REACTOR PRESSURE-VESSEL FUNCTIONALITY ISSUES AND THE DEVELOPMENT OF ALOW-TEMPERATURE OVERPRESSURIZATION TRANSIENT LIMIT FOR PRECLUDING CRACK INITIATION

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. T he difficulty has emerged due to the embrittlement of the pressure ves sel steel from irradiation, and affects both reactor vessel integrity and functionality issues. Since the functional life of the reactor ves sel is dependent upon the ability to adhere to the RPV pressure-temper ature (P-T) limits and the low temperature overpressure protection req uirements, these normal operation constraints become quite important a nd control the reactor coolant system (RCS) operating window. This win dow is anticipated to close for many PWRs during their design life, pr eventing effective operation of the RCS and the RPV. To address this p roblem, the margin to incipient crack initiation associated with ASME Code Section III, Appendix G, fracture mechanics procedures has been e valuated and alternate brittle fracture limits for low temperature ove rpressure transients have been developed. These limits permit higher p ressures for transient conditions than those associated with the norma l operation P-T limits of Appendix G. The P-T limits for low temperatu re transient conditions have been developed, using the principles of l inear elastic fracture mechanics. The stress intensity factors, K(I) h ave been developed through the use of a superposition technique with i nfluence coefficients. The low temperature transient P-T limits preclu de 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 o verpressurization transient P-T limit for resolution of this operation al problem.