Finding leaking fuel in the core based on 134Cs and 137Cs activities during spiking events

Abstract

At present, the ratio of ¹³⁴Cs and ¹³⁷Cs activities during spiking events is widely considered to be the best indicator of fuel burnup in leaking fuel rods. Evaluations are performed by comparison of the ratio between ¹³⁴Cs and ¹³⁷Cs activities in primary coolant with a reference function of fuel burnup. However, there is no universal correlation between the ¹³⁴Cs/¹³⁷Cs activity ratio and fuel burnup. Since ¹³⁴Cs is produced through neutron capture, the ¹³⁴Cs/¹³⁷Cs activity ratio in fuel depends on neutron energy spectrum. The spectrum is sensitive to fuel enrichment and burnup as well as to fission, moderation and absorption characteristics of the surrounding environment. For this reason, the ¹³⁴Cs/¹³⁷Cs activity ratio as a function of burnup differs for different fuel types and changes from cycle to cycle every time when the fuel loading pattern in the core is varied. Using a unified correlation in practice introduces significant errors in the evaluation of leaking fuel burnup. A new approach is developed for the identification of leaking FAs in the core. In this approach, ¹³⁴Cs and ¹³⁷Cs inventory is calculated with low computational costs for each fuel rod in reactor. A software application has been developed to automatically identify fuel rods in the core that provide the closest agreement with the ¹³⁴Cs/¹³⁷Cs activity ratio measured during the spiking event. The developed approach was validated on NPP data for 14 fuel cycles, each containing a single leaking FA. None of the 14 leaking FAs matched the burnup range predicted by the standard technique, with difference ranging from ∼ 10 to 35 MWd/kgU. In contrast, the new approach accurately identified all 14 leaking FAs and prioritized them for leakage testing during reactor outage.