Development of a new methodology for stability analysis in BWR NPP

In this work, a new methodology to reproduce power oscillations in BWR NPP is presented. This methodology comprises the modal analysis techniques, the signal analysis techniques and the simulation with the coupled code RELAP5/PARCSv2.7. Macroscopic cross sections are obtained by using the SIMTAB methodology, which is fed up with CASMO-4/SIMULATE-3 data. The input files for the neutronic and thermohydraulic codes are obtained automatically and the thermalhydraulic-to-neutronic representation (mapping) used is based on the fundamental, first and second harmonics shapes of the reactor power, calculated with the VALKIN code (developed in UPV). This mapping was chosen in order not to condition the oscillation pattern. To introduce power oscillations in the simulation a new capability in the coupled code, for generate density perturbations (both for the whole core and for chosen axial levels) according with the power modes shapes, has been implemented. The purpose of the methodology is to reproduce the driving mechanism of the out of phase oscillations appeared in BWR type reactors. In this work, the methodology is applied to the Record 9 point, collected in the NEA benchmark of Ringhals 1 NPP. A set of different perturbations are induced in the first active axial level and the LPRM signals resulting are analyzed. (authors)