Some Remarks on Time Series Analysis for BWR Stability Studies

The results of an experimental or analytical BWR stability analysis are local or global reactor parameter time series, in particular power time histories that are measured or calculated at the locations of the in-core detectors. The time series reflect a stable or unstable oscillatory power evolution at any operational point of the BWR core after a specific or non-specific (noisy) parameter perturbation. In particular, the analysis of measured noisy time series concerning the stability information is a crucial task, because it is this procedure which mainly determines the experimental uncertainty of the BWR stability analysis. In the framework of an international benchmark exercise (NEA time series analysis benchmark, 1999), it was revealed that, at present, in the field of signal analysis, many different methodologies are used and the uncertainties of the various approaches are in some cases very different. Hence, a code containing seven time series analysis approaches for the calculation of the linear stability characteristic “Decay Ratio” (DR) (the so-called asymptotic DR at the natural reactor frequency NF) has been developed at the Paul Scherrer Institut (PSI). This paper is an overview of our experiences and results obtained on the basis of the application of this method, and a preliminary assessment of the uncertainties.