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Attainment of the design service life of the reactor pressure vessel (RPV) has become a difficult goal for some pressurized water reactors (PWRs) and consequently life extension may be precluded as an option. The difficulty has emerged due to the embrittlement of the pressure vessel steel from irradiation, and affects both reactor vessel integrity and functionality issues. Since the functional life of the reactor vessel is dependent upon the ability to adhere to the RPV pressure-temperature ( P-T ) limits and the low temperature overpressure protection requirements, these normal operation constraints become quite important and control the reactor coolant system (RCS) operating window. This window is anticipated to close for many PWRs during their design life, preventing effective operation of the RCS and the RPV. To address this problem, the margin to incipient crack initiation associated with ASME Code Section III, Appendix G, fracture mechanics procedures has been evaluated and alternate brittle fracture limits for low temperature overpressure transients have been developed. These limits permit higher pressures for transient conditions than those associated with the normal operation P-T limits of Appendix G. The P-T limits for low temperature transient conditions have been developed, using the principles of linear elastic fracture mechanics. The stress intensity factors, K I have been developed through the use of a superposition technique with influence coefficients. The low temperature transient P-T limits preclude crack initiation for a range of postulated defect sizes and account for actual mechanical and thermal loadings, including those neglected in the simplified ASME Code procedures. This paper chronicles the RPV functionality issue along with the development of a low temperature overpressurization transient P-T limit for resolution of this operational problem.

A new fracture test specimen has been developed to study crack extension and arrest in nuclear reactor vessel steels subjected to stress-intensity factor and toughness gradients similar to those in postulated pressurized thermal shock situations. A summary of the results of all the tests performed is presented to illustrate the range of crack arrest and crack reinitiation conditions observed. One test of this specimen with the corresponding stress analysis is described in detail. During this test the crack initiated, extended, arrested, reinitiated, extended again, and reached a final arrest. Comparison of detailed dynamic elastic-plastic finite-element analyses and dynamic strain and displacement measurements of the crack extension, arrest, and reinitiation events, combined with topographic analysis of the fracture surfaces, has led to a new understanding of the crack extension and arrest process. The results of the tests demonstrate crack arrest in a rising stress-intensity field at near-upper-shelf temperature conditions and show that the toughness required for arrest is lower than would be predicted by the analysis procedures usually employed for pressurized thermal shock evaluations.

Deformation and stress calculation programs for three dimensional elastic plastic solids with slits