A theoretical analysis of the reliability of PWR pressure vessels

In a recent paper a simplified analysis of the probability of failure of a population of reactor vessels by the unstable extension of pre-existing cracks and flaws was proposed. The analysis, which is based on the methods of linear elastic fracture mechanics explicitly incorporates various effects, including (1) pre-service pressure tests, (2) the incidence and size distribution of cracks existing in vessels as they enter service, (3) the dispersion in material toughness and (4) crack growth in service by fatigue and corrosion-fatigue. In the present paper the sensitivity of the calculated failure rates throughout the life of the vessel to changes in certain of the basic assumptions is further examined. In particular, the variations coming from changes in mean toughness at both operating temperature and during the pre-service cold-hydro test, and in crack shape and size distribution are examined. The influence of ultrasonic examination during fabrication is also assessed. The numerical calculations have been made principally in the context of the conditions governing the manufacture and operation of pressurized water reactors (P.W.R.). Failure rates decrease the more the toughness at the operating temperature exceeds that pertaining during the cold-hydro test, although they are relatively insensitive to the absolute value of the toughness at operating temperatures. Failure rates directly reflect changes in the crack-size distribution function. They are also notably sensitive to crack growth rates, although only when every crack is assumed to grow by corrosion fatigue under 'wet' conditions do the failure rates enter the range of meaningful values. Repeated overpressurization of the vessel during the operation of the reactor to an extent not greater than that of the cold-hydro test can increase the failure rates by about an order of magnitude. (author)