A COMPUTER MODEL FOR PROBABILISTIC LEAK-RATE ANALYSIS OF NUCLEAR PIPING AMD PIPING WELDS

摘要

This paper describes the development of a computer code titled PSQUIRT for probabilistic evaluations of leak rate in nuclear piping. It is based on (1) Henry-Fauske model of two-phase flow for thermal-hydraulic analysis and (2) estimation model for elastic-plastic fracture-mechanics analysis. In both analyses, uncertainties arise due to the incomplete knowledge of the crack-morphology variables and statistical scatter of the pipe material properties. The relevant parameters required to conduct these analyses were modeled as random variables. Henceforth, the above thermal-hydraulic and fracture-mechanics models were put in a probabilistic format to allow statistical variability of input and determination of their effects in pipe fracture and leak-before-break (LBB) evaluations. A Monte Carlo simulation technique was used to perform the probabilistic analysis. Numerical examples are presented to illustrate the capabilities of the PSQUIRT code. Probabilistic analyses were performed by PSQUIRT for a stainless steel and a carbon steel pipe. Histograms were developed for leakage rate and flaw size in these pipes for LBB applications. The results suggest that the variabilities of flaw size and leak rate can be significant due to statistical scatter of crack-morphology parameters. Using these histograms, the subsequent fracture stability of a leaking crack can be evaluated by either a deterministic or a probabilistic method.