More than 20 years the Reactor Physics Laboratory of the Institute for Nuclear Research and Nuclear Energy – Bulgarian Academy of Sciences (INRNE-BAS) provides technical support of Kozloduy NPP regarding nondestructive neutron fluence determination of the reactor pressure vessel (RPV). The activities on neutron fluence assessment and related evaluation of RPV metal degradation (embrittlement temperature shiftTF ) as well as the consequent RPV lifetime prediction were started in 1986 and are still ongoing. The Reactor Physics Laboratory together with the Radioanalitical Methods Laboratory carried out reactor dosimetry at Kozloduy NPP Units 1 to 6 for validation/verification of the neutron fluence calculation by activation monitors. For this purpose ex-vessel detectors have been installed and irradiated in the air cavity behind the vessel periodically from 1993. The obtained results have provided important information needed for taking decisions on reactor lifetime management. Further improvements of the RPV neutron fluence determination methodology are foreseen. Large scale international effort is searched to validate these improvements.
Keywords: neutron/gamma fluence, reactor pressure vessel, activation detectors, surveillance.