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After a preparation period with VERONA upgrade and lead test assembly program, a new fuel type was introduced at MVM Paks NPP Ltd. This 4.7% average uranium enriched assembly type, together with the former 4.2% uranium enriched fuels, allowed us to lengthen the operating cycles to 15 months. Both fuel types contain gadolinium burnable poison, in six and three pins respectively. All of the four units have been converted to the C15 cycles, and have been being operated without any problems in the last few years. In this paper the test results of core design code HELIOS/C-PORCA, which is the basic model of VERONA, are outlined. C15 cycles were entirely investigated with the comprehensive study of measured and predicted (calculated) values of different reactor states. At first step, in order to prove the capability of the reactivity calculation of the nodal diffusion model, critical boric acid concentrations of different burnup and start up states were calculated and compared with measured values. During the next step of the verification process local in-core parameters were investigated. Measured neutron flux distributions (SPND signals) and coolant outlet temperatures were examined. SPND and thermo couple (TC) signals were predicted as a part of the monitoring system. The results of statistical investigations (average differences and standard deviations) for the applied fuel types are also presented.

Statistical evaluation of C-15 cycles in Paks NPP, based on measured in-core data