Development of an improved reactivity measurement system for reactor physics tests during HANARO startup

During the periodic reload startup physics tests of the HANARO research reactor, accurate evaluation of the critical control rod position and the shutdown margin is essential for operational safety. In this study, the legacy reactivity measurement system, which relied on a single compensated ionization chamber connected to an MS-DOS computer, was replaced with a modernized, multi-channel platform. The new system utilizes three spatially distributed fission chambers from the reactor regulation system to enhance measurement redundancy and robustness. Reactivity was evaluated using both the inverse kinetics equation and the least square inverse kinetics method to mitigate numerical drift under deep subcritical conditions. The developed system was validated during the 116th operation cycle of HANARO. In the approach-to-criticality test, the critical rod position was measured to be 180.4 mm. During the shut-off rod drop experiments, the multi-channel configuration effectively covered the localized spatial dependence associated with the detector geometry, confirming a conservative minimum shutdown margin of 11.182 $. The results demonstrate the practical applicability of the proposed multi-channel framework for supporting future startup physics tests and reactor operations.