The neutron fluence determination methods and approaches applied for Kozloduy NPP reactors of type VVER-1000, Units 5 and 6, and VVER-440, Units 1 to 4, according to the Reactor Pressure Vessel Surveillance Program, are presented. The verification/validation of calculated neutron fluence is carried out by measured activity of ex-vessel detectors installed in the air cavity behind the vessels on units 1 to 6. A methodology for estimation of the neutron fluence for VVER-1000/320 reactor has been developed and applied to surveillance assemblies' sets 1L and 2L of Unit 5 for an assessment of the vessel current embrittlement and prognosis.