Decay heat effects on standards-compliant vacuum drying of spent nuclear fuel

The vacuum drying of spent nuclear fuel (SNF) canisters is a critical safety operation that requires the rapid and reliable removal of residual water while simultaneously mitigating the risks of ice formation and excessive thermal excursions. This study presents a comprehensive experimental evaluation comparing the conventional step-wise (STEP-WISE) depressurization protocol against a sophisticated pump-modulated strategy (NEW METHOD) across four distinct decay-heat levels ranging from 0 to 800 W. Unlike traditional methods that rely on static pressure holds, the NEW METHOD utilizes a targeted flow-throttling technique (reducing exhaust from 600 to ∼100 L·min −1 ) specifically within a ∼ 40-Torr transport-control corridor. This operational approach ensures a continuously sub-saturated headspace, which effectively prevents the re-attachment of detached moisture and facilitates the purging of non-condensable gases from complex micro-geometries. Our quantitative results demonstrate that the NEW METHOD consistently accelerates early-stage evaporation kinetics, achieving average evaporation rate improvements of 34–56% (ratios of 1.34–1.56) compared to the STEP-WISE baseline. Notably, at a high decay heat of 800 W, the NEW METHOD achieved a significantly lower final residual moisture of approximately 160 mg, whereas the STEP-WISE method left nearly double that amount (∼305 mg). This divergence is attributed to a ‘dry hot endgame’ mechanism that promotes the de-hydroxylation of surface-bound water under uninterrupted venting conditions. Throughout all tests, the peak cladding temperatures remained well within established safety margins (≤ ∼ 203 °C), proving that the proposed strategy optimizes drying completeness without compromising thermal integrity. These findings underscore that dynamic operational control is the primary governing factor in ensuring standards-compliant SNF drying. • Evaporation kinetics showed a quantified enhancement of 34–56% across all decay heat levels, improving overall drying efficiency. • Continuous flow modulation provides a mechanistic advantage by preventing moisture re-attachment and icing during the process. • Final residual moisture was reduced by approximately 50% at high decay heat, significantly ensuring long-term storage safety. • Validation of thermal safety margins demonstrates the practical feasibility of the protocol for actual industrial applications. • Empirical validation of surface de-hydroxylation provides a deeper physical understanding of the dry-hot endgame phase behavior.