Comparison of the high Ni VVER-1000 weld microstructure under the primary irradiation and re-irradiation

This paper presents the first comparative experimental assessment of the microstructures of the same surveillance specimens of VVER-1000 high Ni weld: after the primary irradiation, recovery annealing and re-irradiation in the same industrial NPP (Balakovo NPP unit 1) to assess the possibility of its lifetime extension after annealing up to 60 years. Therein, studies of both the initial (carbides) and radiation-induced hardening phases (precipitates and dislocation loops), and the behavior of grain boundary segregation as well are presented. It has been shown that the microstructures of high Ni VVER-1000 weld after the primary irradiation and re-irradiation after recovery annealing up to the close values of fast neutron fluences are identical: the characteristics of both initial and radiation-induced hardening phases, as well as grain boundary segregation, are the same. •VVER-1000 high Ni weld is studied by APT, TEM, AES in primary and re-irradiated states.•parameters of hardening elements are equal in primary and re-irradiated states.•Grain boundary segregation level is the same in primary and re-irradiated states.•Re-embrittlement rate of high Ni VVER-1000 welds should not exceed the primary one.