(a) Film boiling considerably reduces heat transfer at the cladding surface. When combined with a power surge, it causes a sharp rise in fuel rod temperature, making it a critical factor in accident modeling. (b) Reactivity-initiated accident (RIA) simulation tests were conducted for unirradiated and irradiated fuels. Analytical cladding surface temperatures obtained using FEMAXI-8.1, incorporating a boiling heat transfer model for accident conditions, were compared with measured maximum cladding surface temperatures.

Fuel rods used in light-water reactors comprise uranium dioxide pellets and zirconium alloy cladding tubes. During a reactivity-initiated accident (RIA) triggered by the abnormal withdrawal of control rods from a reactor, the fuel rods experience rapid power surges and high temperatures. To appropriately understand the behavior of the fuel rods under such severe conditions and thereby ensure robust safety evaluations, a simulation tool must be developed that can accurately predict overall fuel rod behavior during an actual accident using limited experimental data.

Considering the aforementioned requirement, the Japan Atomic Energy Agency (JAEA) developed FEMAXI, a fuel performance analysis program designed to evaluate both steady-state and transient reactor conditions. The reliability of this program was validated through comparisons with 168 irradiation cases^*2. In this study, we developed new analytical models to predict fuel behavior under accident conditions, such as film boiling (Fig. 1a), and integrated them into FEMAXI. In addition, by enhancing numerical stability, we created the fuel behavior analysis program package FEMAXI-8.1, which can also perform RIA analyses. The accuracy of RIA analyses was confirmed through 141 RIA simulation tests conducted at the Nuclear Safety Research Reactor (Fig. 1b). All calculations were completed without errors, demonstrating an improvement in numerical stability.

FEMAXI-8.1 is available through the JAEA-maintained computer program database PRODAS. It includes a recommended set of analysis conditions, enabling users to conduct RIA analyses with a specified level of predictive accuracy by defining fundamental parameters such as fuel design and power generation.

 

^*1 Saito, S. et al., Development of In-Reactor Fuel Behavior Observation System, Journal of Nuclear Science and Technology, vol.18, issue 6, 1981, p.427–439.
^*2 Udagawa, Y. et al., Model Updates and Performance Evaluations on Fuel Performance Code FEMAXI-8 for Light Water Reactor Fuel Analysis, Journal of Nuclear Science and Technology, vol.56, issue 6, 2019, p.461–470.

Tasaki, Y. et al., Extension of Fuel Performance Code Package FEMAXI-Development and Validation of Reactivity-Initiated Accident Analysis Module RANNS for Light Water Reactor Fuels, JAEA-Data/Code 2024-012, 2024, 76p. (in Japanese).

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