Reactivity measurement in the JUPITER experiment and Its accuracy

not registered; Aoyama, Takafumi ; not registered; Shirakata, Keisho

This report describes the reactivity measurement method and its accuracy employed in ZPPR (Zero Power Physics Reactor) test facility at Argonne National Laboratory, USA. In ZPPR, reactivity measurements were conducted by means of MSM (Modified Neutron Source Multiplication) method using the fission reaction rates by 64 U fission chambers which were uniformly located in the core region. The MSM method can evaluate the subcritical reactivity considering the spatial distribution of neutron flux, although its basic equation is the one point approximation reactor dynamic theory. As each detector response varies depending on the core configuration, the uncertainty of the detector response affect directly the accuracy of measured reactivity. Therefore, the average value of many detectors had been conventionally used to reduce the statistical error in the measurerment. In this study, this method was modified to use the linear fitting between measured reaction rates and detector responses. Using this modified method, the uncertainty of each detector response affect the measured reactivity less than the simple averaging method. This new method was verified by applying to the ZPPR control rod worth measurements. It was found that the ratios of neutron source intensities and detector responses could be corrected accurately by the linear fitting in case of reactivity change without or with little fuel movement such as a unit control rod worth measurement.