B. Štok, P. Koc, N. Mole, T. Rojc

This contribution presents results of analyses that were performed in order to evaluate the degree of safety and serviceability of the reactor coolant pump (RCP) casing of the Krško NPP, according to the ASME Code Case N-481. The corresponding analyses included the determination of stress fields, location of critical points with respect to the flaw existence, and finally, investigation of the stability and fatique growth of the postulated flaws.

The thermo-mechanical response of RCP casing under normal, upset, emergency and faulted operating conditions has been considered. To simulate those conditions within a system, determined by RCP itself, proper boundary conditions should be set on the RCP nozzles and RCP supports. However, to avoid speculations regarding the uncertainty of those boundary conditions, an adequately enlarged system, containing large components of the RCS, such as reactor pressure vessel, steam generator and RCP, together with their piping connections and supports, was considered first. Accordingly, the structural components of the enlarged model were subjected to dynamic analysis by simulating typical operating conditions. In the investigated cases thermal and pressure transient conditions, as met during regular exploitation of the power plant, as well as those associated with seismic exitation and two accidents, the loss of coolant accident (LOCA) and the steam line break accident (SLB), respectively, were taken into account. From a series of time dependent load cases several critical combinations, relevant for the fracture mechanics analysis, were identified.

With the numerical model, corresponding to the enlarged system, loads, as resulted from the interaction with the remaining part of the reactor coolant system (RCS) and acting on the RCP casing, were identified with a greater degree of reliability. Thus, for the subsequent analysis of the RCP casing itself the boundary conditions were prescribed in accordance with the computed thermo-mechanical response on the boundary of the sub-system, appertaining to the RCP casing. Then, the thermo-mechanical analysis of the RCP casing, based on the refined numerical model, was performed by considering loading cases identified in the previous investigations as critical. In view of the possible flaw existence the results of this analysis were used for the identification of most stressed parts of the casing.

Crack stability analysis and fatigue growth investigation for several postulated cracks were finally carried out. In order to assess the stress distribution in the surrounding of a postulated crack a localized sub-domain, cut off the RCP casing model, was correspondingly discretized. Again, for the analysis of the cracked model the boundary conditions were taken from the computed thermo-mechanical response of the global RCP casing model. For both, crack stability analysis and fatigue growth, the J-integral concept was used. For the evaluation of the crack growth, the J-integral values were transformed, according to the linear fracture mechanics, to the stress intensity factors K, and then, using the Paris law, the progress of each of the postulated cracks was calculated.