F. H. E. de Haan-de Wilde’s scientific contributions

For many nuclear power plants worldwide the operation period will be extended from 40 to 60 years. As the operation period increases the knowledge of ageing mechanisms like fatigue becomes more important. Knowledge of the influence of the environment is crucial, since environmental fatigue results in a modification of the existing fatigue assessment method and possible reduction of the fatigue life. This paper contains the results of an extended literature survey of environmentally assisted fatigue in nuclear power plants. It describes the current status and developments in the world. The main regulatory rules, guidelines and methods from the US, Germany, Japan, Finland, France and Czechia are presented. Although the general trend towards a more uniform approach worldwide has begun, considerable different approaches for incorporating the effect of coolant water environment exist. The most common approach is the incorporation of an environmental fatigue correction factor (Fen) in the fatigue derivation of the cumulative usage factor. The Fen formulas and the S-N fatigue curves differ but the general equations are: Fen = Nair/Nwater and CUF = Σ Upartial * Fenpartial Alternatives like using fatigue curves including the environmental effects, using threshold criteria and calculation of an allowable Fen based on testing, are described. Research and material tests are on-going and subject of international development. An overview of the current international state-of-the-art is presented.

The reactor vessel of the High Flux Reactor (HFR) in Petten has been fabricated from Al 5154 - O alloy grade with a maximum Mg content of 3.5 wt. %. The vessel experiences large amount of neutron fluences (notably at hot spot), of the order of 1027 n/m2, during its operational life. Substantial damage to the material’s microstructure and mechanical properties can occur at these high fluence conditions. To this end, a dedicated surveillance program: SURP (SURveillance Program) is executed to understand, predict and measure the influence of neutron radiation damage on the mechanical properties of the vessel material. In the SURP program, test specimens fabricated from representative HFR vessel material are continuously irradiated in two specially designed experimental rigs. A number of surveillance specimens are periodically extracted and tested to evaluate the changes in fracture toughness properties of the vessel as a function neutron fluence. The surveillance testing results of test campaigns performed until 2009 were already published by N. V. Luzginova et. al. [1]. The current paper presents results from the two recent surveillance campaigns performed in 2014 and 2015. The fracture toughness and tensile testing results are reported. Changes in mechanical properties of Al 5154-O alloy with an increase in neutron fluence are discussed in correlation with the irradiation damage microstructure observed in TEM and the fracture morphology observed in SEM. The HFR surveillance testing results are compared to the historically published results on irradiated aluminum alloys and conclusions about the evolution of embrittlement trends in relation with irradiation induced damage mechanisms in HFR vessel are drawn at the end.

NPP Borssele is in operation since 1973. In the framework of the long term operation (LTO) of the NPP Borssele until 2034, the integrity of the safety relevant systems, structures and components in terms of physical ageing will have to be demonstrated during the years beyond 2013. Most of the fatigue analyses in the current safety report are based upon 40 years of operation. Beyond 2013, this lifetime will be passed, hence the safety report has to be adapted for the extra 20 years of operation. In the beginning of 2011 Elektriciteits-Produktiemaatschappij Zuid-Nederland (EPZ) has planned to issue a application in order to adapt the safety report. Renewed ageing analysis reports and supporting documentation will outline the basis for the application. The strategy report on the revalidation of the fatigue assessments at the NPP Borssele is part of the supporting documentation. EPZ has initiated several projects on the assessment of fatigue for LTO. The first fatigue project was the review of the existing fatigue analyses of NPP Borssele. The second project is subject of this compact and is called the strategy report on the revalidation of the fatigue assessments far beyond 2013. An investigation of the necessary measures for prolongation of the operation considering fatigue will be made. During this further work the fatigue analyses will be revalidated until 2034 and more necessary precautions can be taken to guarantee the safe operation until 2034 considering the fatigue effects.