Feasibility of Continuous Sub-criticality Monitoring using a Digital Reactivity Meter to Overcome the Criticality Accident

Abstract

In the foregoing studies, it has been proved that the digital reactivity meter can be used for not only sub-criticality measurement but also continuous sub-criticality monitoring during criticality approach. Based on these studies, we investigated the applicability of a digital reactivity meter for continuous sub-criticality monitoring to intervene before a criticality accident occurs that is similar to the Tokai-mura accident in 1999. In our mock up numerical simulation, there are three calculation steps that are (1) reactivity transient calculation, (2) neutron transient calculation and (3) sub-criticality monitoring calculation. The reactivity transient was calculated using two-group diffusion nuclear constants and the neutron transient was calculated using the one-point reactor kinetics. In sub-criticality monitoring, three algorithms for reactivity evaluation were compared. We have investigated which algorithm is the most suitable to use in an actual system. In practice, we also advise use of some filtering algorithm to reduce the neutron transient fluctuation and a warning reactivity to know estimated sub-criticality as earlier as possible.