Analysis of the X2 VVER-1000 benchmark with FENNECS

The X2 VVER-1000 benchmark contains operational data from the Khmelnitsky (X2) Nuclear Power Plant and offers the opportunity to validate nuclear simulation codes in hexagonal geometry. The first part to the benchmark is dedicated to the fresh core at hot zero power (HZP) state. During the start-up experiments, several quantities have been evaluated such as the worth of the control rod banks and the isothermal temperature reactivity coefficient. As there is no thermal-hydraulic feedback, this benchmark is ideal for the validation of standalone neutronics methods and codes. In this paper, the HZP state is reproduced with the FENNECS code and with the MonteCarlo (MC) code SERPENT. The modelling with SERPENT is twofold: generation of fewgroup cross sections for FENNECS and generation of a reference solution in terms of power distribution since there is no experimental data available for it. Prediction of FENNECS for the HZP are in fair agreement with the reference MC solution, both multiplication factor and radial power distribution discrepancies are within common acceptance margins. FENNECS results are also in good agreement with experimental values such as the isothermal temperature reactivity coefficient. Some weakness in the prediction of control rod worth has been identified. To mitigate shortcomings resulting from the cross-section energy condensation, the superhomogeneisation method has been applied. It results on a better agreement between FENNECS and the reported measured worth.