Pressurized Thermal Shock Analysis For Reactor Pressure Vessel

The technical basis of the pressurized thermal shock (PTS) rule andreguide,10CFR50.61and R.G1.154developed by the U.S.NRC in the1980s,contained significant conservatisms. Thus, the screening limits restricted boththe licensable and economic lifetime of PWRs.From1999, the NRC conducted a study to develop the technical basis,including the refinement of approaches, models, parameters and treatment ofuncertainties. NRC realized transients and material factors that dominate PTSrisk, and recommended to adopt the new through-wall cracking frequencylimit1x10-6/r y and screening criteria based on the reference temperature ofnil-ductility transition. The PTS screening limit10CFR50.61（a）was passedin2010.For a series of PTS transients, the thermal-hydraulic responses of atypical conventional PWR were calculated with the thermal-hydraulic systemanalysis program RELAP5, and fracture mechanics analyses of the RPVmodel were calculated with the finite element analysis software ANSYS. Furthermore, the risk assessment was performed under these PTS transients,and the effect of RPV material embrittlement was studied.The analysis indicated that LBLOCAs were more dangerous than stuckopen valves without repressurization and MBLOCAs and that stress intensityfactors were dominated by thermal stresses in MBLOCAs and LBLOCAs,while stress intensity factors were dominated by tensible stress in stuck-openvalves that later reclosed. Surface flaws were more vulnerable to producecrack initiation than buried cladding flaws, while those deeper in the RPVwall were not going to to produce crack initiation. Besides, axial flaws weremore vulnerable to produce crack initiation than circumferential flaws underthe same conditions, and axial flaws were much more likely to propagatethrough the RPV wall than circumferential flaws in MBLOCAs andLBLOCAs.