Effect of changes in fuel characteristics on neutron parameters of refuelling in WWER-440 reactors at Kozloduy NPP

Effect of change in thickness of the cassette wall from 2.1 to 1.5 mm on the critical core parameters of the fuel cycles has been studied. Calculations were performed by SPPS-1.6 and BIPR-7A codes. The following main differences of core parameters are found: the duration of the fuel cycle increases in 2-6 % (8-15 effective days); the differences in inhomogeneity coefficient are 2-3%; the temperature reactivity coefficient changes by 2 pcm/grad C; the critical concentration of boric acid in the process of fuel burnup is by 2 5 % higher in the cassettes with 1.5 mm wall. The differences in fuel burnup characteristics found are within the limits of the safety standards for the Kozloduy NPP units 1-4