Results on research of templates from Kozloduy-1 reactor pressure vessel Original Research Article

The studies on the specimens manufactured from the templates cut out from the weld 4 of Kozloduy NPP Unit 1 reactor vessel have been conducted. The data on chemical composition of the weld metal have been obtained. Neutron fluence, mechanical properties, ductile to brittle transition temperature (DBTT) using mini Charpy samples have been determined. The phosphorus and copper content averaged over all templates is 0.046 and 0.1 wt.%, respectively. The fluence amounted up to 5×1018 n cm−2 within 15–18 fuel cycles, and about 5×1019 n cm−2 for the whole period of operation. These values agree well with calculated data. DBTT was determined after irradiation (Tk) to evaluate the vessel metal state at the present moment, then after heat treatment at the temperature of 475°C to simulate the vessel metal state after thermal annealing (Tan), and after heat treatment at 560°C to simulate the metal state in the initial state (Tk0). As a result of the tests the following values were obtained: Tk, +91.5°C; Tan, +63°C; and Tk0, 54°C. The values of Tk and Tan obtained by measurements were found to be considerably lower than those predicted in accordance with the conservative method accepted in Russia (177°C for Tk and 100°C for Tan). Thus, the obtained results allowed to make a conclusion that it is not necessary to anneal Kozloduy NPP Unit 1 reactor vessel for the second time. The fractographic and electron-microscopic research allowed to draw some conclusions on the embrittlement mechanism.