A reactor noise analysis methodology for BWR core stability evaluation: application and assessment to Leibstadt stability tests

As a first step towards establishing a best-estimate methodology for the evaluation of BWR core stability parameters, i.e. the decay ratio and resonance frequency, from measured reactor noise signals, a systematic approach has recently been developed and adopted at PSI for the analysis of the Swiss BWRs. The aim is to evaluate stability tests in a consistent manner at any operating condition in the power/flow map and for any operating cycle. This methodology principally consists in a general reactor noise analysis where as much as possible information recorded during the tests is investigated prior to determining 'core representative' stability parameters along with an associated uncertainty range. A central part in this approach is that a time series analysis of all measured neutron flux signals, rather than only one or few signals, is performed. In addition, for each signal, three different model-order optimization methods are systematically employed to take into account the sensitivity upon the model-order. The adopted methodology is then applied to the evaluation of the core stability measurements performed at the Kernkraftwerk Leibstadt nuclear power plant, Switzerland, during cycles 10, 13 and 19. A total of 28 tests are hence analyzed. This is primarily done in order to obtain a broad range of tests to serve as basis for the validation of the coupled neutronic/thermal-hydraulic codes used at PSI for BWR stability calculations. In addition, in order to assess the results obtained with the current methodology, a comparative study has been carried out with respect to results from previously developed and applied procedures. The results show a good agreement between the current method and the other methods. (authors)