Probabilistic fracture mechanics analysis software

Central to the evolution of risk related criteria for assessing RPV integrity in the USA was the development of the PFM software FAVOR (Williams et al. 2007). The FAVOR software was produced at Oak Ridge National Laboratory under sponsorship of the USNRC and received extensive verification and validation as part of the USNRC PTS study. Because the 

FAVOR software was recently employed to define licensing criteria (NRC 2010b) and ASME code guidelines (ASME, 2010) for RPV integrity the description of the PFM methodology in this chapter is based on the methodology in the FAVOR software. 

The CPI is computed for each flaw in a distribution of flaws in a trial vessel. Flaws that have a non-zero probability of extending under the applied loading conditions are then evaluated to compute the probability that the flaw will grow through the vessel wall. The CPI and CPF computations are repeated for a large number of trial vessels to obtain CPI and CPF distributions. The CPI and CPF distributions are combined with frequency distributions for each transient included in the analysis to obtain distributions of crack initiation and TWCF. 

The load module computes the stress and temperature distributions in the vessel wall as a function of time during the transient. These time histories then are used to compute the applied stress intensity factor, Ki, time history for surface flaws. The PFM module uses the stress and temperature time histories to compute the Ki time history for embedded flaws, and then uses the Ki time histories for either surface or embedded flaws and the irradiation 

The flaw, irradiation degradation and toughness related variables are sampled from statistical distributions. The computations include consideration of uncertainty in the distributions and in the values sampled from the distributions. Pressure, stress, temperature, and geometry related variables are not sampled during the analyses. The software uses deterministic models for the temperature dependence of the appropriate material physical and mechanical properties. 

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