Material properties for structural integrity assessment of reactor pressure vessel shells affected by hydrogen flaking

Robert Gérard, Rachid Chaouadi

    Research outputpeer-review


    During the summer outages of 2012, large numbers of nearly-laminar indications were found in the core shells of the Doel 3 and Tihange 2 reactor pressure vessels (RPV). As a consequence, both units remained in cold shutdown with their core unloaded. A series of examinations, tests and inspections were performed leading to the conclusion that the indications are hydrogen flakes and that they do not affect the structural integrity of the RPV, regardless of the operating mode, transient or accident condition. All this was documented in the Safety Case reports issued in December 2012 and in the Safety Case Addenda issued in April 2013. Based on those reports, the Belgian Federal Agency for Nuclear Control (FANC) authorized the restart of both units which went back on-line in June 2013. A key input required for this Safety Case was the definition of the material properties for the RPV shells affected by hydrogen flakes, to be used for the structural integrity analyses. Of particular importance in this respect is the fracture toughness, defined in the current regulatory approach by generic curves given in the ASME code in function of an equivalent temperature T-RTNDT. A material testing program on non-irradiated materials evaluated aspects like the effect of specimen orientation on the fracture toughness, the effect of segregations and ghost lines and the properties of a flaked material in unirradiated condition. This was the basis of a conservative trend curve of RTNDT evolution vs. fluence used in the Safety Cases submitted in 2012-2013. The restart authorization in 2013 was accompanied by a number of “mid-term” requirements, to be completed during the first operating cycle after the restart. One of these requirements was the mechanical testing of irradiated specimens containing hydrogen flakes, in order to confirm the conservativeness of the RTNDT trend curve used for the structural integrity analyses. After a first irradiation campaign of a material containing hydrogen flakes in the BR2 reactor of the Belgian Nuclear Research Center SCK.CEN, atypical results were obtained and the utility decided to shut down the units in March 2014. Detailed investigations involving three additional irradiation campaigns including other reference materials, among which another material affected by hydrogen flakes, were performed in order to characterize this atypical behaviour and to derive a new conservative RTNDT trend curve. The resulting trend curve was accepted by the FANC. The restart of the units was authorized in November 2015. The paper summarizes the results of the investigations and tests on non-irradiated and irradiated materials and the process leading to the definition of this conservative RTNDT trend curve. A more detailed description can be found in [3]. An overview of the Safety Cases is presented in [2] and the full reports as well as the FANC reports can be found on the FANC website [1].
    Original languageEnglish
    Title of host publicationEuropean Nuclear Society
    PublisherENS - European Nuclear Society
    Number of pages10
    StatePublished - 10 Oct 2016
    Event2016 - ENC - European Nuclear Conference - Sofitel Warsaw Victoria, Warsaw
    Duration: 9 Oct 201613 Oct 2016

    Publication series

    NameSCK•CEN Reports
    PublisherStudiecentrum voor Kernenergie


    Conference2016 - ENC - European Nuclear Conference
    Abbreviated titleENC2016
    Internet address

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