Modal decomposition method for BWR stability analysis using Alpha-modes

•The Lambda and the Alpha modes are reviewed.•LPRM signals are decomposed using a power modal decomposition.•Both Lambda modes and Alpha modes are suitable for this decompositions.

In-phase and out-of-phase oscillations have been observed in BWR reactors. To improve the safety of these reactors it is necessary to detect in a reliable way these oscillations from the neutronic signals. In this paper, a methodology to decompose the neutronic signals in its modal amplitudes is proposed. Usually, to compute this decomposition the Lambda eigenfunctions are used as expansion functions and their adjoint modes are used as weight factors. Different approaches using the Alpha modes are investigated to obtain the LPRM signals modal decomposition for a BWR unstability event. The calculation of Alpha eigenmodes is reviewed and the oscillation parameters for the modal decomposition of the neutronic signals from Ringhals NPP have been calculated.