N. Ghadiali

Probabilistic pipe fracture evaluations for leak-rate-detection applications

Regulatory Guide 1.45, {open_quotes}Reactor Coolant Pressure Boundary Leakage Detection Systems,{close_quotes} was published by the U.S. Nuclear Regulatory Commission (NRC) in May 1973, and provides guidance on leak detection methods and system requirements for Light Water Reactors. Additionally, leak detection limits are specified in plant Technical Specifications and are different for Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs). These leak detection limits are also used in leak-before-break evaluations performed in accordance with Draft Standard Review Plan, Section 3.6.3, {open_quotes}Leak Before Break Evaluation Procedures{close_quotes} where a margin of 10 on the leak detection limit is used in determining the crack size considered in subsequent fracture analyses. This study was requested by the NRC to: (1) evaluate the conditional failure probability for BWR and PWR piping for pipes that were leaking at the allowable leak detection limit, and (2) evaluate the margin of 10 to determine if it was unnecessarily large. A probabilistic approach was undertaken to conduct fracture evaluations of circumferentially cracked pipes for leak-rate-detection applications. Sixteen nuclear piping systems in BWR and PWR plants were analyzed to evaluate conditional failure probability and effects of crack-morphology variability on the current margins used in leak rate detection for leak-before-break.

Degraded piping program-phase II progress

Abstract This paper summarizes the results from the NRC Degraded Piping Program over the last year. The objective of the NRC Degraded Piping Program - Phase II, is to verify limit-load analyses and develop elastic-plastic fracture mechanics analyses methods for cracked (degraded) nuclear piping under a variety of loading conditions. These analyses are used in leak-before-break evaluations. Since experimental efforts are conducted at LWR temperatures, failure modes and metallurgical phenomena of concern are also being assessed.

A computer model for probabilistic leak-rate analysis of nuclear piping and piping welds

Abstract This paper describes the development of a computer code entitled PSQUIRT for probabilistic evaluations of leak rate in nuclear piping. It is based on (1) the Henry-Fauske model of two-phase flow for thermal-hydraulic analysis and (2) an estimation model for elastic-plastic fracture-mechanics analysis. In both analyses, uncertainties arise due to the incomplete knowledge of the crackmorphology 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 standard 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 variability of leak rate can be significant due to statistical scatter of crack-morphology parameters. Using these histograms, the subsequent fracture stability of a leaking crack, actural or hypothetical, can be evaluated by either a deterministic or a probabilistic method.

Effects of off-centered cracks and restraint of induced bending caused by pressure on the crack-opening-area analysis of pipes☆

Abstract Current models for the crack-opening-area analysis of pipes with circumferential through-wall cracks are based on various idealizations or assumptions which are often necessary to simplify the mathematical formulation and numerical calculation. This paper focuses on the validity of two such assumptions that involve off-centered cracks and the restraint of induced bending caused by pressure, and quantifies their effects on the crack-opening area analysis of pipes. Finite element and/or simple estimation methods were employed to compute the center-crack-opening displacement and crack-opening shape for a through-wall-cracked pipe, considering off-centered cracks and the restraint of induced bending caused by pressure. The results of the analyses show that, for both cases, the crack-opening area can be reduced significantly. For pipes with off-centered cracks, the crack-opening area can be evaluated from analyses of symmetrically centered cracks and assuming elliptical profile. For pipes with complete restraint of the induced bending caused by pressure, the reduction in crack-opening area depends on the crack size. When the crack size is small, the restraint effects can be ignored. However, when the crack size is large, the restrained crack opening can be significantly smaller than the unrestrained crack opening, depending on the length of pipe involved; hence, it may be important for the crack-opening-area and leak-rate analyses.

Deterministic and probabilistic evaluations for uncertainty in pipe fracture parameters in leak-before-break and in-service flaw evaluations

This report presents new results from deterministic and probabilistic analyses to evaluate the significance of a number of technical aspects that may affect LBB or in-service flaw evaluations. The following summarizes the objectives and results from both the deterministic and probabilistic studies. The reasons for including each technical aspect being evaluated are given first. Then a table is given that summarizes the relative significance of each technical aspect. In most cases there are both deterministic and probabilistic results. The deterministic analyses were conducted independently of the probabilistic analysis, which offered the opportunity to validate conclusions from each of these studies.