In case of transient changes or minor accidents during reactor operation, the fuel temperature may temporarily exceed the critical threshold, thus forming bubbles on the fuel plate. Bubbling can significantly affect the temperature distribution and mass flow balance in rectangular channel of the fuel plate, which may lead to the rupture of the fuel plate and even the damage of the whole reactor core. The phenomenon of bubbling in plate-type fuel assemblies within nuclear reactors includes fission gas bubbles and solid bubbles.

This study aims to investigate the effects of air gap on the flow and heat transfer behavior in rectangular channel of fuel plate during bubbling conditions.

Firstly, a fuel plate and two adjacent flow channels were selected as the calculation domain, and Fluent software with dynamic mesh technology was utilized to simulate gas bubbling and solid bubbling phenomena within nuclear reactor fuel plates. Then, the dynamic mesh was employed to accurately adapt to the geometric changes during bubble formation and development, and the Realizable k-ε turbulence model was used to handle complex fluid dynamics, with boundary conditions set as inlet velocity and outlet pressure to reflect real operational environments. Finally, the differences between fission gas bubbling and solid bubbling were compared, and all solid surfaces were designated as no-slip and adiabatic, enhancing the predictions of interactions between heat transfer and fluid flow.

The findings reveal that gas bubbles cause a local increase in temperature, with the heat flux around the bubbles tripling, though the overall heat flux of the fuel plate remains largely unchanged. The formation of bubbles locally enhances heat transfer capability by approximately 10%, with a 4% increase in heat flux on the bubble side. Under conditions of high flow rates, the presence of bubbles leads to a significant pressure difference across the fuel plate, causing deformation of the fuel plate and potentially leading to the blockage of the flow channel.

Results of this study provide significant references for the design and safety assessment of nuclear fuel plates, highlighting the importance of considering the effects of gas bubbling on thermal-hydraulic characteristics in the design and operation of nuclear reactors.