• decomposizione modale del segnale
• analisi modale
• instabilità
• accoppiamento di codici termoidraulici e neutroni

In this work, three dimensional time-domain BWR stability analyses were performed on test point 3 (PT3) of the Low Flow Stability Tests for the core wide oscillation mode. In order to obtain detailed information regarding the state of the reactor for each time step the analyses have been carried out with the coupling codes RELAP5 Mod3.3/ PARCS.
With the aim of better understanding the instability development process, it has been studied the stability response of this operational point to several type of disturbance obtaining realistic and meaningful information of the reactor behavior at the stability boundary of the Power/Flow Map apart from demonstrating that the small pressure perturbation tests offer an operationally simple and precise technique for determining BWR core stability margins.
In order to confront the results achieved with different thermalhydraulic nodalizations all the cases have been performed with two different models obtaining important differences.
However, the most important and innovative contribution of this study is the use, for the first time, of the data provided by the coupling codes RELAP5/PARCS transient calculations to perform signal modal analyses with the VALKIN code with results very satisfactorily. Moreover, with the aim to characterize the studied transient as in-phase or out-of-phase and also to study the importance of different modes during the transients the amplitudes of the different power modes have been computed with VALKIN.
Additionally, for two perturbation tests, the results for the power modal decomposition have been complemented with the informations provide by the simulation of the LPRM signals by RELAP5/PARCS coupled codes obtaining results that are of great practical importance because demonstrates that, in theory, with this methodology, it is possible in a nuclear plant to obtain on-line information regard to the reactor stability.
Finally one perturbation analysis has been performed also with the coupling codes TRAC-BF1/VALKIN obtaining a very good agreement between the results achieved with the two distinct thermalhydraulic-neutronic coupled codes.