OSKARSHAMN-2 BWR INSTABILITY EVENT MODELLING BY RELAP5-3D CODE

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 nuclear-thermal-hydraulic (CNTH) codes on challenging situations. The ENEA “Casaccia” Research Center is participating to this benchmark, developing a computational model using RELAP5-3D© code. The model was developed devoting particular attention to the core nodalization which is a critical point in reproducing the instability event. All the 444 core fuel assemblies were independently modeled as a single thermal-hydraulic channel coupled to an equal number of neutron kinetics nodes. Thus a complete thermal-hydraulic and 3D neutron kinetics description of the core, including the bottom and top reflector was achieved. Auxiliary software was also developed for handling the large amount of geometrical data and for performing the CASMO code cross section libraries interpolation. Operating plant data was used to perform the model steady state validation. The instability transient and several sensitivities studies were carried out, using benchmark defined boundary and initial conditions. The effects of the uncertainties of specific physical parameters on the transients results was also assessed. Finally, the simulation results demonstrate the model suitability in reproducing the instability event with a good level of accuracy, while the sensitivity analyses confirm the expected behavior to each perturbation.