Irradiation Embrittlement Modelling of Linde 80 Weld Metals

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Abstract
Linde 80 weld metals are characterised by a narrow range of chemical compositions, except for copper content, which varies from 0.2–0.42 wt%. The surveillance database for Linde 80 weld metals constitutes a unique and large database on a single class of material for evaluating embrittlement models and property correlations used in the assessment of RPV embrittlement. Correlations between Charpy transition temperature shifts and yield strength, which best characterises the hardening behaviour, were tested and it was concluded that fracture appearance transition temperature (FATT) provided less scatter and a more representative indicator than either lateral expansion or Charpy 30 ft-lb energy. This is believed to be a consequence of the low upper shelf of Linde 80 weld metals distorting the latter two transition temperatures, especially after irradiation, which induces a significant reduction in upper-shelf to levels approaching the indexing values. The main purpose of the present study has been to obtain a simple two component barrier hardening model form which best describes the behaviour of Linde 80 weld metal. To this end, the UK Magnox Embrittlement Model, often referred to as the Fisher Model, has been used and the key parameters, such as activation energies, dislocation densities and damage cross sections, were used as fitting parameters. Good agreement was found between model predictions and the surveillance yield strength data following optimisation of model parameters. Applying the latter optimised form produced equally good prediction of HSSI results at a significantly higher damage rate demonstrating the “rate effect” inherent in the irradiation enhanced copper precipitation process. The best agreement was obtained assuming a matrix copper content of about 0.23 wt% which is consistent with recent solubility data for Linde 80 welds. For these materials an insensitivity to bulk copper content was demonstrated even though this varied from 0.21–0.42 wt%. Additionally, the parameters optimised for high copper welds provided a good prediction for low-copper materials indicating that the matrix hardening component was correctly specified. The low matrix hardening and predicted completion of copper precipitation by doses of about 1×1019ncm−2 support the notion of embrittlement saturation at high doses.