Burnup influence on the VVER-1000 reactor vessel neutron fluence evaluation

The neutron fluence of the vessels of the reactors is determined regularly according the RPV Surveillance Program of Kozloduy NPP Unit 5 and 6 in order to assess the state of the metal vessels and their radiation damaging. The calculations are carried out by the method of discrete ordinates used in the TORT program for operated reactor cycles. Reactor spectrum corresponding to fresh U-235 fuel is used as an input neutron source. During the burning up of the fuel Pu 239 is accumulated. The impact of the Pu spectrum on the neutron source is evaluated. The impact of the burn up of the fuel on the neutron fluence of VVER-1000 reactor vessel is evaluated. The calculations of isotopic concentrations of U-235 and Pu-239 corresponding to 4 years burn up were performed by the module SAS2H of the code system SCALE 4.4. Since fresh fuel or 4 years burn up fuel assembly are placed in periphery of reactor core the contribution of Pu-239 of first year burn up and of 4 years burn up is taken in consideration. Comparative calculations of induced activities of Mn-54, Co-60, Nb-93m of neutron detectors in the air cavity behind the RPV were carried out without and with taking into account the contribution of Pu-239 in nuclear fuel. Calculations of neutron fluence were performed with neutron spectrum for fresh fuel, for 1 year and for 4 years burn up fuel. Correction factors for neutron fluence at the inner surface of the reactor vessel, in 1/4 depth of the vessel and in the air behind the vessel were obtained. The correction coefficient could be used when the neutron fluence is assessed so in verification when the measured activity of ex-vessel detectors is compared with calculated ones. (authors)