Enhancement of the quality of the reactor pressure vessel used in light water power plants by advanced material fabrication and testing technologies

The fracture safe assessment of nuclear pressure vessels (RPV) is based upon: (i) an adequate stress analysis, (ii) reliable material characteristics, (iii) acceptable defect sizes. There may arise problems which are related to the inhomogeneity, low toughness and crack phenomena sometimes observed in the base material and heat affected zone (HAZ). Due to this it is difficult and in some respects even impossible to measure the decisive values of (fracture-) toughness and defects. Apart from the significance of those facts for existing RPVs, all efforts were directed to provide a steel which should be non-susceptible to embrittlement and/or cracking in the HAZ, and simultaneously yielding in a higher upper shelf toughness of base and HAZ material. These objections were pursued in cooperation with manufacturers, vendors and inspection authorities by the following activities: (i) detailed investigations to obtain information on: occurence and size of inhomogeneities and defects, especially stress relief cracking (SRC), toughness properties adjacent to defects; (ii) improvement of chemical composition, steel making processes, welding procedures, optimum temperature cycle and level for stress relief heat-treatment