Accuracy of the programme package for neutron-physical characteristics forecasting of the refuelling cycles in Kozloduj NPP WWER 440

Based on calculations of some important physical parameters of the WWER-440 cores, the accuracy of the following currently used computer codes is discussed: the 3D neutron diffusion large-nodes code SPPS-1 (modeling of the major physical processes in the core); BIPR-5AK (calculations of the core state at low power levels); KR (reactivity coefficients calculations) and HEXAB2DB (calculations of the pinwise power density distribution). Comparisons of the calculated results with real measured data are systematically carried out. The errors in determination of major parameters are assessed as follows: for thermal power values the differences in the balances between primary and secondary coolant circuits - < ±2%; for the power distribution in WWER-440/230 by in-core thermocouple measurements - 0.8oC; for determination of the peaking factors - 3-5%; for boron concentration (mean square root error) - 0.06-0.1 g/kg; for control rod position - several cm. For the critical core state parameters and the continuity of the fueling cycle the accuracy of the codes is ±0.4% of reactivity. For the assembly power distribution the maximum errors are in the order of ±8%, the average error for assemblies of different enrichment and operation period - in the order of ±3%. The accuracy of the SPPS-1 code practically does not depend on the possible variations of operation parameters and is considered as a warrant for an efficient use of the fuel ensuring the maximum load peaking factors of the core power distribution which are stable and lower than the permissible values. 10 refs., 5 figs., 4 tabs. (R.Ts.)