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The main tasks and problems of VVER reactor pressure vessel (RPV) dosimetry are reviewed taking into account the RPV metal characteristics, the peculiarities of surveillance programs and some features of irradiation embrittlement of VVER-440 and VVER-1000 RPV materials. The problems of dosimetry of VVER-1000 surveillance program are discussed separately. The experimental dosimetry programs, which are carried out in this time, are briefly summarized. It's concluded that there are the serious reasons for improvement of VVER dosimetry.

The influence of impurity grain boundary segregation on mechanical properties of low-alloy Cr-Ni-Mo steels is discussed. Mechanisms of impurity and alloying effects on grain boundary embrittlement of low-alloy steels are considered. An approach for estimation of radiation enhanced grain boundary segregation kinetics is proposed. The approach takes into account the reduction of dislocation density (from 1×1014 to 1×1013 m-2) due to irradiation that was observed for low-alloy Cr-Ni-Mo steels. The influence of collision cascades on the radiation damage is also considered. Auger spectroscopy is used for grain boundary segregation detection. The contribution of grain boundary embrittlement to mechanical property degradation due to neutron irradiation is assessed.

The DBTT shift in irradiated VVER pressure vessel steels is represented by standard guidelines with a power law dependence on fast neutron fluence multiplied by a material constant. However, from recent data on surveillance specimens for several units of VVER-440, an irradiation embrittlement enhancement has been observed at neutron fluences above (2–3)×1020 n/cm2 (E>0.5 MeV) with a significant deviation from the standard guideline. This suggests an additional mechanism of irradiation embrittlement at high fluences. In the present work it was supposed that such a mechanism could be intergranular embrittlement caused by grain boundary phosphorus segregation. A threshold dose was used that accounts for intergranular embrittlement, that occurs after sufficient phosphorus segregation, when the critical stress for intergranular fracture falls below that for cleavage fracture. The modeling of phosphorus accumulation on grain boundaries is performed taking into account the radiation-enhanced phosphorus diffusion by both vacancy and interstitial mechanisms, the radiation-induced segregation in the matrix near grain boundaries, and the Gibbsian adsorption on grain boundaries. Predictions of such a composite model are in agreement with surveillance data for VVER-440.

Radiation embrittlement and its mitigation by annealing of VVER-440 reactor pressure vessel steels are studied. The Russian regulatory approach for prediction of radiation embrittlement of VVER-440 steels is considered. Results of an investigation of materials cut out of operating nuclear power plants are discussed. Different models of re-irradiation embrittlement are compared. A new model for assessment of embrittlement under re-irradiation is developed.