Assessment of neutron fluence onto the reactor vessel of Unit 1 at Kozloduy NPP after the 18th cycle

According to the Surveillance Programme the neutron fluence onto the reactor pressure vessel of Unit 1 at Kozloduy NPP has been evaluated for the 15-18th cycles for the indirect assessment of the vessel irradiation embrittlement after the vessel annealing. The neutron fluence verification has been performed by comparative analyses of calculated and experimental induced activity values of the scraps from reactor vessel weld 4 and detectors irradiated in the cavity behind the vessel. The experimentally obtained on/behind attenuation of neutron flux permits to reduce the uncertainty of reactor vessel neutron fluence estimation by measured activity of detectors irradiated behind the vessel during the next cycles with dummy cassettes loading. The neutron fluence onto the reactor vessel of Unit 1 at Kozloduy NPP has been conservatively evaluated for the 15-18th after the annealing taken into account the loading patterns and operation regimes. The experimentally determined activities of the scraps from the inner wall of RPV have provided an extreme opportunity to prove the calculational methods applied to evaluation of neutron fluence onto the VVER-440 vessel. The good consistency between experimental and calculation results demonstrates the reliability of the calculations. (authors)