Abstract. Following the requirement of safety operation an assessment of fast neutron fluence onto the RPV is being carried out on Kozloduy NPP for evaluation of reactor pressure vessels (RPVs) steel degradation and to revise the safety margins limits of all Units 1-6 of Kozloduy NPP. For this purpose neutron fluence calculation methodology has been developed using the neutron transport equation for describing the neutron penetration through heterogeneous reactor system, from the reactor core trough the reactor vessel toward the shielding. The main task is obtaining "best estimated" neutron fluence at the critical locations of reactor vessel. The verification of calculated neutron fluence is being carried out by measured activity of ex-vessel detectors installed in the air, cavity behind the vessels on all units from 1 to 6, and by surveillance sets on units 5 and 6 of Kozloduy NPP.

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