Subcriticality measurement of pressurized water reactors by the modified neutron source multiplication method

Applicability of the modified Neutron Source Multiplication (NSM) method with extraction of the fundamental mode to subcriticality measurement has been investigated. This concept can be applied to the control rod worth measurement that is performed during reactor physics tests at periodical inspection of commercial PWRs. Following the feasibility verification in the previous study based on numerical analyses, its applicability has been proven in a more realistic situation; Regarding a withdrawal sequence of control rod banks during the PWR startup as a mockup experiment, subcriticalities with various control rod insertion patterns were estimated based on the modified NSM method. The subcriticality could be evaluated with good accuracy even with the mockup experiment where any special treatments for accurate measurement were not taken into account and furthermore the insensitivity of measuring signals by reactivity changes and their large fluctuations were seen. Some vital treatments for accurate estimation were identified: The reference subcritical state must be as closer to zero as possible. It is necessary to wait long enough for the delayed neutron effect to attenuate after a reactivity change by control rod bank withdrawal. Gamma-ray contamination contained in the detector signals should be corrected. (author)