Uncertainty evaluation in multi-state physics based aging assessment of passive components

A methodology is presented to evaluate aging degradation of passive components under uncertainty. Stress corrosion cracking (SCC) degradation is selected as the example aging phenomenon and the methodology is implemented on the pressurizer surge line pipe weld of a pressurized water reactor. The degradation is described as a multi-state model consisting of six differential equations with system history dependent transition rates. The input data to the model include operating temperature, weld residual stress, stress intensity factor, thermal activation energy for crack initiation and crack growth. The associated uncertainties are represented by probability distributions derived from historical data, experimental data, expert elicitation, physics, or a combination of these. Latin Hypercube Sampling is used to generate observations from the distributions governing these parameters with a two-step approach that distinguishes between aleatory and epistemic uncertainties. The degradation model is solved by a semi-Markov approach using the concept of sojourn time to account for system history dependence of transition rates. The results are compared to a single step sampling process. The results show highest sensitivity of damage to weld residual stress.