Effects of cross sections libraries parameters on the oecd/nea oskarshamn-2 benchmark solution

The OECD/NEA proposes a new international benchmark based on the data collected from an instability transient occurred at the Oskarshamn-2 NPP with the aim to test the coupled 3D Neutron Kinetic/Thermal Hydraulic codes on challenging situations. The ENEA “Casaccia” Research Center is participating to this benchmark, developing a computational model using RELAP5-3D^© code. The 3DNK model was developed starting from the cross sections datasets calculated by OKG, the Oskarshamn-2 licensee, using the CASMO lattice code. Integration of neutron cross sections database in RELAP5-3D^© required data fitting by a n-dimensional polynomials, calculations of the various polynomial coefficients and of the base cross sections values. An ad-hoc tool named PROMETHEUS has been developed for automatically generate the RELAP5-3D^©-compatible cross sections libraries. Thanks to this software it has been easily possible to visualize the complex structure of the neutronic data sets and to derive different cross sections libraries for evaluating the effects of some neutronic parameters on the prediction of the reactor instability. Thus, the effects of the fuel temperature and control rod history, of the discontinuity factors (averaged/not averaged), and of the neutron poisons has been assessed. A ranking table has been produced, demonstrating the relevance of the not-averaged discontinuity factors and of the on-transient neutron poisons calculations for the correct prediction of the Oskarshamn-2 event.