The Huge Missing Factor in LBB Analysis - How A Circumferential Through-Wall-Crack in A Pipe System Changes the Flexibility and Reduces the Applied Moments

In typical leak-before-break (LBB) analyses in the nuclear industry, the uncracked piping normal operating forces and moments are applied in a cracked-pipe analytical procedure to determine normal leakage, and the combined forces and moments under normal operating condition and safe shutdown earthquake seismic loading are used in a fracture analysis to predict margins on "failure". The International Piping Integrity Research Program (IPIRG) performed in 1990 to 1998 provided some insights to typical LBB behaviors where pipe system tests were conducted with simulated seismic loadings. The test results showed a large margin on LBB which was also recognized in 2011 when the Argentinian Atucha II plant was analyzed using a robust full FE model. It was found that when circumferential through-wall cracks were put in the highest stressed locations, the applied moment dropped for both normal operating and N+SSE loading as the crack length increased. The through-wall crack size for causing a double ended guillotine break (DEGB) was greater than 90%-percent of the circumference. Similar results were also found for a petrochemical pipe system where thermal expansion stresses are much higher than the primary stresses. Even with very low toughness materials, the critical crack size leading to DEGB was greater than 80% of the circumference. The implication of this work is that pragmatically there is much higher margin for DEGB failure in nuclear plant operation, and efforts would be better focused on the potential for a small-break loss-of-coolant accident (SB-LOCA).

Modeling and Simulation of Subcooled Coolant Loss Through Circumferential Pipe Leakage

The present work demonstrates the leak flow behavior of subcooled water at high pressure and high temperature through a narrow slit analogous to a pipe crack. The modeling and simulations are based on the loss of coolant accident in the primary loop piping of pressurized water-cooled reactors where a subcooled liquid is subjected to a rapid depressurization. Prediction of critical leak flow pattern is crucial in the design methodology of costly high energy pipelines in the perspective of leak before break consideration. Computational techniques have been used to replace costly experiments required for simulating leak flow conditions. For a variety of entry and exit conditions, the interphase mass transfer was studied with a change of boundary conditions. Presence of thermodynamic nonequilibrium has been detected on several occasions due to high transit velocities. A comparison with experimental findings indicates the validity of the flashing model for safety analysis of similar high energy thermal systems.

Predicting Pipe Rupture Frequencies Using xLPR

Over the last several years, the U.S. Nuclear Regulatory Commission (NRC), in cooperation with the Electric Power Research Institute (EPRI), conducted a multi-year project that focused on the development of a viable method and approach to address the effects of primary water stress corrosion cracking (PWSCC) in primary piping systems approved for leak-before-break (LBB). This project, called eXtremely Low Probability of Rupture (xLPR), defined the requirements necessary for a modular-based probabilistic fracture mechanics assessment tool to directly assess compliance with the regulations. Version 2.0 of this code has been completed and is currently awaiting public release. Since the focus of xLPR Version 2.0 is investigating the impacts of active piping degradation on the leak-before-break behavior of reactor coolant piping, questions have been raised to whether xLPR can be used to confirm pipe rupture frequencies developed in other efforts, such as NUREG-1829, “Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process.” This paper discusses an initial study focused on whether xLPR can be used to estimate pipe rupture frequencies. A series of analyses were conducted, based on inputs developed by the xLPR program team, focused on the reactor pressure vessel outlet nozzle geometry of a typical pressurized water reactor. Additional analyses were conducted using the same radius-to-thickness ratio but decreasing the pipe diameter. Due to computer memory restrictions, it was difficult calculating low probability events when considering PWSCC initiation, typical residual stresses, leak detection and in-service inspection. Therefore, to bound the problem, an aggressive weld residual stress was assumed with multiple pre-existing defects. By modifying the size and number of these initial defects, results were generated that indicated the conditional probability of rupture was related to the percentage of the inner circumference cracked and the pipe diameter. Using the PWSCC initiation model from xLPR Version 2, the yearly rupture frequency with leak detection and in-service inspection was calculated. The results indicate that the rupture frequencies in NUREG-1829 appear conservative relative to the results from this study. Due to the limited scope of this study, the assumptions used in these analyses were limited or conservative; therefore, additional analyses are needed for a more robust comparison. However, the results suggest that conducting xLPR analyses with pre-existing defects may be useful in bounding LBB applicability with active degradation.

Probabilistic Leak Before Break Using the R6 Methodology

The integrity of a component in a safety critical industry is determined by carrying out Engineering Critical Assessments (ECA). These are designed to provide a conservative estimate of the life of a component based on conservative inputs/methodology. It is becoming increasingly apparent that for many applications these methods are overly conservative. The only physical way to really assess the reliability of a component is by producing many thousands, if not millions of a specific component and calculating a failure probability based on testing/OPEX. This is simply not feasible for the components in, for example, a nuclear reactor, and probabilistic techniques are becoming increasingly important as a means to understand the reliability of a component. This information can then be used to assess risk and inform inspection programmes. Typically a probabilistic method relies on assigning distributions to various input parameters and evaluating a probability integral, usually by Monte-Carlo analysis. A previous PVP paper developed Monte Carlo methods using the R6 fracture mechanics procedure. Although providing good insight into the likelihood of failure, these analyses were simplified and not readily applied to realistic plant situations. Further development would enable much more of the technology contained within R6 to be applied within probabilistic software. The following new features of the software are presented in this paper: • the latest K and limit load solutions from R6 for through wall circumferential defects • Simplified V factor approach to account for secondary stresses • two phase flow (water) based on the latest SQUIRT methodology • global bending, through wall bending, weld residual stress This enables a full probabilistic leak detection calculation for circumferential through wall cracks in pipes. Examples of probabilistic Leak-before-Break calculations for PWR pipework are presented in the paper.

Crack Opening Areas Under Combined Primary and Secondary Loading

Leak-before-break is a methodology to assess whether a leak through a defect in a pressurised component can be detected prior to the defect attaining a critical size. Developing leak-before-break arguments in non-stress relieved piping components can be challenging, in part due to the lack of solutions available, including in R6, to predict the crack opening area (required to evaluate leak rate) for combined primary and secondary stresses under elastic-plastic conditions. This is because the nature of the secondary stresses is to relax with plasticity, which can be captured in the calculation of the crack driving force (elastic-plastic stress intensity factor), but methods to account for the additional crack-tip strain this induces and its influence on crack opening are not available. Here primary stresses are those resulting from an applied force, such as pressure, and secondary stresses are those which result from an internal mismatch and do not contribute to plastic collapse, such as thermal or residual stresses. There is, of course, potential for a higher accuracy of crack opening area evaluations from finite element analysis modelling approaches, which include elastic-plastic material properties, in the presence of combined loading scenarios. In this study, a series of finite element analyses have been conducted whereby crack opening area and stress intensity factor have been calculated from circumferential through-wall defects, under the influence of combined primary and secondary stresses, where the magnitude and order of the combined stress has also been varied. The crack opening areas have been compared to current elastic ‘handbook’ solutions, which are conservative for primary stresses, to better understand the effect of plasticity on crack opening area and to help inform assessments when accounting for the inclusion of plasticity with secondary stresses.