Mikko I. Jyrkama

Probabilistic Estimation of Flow-Accelerated Corrosion Rate at the Welded Joints of the Nuclear Piping System

Feeder piping is an integral part of the heat transport system (HTS) that supplies the primary coolant from the reactor to the steam generator in CANDU reactors. One of the life limiting factors of the feeder pipes is the highly localized wall thinning caused by the flow-accelerated corrosion (FAC) at the welded joints of the pipes. To ensure the fitness-for-service of the piping system, periodic inspections of the pipe wall thickness and estimation of the FAC thinning rate at the welded joints are needed.A major challenge of FAC rate estimation at the welded joints is that the initial wall thickness is known precisely, since the grinding process before the welding introduces initial thinning in the pipes. Using the nominal wall thickness without considering this initial thinning is likely to overestimate the FAC rate. Another difficult is the sizing error in the wall thickness measurements, which also needs to be properly accounted.This paper develops a sound probabilistic method for the FAC estimation for the welded joints considering both the initial thinning and sizing error. Predictions regarding the lifetime of individual welded joints are also obtained from the proposed method. A practical case study of the problem in a nuclear plant is presented.© 2012 ASME

Integration of Degradation Models Into Generation Risk Assessment: Challenges and Modeling Approaches

The main objective of generation risk assessment (GRA) is to assess the impact of equipment unavailability and failures on the ability of the plant to produce power over time. The system reliability models employed for this purpose are based on the standard fault tree/event tree approach, which assumes failure rates to be constant. However, this ignores the impact of aging degradation and results in static estimates of expected generation loss. Component and equipment degradation not only increases the probability of failure over time, but also contributes to generation risk through increased unavailability and costs arising from greater requirement for inspection and replacement of degraded components. This paper discusses some of the key challenges associated with integrating the results of component degradation models into GRA. Because many analytical and simulation methods are subject to limitations, the methodology and modeling approach proposed in this work builds on the current GRA practice using the fault tree approach. The modeling of component degradation can be done separately at the fault tree cut set level, assuming the cut sets are independent and the component unavailabilities are relatively small. In order to capture the joint contribution of equipment failure and unavailability to generation risk, new risk-based importance measures are also developed using the concept of net present value.

The Impact of Flow Accelerated Corrosion (FAC) on Feeder Life Cycle Management

Wall thinning of outlet feeder piping by flow accelerated corrosion (FAC) is a serious form of degradation affecting some CANDU® stations. The general and localized loss of wall thickness is typically highest at or near welds and changes in pipe geometry due to increased velocity or turbulence. While the process is not a high safety concern because catastrophic failure is unlikely, feeder wall thinning may result in significant economic losses as a result of forced shutdowns for repair and replacement. Accurate modelling and prediction of feeder replacements and the probability of feeder failure is not only important for continued fitness-for-service, but essential for feeder life cycle management (LCM). This paper discusses some of the key issues related to feeder FAC, and presents a probabilistic method for modelling the wall thinning process. The wall thickness loss due to FAC is modelled using a random rate model, while the probability of feeder failure is based on an empirical approach. The developed methodology allows the estimation of the remaining life of both inspected and uninspected feeder populations, while methodically accounting for the uncertainties in the problem.Copyright

Integration of Degradation Models Into Generation Risk Assessment (GRA): Challenges and Modeling Approaches

The main objective of generation risk assessment (GRA) is to assess the impact of equipment unavailability and failures on the ability of the plant to produce power over time. The system reliability models employed for this purpose are based on the standard fault tree /event tree approach, which assumes failure rates to be constant. However, this ignores the impact of aging degradation and results in static estimates of expected generation loss. Component and equipment degradation not only increases the probability of failure over time, but also contributes to generation risk through increased unavailability and costs arising from greater requirement for inspection and replacement of degraded components. This paper discusses some of the key challenges associated with integrating the results of component degradation models into GRA. Because many analytical and simulation methods are subject to limitations, the methodology and modeling approach proposed in this work builds on the current GRA practice using the fault tree approach. The modeling of component degradation can be done separately at the fault tree cut set level, assuming the cut sets are independent and the component unavailabilities are relatively small. In order to capture the joint contribution of equipment failure and unavailability to generation risk, new risk-based importance measures are also developed using the concept of net present value (NPV).© 2009 ASME