Fluence rate effect semi-mechanistic modelling on WWER-type RPV welds

Effort at JRC-IE is ongoing in order to develop a semi-mechanistic model to forecast radiation embrittlement. The understanding and the quantification of the influence of the fluence rate is of particular importance for the correct interpretation of data obtained in material testing reactors or in surveillance capsules, which are accelerated with respect to embrittlement of the reactor pressure vessel wall itself. To verify the applicability of the fluence rate as included in the semi-mechanistic model and tuning the model parameters various WWER-type vessel weld material have been studied. For the selected welds, copper ranges from 0.08 to 0.18 mass%, while phosphorus variation is from 0.013 to 0.036 mass%. The fluence range is up to 2 × 1020 n cm−2 obtained at two fluence rates of 4 × 1011 and 3.5 × 1012 n cm−2 s−1, typical for WWER-440 surveillance positions. Significant fluence rate effect has been observed for the welds containing low copper and moderate phosphorus, and adaptation of the semi-mechanistic model's parameters for the high flux data is required. To verify the consistency and the limits of the findings other similar data coming from RPV surveillance programmes are also included in this analysis.