Evaluation of Control Rod Worth in Pressurized Water Reactors Using Neutron Count Rate during a Control Rod Drop Testing

摘要

Reactor Physics test are carried out at the startup after refueling the commercial Pressurized water Reactors (PWRs). Among the tests, control rod worth measurement is one of the important test items. For the control rod worth measurement, the boron dilution method is conventionally used in Japan. However, recently the Dynamic Rod Worth Measurement (DRWM) without boron adjustment has been widely adopted in reactor physics tests at hot zero power condition. Furthermore in order to reduce the interval of the tests from the economical point of view, one of the methods is to evaluate the control rod worth from subcriticality measurement. To estimate the subcriticality exactly, the authors introduced the modified Neutron Source Multiplication (NSM) method with extraction of the fundamental mode. The principles of corrections of this method are: (1) extraction of the fundamental mode from measuring data and (2) importance and spatial corrections for perturbations induced in the distributions of neutron importance field and the fundamental mode. For the evaluation of the correction factors, the neutron source distribution, the position of neutron detector and the control rod insertion configuration must be taken into account We have shown that the neutron count rate data obtained during the criticality approach could be used for subcriticality evaluation or control rod worth evaluation, in which control rods are withdrawn in sequence by group. The authors have applied the modified NSM method in a numerical simulation to subcriticality estimation using the neutron count rate data during the control rod drop testing. We will present an example of control rod worth estimation in a typical Westinghouse type two loop nuclear power plant. Although the measured data were reduced from a chart recorder, reasonable results were obtained. Thus, we expect that the modified NSM method could be also applied to control rod worth measurement based on the neutron count rate data during the control rod drop testing, if the data were to be acquired digitally.