Recent results on neutron-irradiated RPV submerged-arc welds have revealed grain boundary segregation of phosphorus during irradiation, which may lead to intergranular fracture. However, the experimental database is insufficient to define the dependence of the process on variables such as dose, dose-rate and temperature. This paper describes work in which two existing models of phosphorus segregation, under thermal or irradiation conditions, have been developed to obtain predictions of these dependencies. The critical parameters in the models have been adjusted to give consistency with the available reference data, and predictions have been made of the dependence of segregation on a number of variables.

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This paper provides details of accelerated irradiation experiments on a Magnox mild steel submerged-arc weld at mean irradiation temperatures of 165°C and 269°C. The irradiation-induced changes in mechanical properties and weld microstructure have been determined. Irradiation increased the 40J transition temperature and the yield stress. The increase in transition temperature at a given dose was rather higher in the lower temperature irradiations. Irradiation-induced precipitates of about 2nm diameter were produced in both series of irradiations. Small angle neutron scattering data indicated a precipitate composition of about 75Cu25Mn (at%). Electron microscopy confirmed the presence of copper-rich precipitates of that size. The irradiation dose ranges covered the development stage of the precipitation. The directly measured and (yield stress) derived transition shift data were interpreted satisfactorily using a mechanistically based model involving a matrix damage component and a component attributable to copper precipitation.

Until very recently, the irradiation embrittlement behaviour of submerged-arc welds has been interpreted in terms of two mechanisms, namely a matrix damage component and an additional component due to the irradiation-enhanced production of copper-rich precipitates. However, some of the weld specimens from a recent accelerated re-irradiation experiment have shown high Charpy shifts which exceeded the values expected from the measured shift in yield stress. Microstructural examination has revealed the occurrence of intergranular fracture (IGF) in these specimens, accompanied by grain boundary segregation of phosphorus. Theoretical models were developed to predict the parametric dependence of irradiation-enhanced phosphorus segregation on experimental variables. Using these parametric forms, along with the concept of a critical level of segregation for the onset of IGF instead of cleavage, a three mechanism trend curve has been developed. The form of this trend curve, taking into account IGF as well as matrix and copper embrittlement, is thus mechanistically based. The constants in the equation, however, are obtained by a statistical fit to the actual Charpy shift database.

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