Embrittlement of Materials

The ageing materials European strategies (AMES) network was set up by the EC to bring together expertise on nuclear reactor materials (Gerard et al., 2001; Sevini et al., 1999). The most important area for research for effective plant life extension and management is the reactor pressure vessel (RPV), but metallic components in general (e. g. internals, pressuriser and piping) were targeted in AMES. The other principal areas are irradiation embrittlement and thermal ageing. In recent years, the network has been enlarged to include representatives from the Central and East European countries. AMES members collaborate in the TACIS and PHARE programmes to integrate findings for PWR and VVER LWRs.

The phosphorus influence on steel ageing (PISA) programme (English et al., 2001) is an experimental study to investigate the influence of phosphorus on RPV steel irradiation embrittlement. The objective is to improve understanding by segregating the phosphorus to grain boundaries and determining the effect of brittle inter-granular failure mechanisms on the RPV properties. The experiments focus attention on investigating various irradiated steels and metal alloys. The lack of phosphorus segregation data on certain steels under irradiation conditions relevant to end-of-life was recognised in a recent review (English et al., 2002). Both PWR and VVER reactor designs are covered in the project. The understanding of phosphorus segregation in irradiated and thermally aged fuels is now advancing significantly.

Another EC programme, fracture mechanics based embrittlement (FRAME) (Valo et al., 2001) aims to irradiate a relatively large number of different materials, chosen to determine the effects of chemistry on embrittlement. The objective is to develop fracture mechanics based trend curves. Irradiation shifts are measured and these are compared with existing Charpy-V (CH-V) based regulatory and other trend curves. Since the cleavage initiation fracture toughness material property KJC, is required for pressurised thermal shock (PTS) safety analyses, the availability of directly measured data will help to remove uncertainties (Sokolov and Nanstad, 2000) from the utilisation of CH-V test data.

The need for accurate data on neutron fluence to be used in conjunction with materials data is important for determining the life of nuclear power plant components, particularly the RPV. The RETROSPEC Dosimetry programme of the EC (Voorbraak et al., 2001) aims to provide retrospective fluence data by focussing on the niobium reaction 93Nb(n, n0)93Nbm. The methodology is being developed by examining specimens from material test programmes in research reactors, the Petten High Flux Reactor (HFR) and from specimens in surveillance capsules from the Dukovany NPP and the Loviisa NPP. Four steels have been selected, which are representative of the RPV in East European VVERs. The methodology is validated by comparing results from the retrospective analysis with the measured fluence at the locations of the specimens. It is concluded that retrospective dosimetry is useful in determining the neutron fluence at various locations inside a nuclear reactor, e. g. at RPV welds. Retrospective dosimetry has been reported previously by a number of researchers, see also van Aerle et al. (2000).

The EC GRETE programme (Delnondedieu et al., 2001) is concerned with the development of innovative non-destructive techniques for the inspection of critical components that may affect decisions on the lifetime of the plant. The objective is to assess techniques that aim to detect changes in materials before macro-structural defects occur, thus allowing remedial action to be taken. The techniques are evaluated in relation to neutron irradiation damage of the reactor vessel and the thermal fatigue of piping of the primary loops. Aged samples are being examined metallurgically and mechanically and then tested using various non-destructive techniques. All the known NDT techniques and their limits and limitations have been listed within the frame of the AMES project, see Delnondedieu et al. (2001) and Series of AMES reports (1975).

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *