Susceptibility of boiling water reactor pressure vessel and its internals to degradation

Otso Cronvall

Research output: ThesisDoctoral ThesisMonograph


This work concerns the susceptibility of boiling water reactor (BWR) unit reactor pressure vessel (RPV) and its internals to significant degradation modes. In the top level, this work consists of the following three parts: 1) literature survey and review, 2) component specific survey on susceptibility to degradation, 3) computational analysis approaches, tools and examples. The ageing of nuclear power plants (NPPs) emphasises the need to anticipate the possible degradation modes. The degradation potential of the NPP components is an important issue both in Finland and in other countries. The survey on the susceptibility to degradation concerns BWR RPV and all its significant internals as well as an investigation of loads they experience. A screening process is performed on the need to carry out computational degradation potential analyses. The computational part consists of a description of applicable analysis approaches as well as of several representative computational examples. The covered approaches are both analytical and numerical, as well as both deterministic and probabilistic. New procedures are also developed, which are also used in the analysis examples. For the screening process, the component specific load induced stresses, strains and temperatures are computed. Using that and other data, the potential to brittle, ductile or other degradation is analysed for the selected components. The analysis targets are the Finnish Olkiluoto NPP units OL1 and OL2 run by the power company TVO. Based on the degradation analysis results, it is concluded that the operational lifetime of the internals of the OL1/OL2 RPVs can be safely continued from 40 to at least 60 years. Importantly, it is concluded that the operational lifetime of the OL1/OL2 RPVs and connecting main nozzles can be safely continued from 40 to even 80 years. According to the conservative degradation potential analysis results, the degradation in terms of crack growth is in most cases very or extremely slow. In the few cases with faster crack growth the cracks would be detected in the inspections well before they grow to any significant size. The structural risk assessment results for the OL1/OL2 RPVs and their internals show that for all components but one the computed risk class is moderate or lower. It is concluded that for the OL1/OL2 RPV and their internals the overall structural risks are small and even in the case with highest risk acceptable.
Translated title of the contributionKiehutusvesireaktorin reaktoripaineastian ja sen sisäosien alttius vaurioitumiselle
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Paavola, Juha, Supervising Professor
  • Wallin, Kim, Thesis Advisor, External person
Print ISBNs978-952-64-0029-7
Electronic ISBNs978-952-64-0030-3
Publication statusPublished - 2020
MoE publication typeG4 Doctoral dissertation (monograph)


  • BWR
  • RPV
  • internals
  • degradation
  • structural mechanics
  • fracture mechanics


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