Time dependent correlation between the criticality and short-half-life noble-gas fission products of fuel debris

Abstract

The activity ratio of short half-life noble gas fission products, ⁸⁸Kr-to-¹³⁵Xe (A(⁸⁸Kr/¹³⁵Xe)), has close relationship with the criticality (k_eff) in the primary containment vessel (PCV) or fuel debris canister, in which fuel debris are accumulated. In this paper, we investigated the time-dependence of the A(⁸⁸Kr/¹³⁵Xe) when the k_eff changes according to an incident such as the change of geometry in the PCV. Our calculation shows a slow change of the A(⁸⁸Kr/¹³⁵Xe) value with the maximum or minimum values before settling in a new stable value after several hours from the k_eff change. The applicability and the suitability of the A(⁸⁸Kr/¹³⁵Xe) time-dependent measurement is discussed for the monitoring of the k_eff in PCV.