Regulatory requirements for determining Kozloduy NPP WWER pressure vessel neutron fluence

Safety assurance of the Kozloduj NPP is provided by the specially developed Pressure Vessel Surveillance Program. The main way to assess the degree of material radiation embrittlement is by numerical calculations of neutron fluence. The author describes the steps of the calculation procedure, as well as standard regulations and methods for evaluation of the results. WWER-440/230 reactors (units 1-4) have no samples-witnesses and the radiation embrittlement is determined indirectly, by embrittlement temperature and neutron fluence. This requires higher reliability and accuracy in assessment. Verification of the neutron fluence by means of activation detectors placed behind the reactor vessel is of crucial importance. WWER-1000 reactors (units 5-6) are equipped with samples-witnesses and threshold activation detectors. Based on measured mechanical properties and activities, and using the relationship between the neutron fluence and radiation embrittlement temperature, the changes in metal brittle are predicted. In this case verification is made by parallel dosimetric measurements