Reactivity measurement at VENUS-II during control rods drop based on inverse kinetics method

Feasibility of subcriticality monitoring with a digital reactivity meter based on inverse kinetics method has been investigated at the lead-based zero power reactor VENUS-II. A series of control rod drop experiments have been carried out in this work. The subcriticality before and after the rod drop were estimated considering the effect of external neutron source. The control rod worth was also extracted from the rod drop experiments. The measurement results exhibited obvious spatial dependence induced by the control rod insertion, and they could be corrected by correction factors calculated with MCNP code. The corrected values of control rod worth were found to be consistent with the reference ones predetermined at critical condition. Moreover, this method could also provide real time subcriticality without any correction when the detectors were placed at appropriate positions at slightly subcritical state. Based on these measurements, it has been proved that the digital reactivity meter could be a useful tool for continuous reactivity monitoring in the operation of ADS in future.