Some Technical Issues on Continuous Subcriticality Monitoring by a Digital Reactivity Meter during Criticality Approach

Abstract

It has been proposed that a digital reactivity meter can be used for not only subcriticality measurement but also continuous subcriticality monitoring during criticality approach. In this study, a mock up numerical simulation was carried out to investigate some technical issues that still exist. These are sampling interval, time lag of estimated subcriticality due to filtering effect as well as due to the error contribution to the initial reference subcriticality and the way to avoid time lag of estimated subcriticality from safety point of view. It was found that there is influence of sampling interval on the reactivity fluctuation. Filter can cause time lag of estimated subcriticality when a large filter time constant is used. In subcriticality monitoring, the error contribution to the initial reference subcriticality obtained by some design calculation should be investigated to detect accurately an unexpected criticality as earlier as possible. It was known that the negative error contribution to the initial reference subcriticality should be used from safety point of view. We have analytically obtained the relation between the error of the initial reference subcriticality, ramp rate of reactivity addition, warning subcriticality and the margin of the time interval for the counter action.