CODE SEC.XI BASED ON FAILURE PROBABILITY

ABSTRACT Based on the faihre probability, the flaw acceptaao~ standard of ASME Code Sec. XI is examined with some concerns weather tbe faihre l~vbabflity is unif~in for flaws with various aspect ratios and failure frcquoneies are small enough. In this paper, the results of lreliminmy case studies are deseff0ed on liae failure probability of n~etor pressure vessels (RPVs) with a surface flaw specified in Scc. XI. PFM code PASCAL was used for case studies. A PTS (Pressurized Thermal Shock) Iransient prescn'bcd by NRC2EPRI ITS Benchmark Study was used as an applied load. Analysis results showcd that the conditional failure probability of a RPV wilh ~m initial flaw of acceptable depth depends on the aspect mlio. h case flaw' shapes ~ze close to semi-circular, the failme probability are higher than that of the eases aspect ration are less than 0.6 by one order of magnitude due to the diffczeace of fracture behavior at the surface point. A case study for determining the acceptable flaws based on failure probability was also eanied on

STRESS INTENSITY FACTOR ESTIMATION FOR EMBEDDED AND SURFACE CRACKS IN AN RPV SUBJECTED TO YIELDING OF CLADDING

ABSTRACT The cladding of an RPV (Reactor Pressure Vessel) at the inner surface may be subjected to a plastic yielding due to a high thermal stress under some severe overcooling events, while the stress in base metal remains elastic. The stress intensity factor estimation at the deepest crack tip of an embedded crack (EC) or a surface crack (SC) under such loadings is essential in the integrity assessment of an RPV. However, an elasto-plastic FE analysis is required to obtain the stress intensity factor on this problem generally. A solution for an under-clad crack (UCC) was developed by EDF based on numerous 2D FE computations. This paper proposes a simplified estimation scheme which takes the yielding of cladding into account. This scheme estimates the stress intensity factor at the deepest crack tip of an embedded crack or a surface crack. It is assumed that the stress singularity does not exist at the shallowest crack tip in the cladding. To estimate the stress intensity factor of an embedded crack, the crack shape is replaced by a semi-elliptical one to utilize existing solutions of SC. Case studies to examine the proposed estimation scheme were carried out for an UCC and SC subjected to the SBLOCA, SLB and PTS transients, which were defined by a international round robin PROSIR (Probabilistic Structural Integrity of a PWR Reactor Pressure Vessel) project being conducted by OECD/NEA/IAGE-WG. It was found that the proposed scheme gives a reasonable estimation of the stress intensity factor for an UCC and a SC

IMPROVEMENT OF PROBABILISTIC FRACTURE MECHANICS ANALYSIS CODE FOR REACTOR PIPING CONSIDERING LARGE EARTHQUAKES

ABSTRACT The probabilistic fracture mechanics analysis code PASCAL-SP is improved by introducing crack-growth evaluation methods based on J-integrals, including calculation functions of J-integral values for semi-elliptical surfaces and through-wall cracks in pipes. Using the improved PASCAL-SP, sensitivity analyses that varied parameters such as earthquake magnitude were carried out on the basis of probabilistic evaluation. Results obtained from sensitivity analyses are also presented, e.g., the effect of earthquake magnitude on failure probability. The improved PASCAL-SP makes evaluation of the failure probability of piping under large seismic loading possible