Vacuum-induced evaporation and heat transfer in a simulated single-assembly spent nuclear fuel canister

Vacuum drying is a critical process for the long-term integrity of spent nuclear fuel (SNF) storage. This study presents a systematic thermal-hydraulic evaluation comparing the conventional step-wise decompression method with a next-generation variable exhaust flow rate operation method in a single-assembly scale simulated canister. Experiments were conducted under various decay heat loads (800 W, 1200 W, 2000 W) and initial residual water masses (25 g to 200 g). The results demonstrate that the conventional step-wise method often suffers from mass transfer bottlenecks, where periodic pump isolation leads to internal vapor saturation and detrimental re-condensation on structural surfaces. In contrast, the variable exhaust flow rate method, which maintains continuous, flow-regulated exhaust, significantly enhances moisture removal efficiency by keeping the vapor-liquid interface in an unsaturated state and maximizing the sweeping effect. Quantitative analysis revealed that the variable exhaust flow rate method achieves a vacuum drying efficiency approximately 2.05 to 2.48 times higher than the conventional approach. While the conventional method paradoxically showed improved dehydration performance at higher heat loads due to a prolonged bake-out effect, this came at the cost of excessive process times and potential structural risks. Conversely, the variable exhaust flow rate method provided robust performance across all moisture loads, ensuring safety margins well within regulatory limits. These findings provide empirical evidence that dynamic, continuous flow control is a superior operational strategy for maximizing drying efficiency and operational stability in SNF dry storage systems.