Experimental determination of subcriticality at subcritical PWRs in Korea

A direct subcriticality measurement system (SMS) based on the Feynman-α method has recently been developed by KEPRI. It was applied to six commercial pressurized water reactors in Korea. However, the obtained Feynman curves failed to give proper multiplication factors. The objective of these tests was to investigate the performance of the Feynman method to predict directly the subcriticality of a given subcritical reactor by using the neutron pulse counts only without any reactor perturbation in the large commercial reactors. Recently, two methods developed by Hokkaido University and Westinghouse Electric Corporation. These methods have a defect due to being based on the modified neutron source multiplication method. To overcome this defect and derive operational benefits is necessary to estimate the subcriticality of a subcritical core directly from the neutron pulse counts only. The performance of the developed SMS was verified in the Kyoto University Critical Assembly and applied to eight 1000 MWe Optimized Pressurized Water Reactors (OPR1000) in Korea. The obtained results show that the SMS based on the Feynman method can be a useful tool to estimate the reactivity of a subcritical power reactor. Although the discrimination level of the signal-processing unit in OPR1000 suffers from noise and gamma ray effects, SMS can provide good Feynman curves and effective multiplication factors. However, the SMS has failed to give the reactivity for the entire measured data set. Improving the SMS and investigating the effects of different discriminator level settings at SPU in OPR1000 will be topics for further study.