This study reassess safety margin of the current Peak Cladding Temperature (PCT) limit of dry storage in terms of hydrogen migration by predicting axial hydrogen diffusion throughout dry storage with respect to wet storage time and average burnup. Applying the hydride nucleation, growth, and dissolution model, an axial finite difference method code for thermal diffusion of hydrogen in zirconium alloy was developed and validated against past experiments. The developed model has been implemented in GIFT – a nuclear fuel analysis code developed by Seoul National University. Various discharge burnups and wet storage time relevant to spent fuel characteristics of Korea were simulated. The result shows that that the amount of hydrogen migrated towards the axial end during dry storage for reference PWR spent fuel is limited to ~50 wppm. This result demonstrates that the current PCT margin is sufficient in terms of hydrogen migration.

The size of hydrogen molecule is not so small as to invade into the lattice of material, and therefore, hydrogen invades into the material as atom. Hydrogen movement is done by diffusion or dislocation movement in the near crack tip or plastic deformation. Hydrogen appeared to have many effects on the mechanical properties of the Cr-Mo steel alloys. The materials for this study are 1.25Cr-0.5Mo and 2.25Cr-1Mo steels used at high temperature and pressure. The hydrogen amount obtained by theoretical calculation was almost same with the result solved by finite element analysis. The distribution of hydrogen concentration and average concentration was calculated for a flat specimen. Also, finite element analysis was employed to simulate the redistribution of hydrogen due to stress gradient. The calculation of hydrogen concentration diffused into the material by finite element method will provide the basis for the prediction of delayed fracture of notched specimen. The distribution of hydrogen concentration invaded into the smooth and notched specimen was obtained by finite element analysis. The hydrogen amount is much in smooth specimen and tends to concentrate in the vicinity of surface. Hydrogen embrittlement susceptibility of notched specimen after hydrogen charging is more remarkable than that of smooth specimen.