An alternative method for local surveillance of coolant flow in BWRs fuel channels

A new method to perform “local surveillance” of fuel channels is proposed, it is based on the reduced model of March-Leuba for BWR dynamics along with the measurements of the characteristic frequency of the neutronic signal in steady state. The steam voids are one of the main characteristics in BWR reactors and their speed is a good indicator to verify that there is not an abnormal restriction blocking the coolant flow through the fuel channel. Previous analysis using the March-Leuba reduced model have been performed by using the transit time of steam voids through the fuel assemblies, in those analysis the whole length of the fuel assembly was considered. In this study, a “local surveillance” of the coolant flow is proposed by assessing the measurements of different monitors allocated at different levels of the fuel assemblies. As a new characteristic for the proposed “local surveillance”, the Prony's method is applied to obtain the frequency from the more unstable pole, which is used to obtain the transit time of steam voids. In addition, a rational approximation is used because the original Padé (2,2) approximation considered in the March-Leuba original reduced model does not follow the asymptotic behavior of the exponential in the range of local monitors. To verify the proposed method LPRM signals are analyzed from an actual power plant and the results are compared against the results obtained by using the method of first minimum of the transfer function between two detectors of neutronic flux, which are placed at different levels. Results show agreement between the first minimum of the transfer function and the proposed local surveillance method (using the rational approximation). However, the proposed local surveillance method shows transit time differences smaller than 5% for different APRMs inasmuch the method of the first minimum of the transfer function shows transit time differences up to 22% for different LPRMs. It is here proposed to use both methods for the reactor characterization and to verify if there is or is not an abnormal restriction blocking the coolant flow through the fuel channel.