it incorporates effects of irradiation on fracture toughness. Dangers associated with extrapolation of data to low probabilities are illustrated. The treatment of fatigue-crack growth is described, in the context of transients that may be experienced in the operation of PWR plant. Detailed attention is paid to the thermal shock associated with a large loss-of-coolant accident. The final section reviews the arguments advanced to justify ‘Incredibility of Failure’ and how these are incorporated in assessments of the integrity of existing plant and proposed ‘new build’ PWR pressure vessels.

Keywords:

• fracture
• steel
• metals

Acknowledgements

The views expressed in this paper have been developed over the 40 years associated with, first, the Marshall/Hirsch Study Groups and then TAGSI. I have benefited enormously from discussions that I have had with colleagues on these bodies; also with those on the MAC and with staff of the IVC. I have supervised many research projects associated with the integrity of RPV steel; several have been referenced, but I would like to acknowledge particularly inputs from Paul Bowen, Milorad Novovic, Xiao Zhong Zhang, Sujun Wu and Maria Balart. Above all, I would like to acknowledge the great fortune that led to Sir Alan becoming my PhD supervisor and to the many opportunities that I had to discuss the topics covered in this paper with him over the period from 1974 until he retired from TAGSI in 1999.