Structural lifetime, reliability and risk analysis approaches for power plant components and systems: Licenciate thesis

Lifetime, reliability and risk analysis methods and applications for structural systems and components of power plants are discussed in this thesis. These analyses involve many fields of science, such as structural mechanics, fracture mechanics, probability mathematics, material science and fluid mechanics. An overview of power plant environments and a description of the various degradation mechanisms damaging the power plant systems and components are presented first. This is followed with a description of deterministic structural analysis methods, covering e.g. structural mechanics and fracture mechanics based analysis methods as well as the disadvantages of the deterministic analysis approach. Often, physical probabilistic methods are based on deterministic analysis methods with the modification that one or more of the model parameters are considered as probabilistically distributed. Several probabilistic analysis procedures are presented, e.g. Monte Carlo Simulation (MCS) and importance sampling. Description of probabilistic analysis methods covers both physical and statistical approaches. When the system/component failure probabilities are combined with knowledge of failure consequences, it is possible to assess system/component risks. Several risk analysis methods are presented as well as some limitations and shortcomings concerning to them. Modelling methods for various degradation (or ageing) mechanisms are presented. These methods are needed in the lifetime analyses of structural systems and components of power plants. In general, the lifetime analyses in question necessitate a thorough knowledge of structural properties, loads, the relevant degradation mechanisms and prevailing environmental conditions. The nature of degradation models of structural systems/components can be deterministic, probabilistic or a combination of these two types. Degradation models of all these kinds are presented here. Some important risk analysis applications are described. These include probabilistic risk/safety assessment (PRA/PSA) and risk informed in-service inspections (RI-ISI). In practise, lifetime and risk analyses are usually performed with a suitable analysis tool, i.e. with analysis software. A selection of probabilistic system/component degradation and risk analysis software tools is presented in the latter part of this thesis. Computational application of probabilistic failure and lifetime analyses to a representative set of NPP piping components with probabilistic codes VTTBESIT and PIFRAP are presented after that. The thesis ends with a summary and suggestions for future research.