Separation of various flux oscillation modes in boiling water reactor (BWR) noise measurements, based on flux factorization techniques (i.e., using orthogonality relations via integrals over the whole core), have been attempted in the past but without much success. One such example is the attempt made in 1990 in connection with the measurements in the Swedish Ringhals Unit 1 (Ringhals-1) BWR where both global (fundamental-mode) and regional (first-azimuthal-mode) oscillations were observed.

Shown here is the reason for the failure of the earlier separation methods, that is, the presence of the local component of the noise with its known axial correlation properties. This component has been ignored in all BWR instability work so far. Further, because of the approximation of the factorization integral by a finite sum, cross-correlations between all detectors will appear in the autocorrelation of the factorized detector signals.

Taking into account the properties of both the noise structure and the approximate factorization, elaboration of a factorization-based flux decomposition is possible. A phenomenological BWR noise model is used here in support of the decomposition technique. The model is also used to explain the success or failure of previous methods. The general factorization method proposed is demonstrated in various examples using the Ringhals-1 measurement data. In particular, the global and regional decay ratios are determined in a consistent way that is more general than the alternative methods used so far.