Simulation of hydrogen diffusion along the axial direction in zirconium cladding tube during dry storage

Abstract

This study presents an updated mHNGD (modified hydrogen nucleation-growth-dissolution) model by improving atomic fraction of solid solution hydrogen with the use of instantaneous solid solution concentration and carefully selecting hydrogen solubility curves specific to reactor cladding materials. An explicit finite difference method was applied to simultaneously solve diffusion and phase change kinetics and ensure stable solution. The code validation with past experiments demonstrate improved accuracy. A prototypic hottest fuel pin in assembly average discharge burnup of 50 MWd/kgU of CASTOR® V/21 dry storage cask was simulated. The result of the reference case shows that the increase in the hydrogen amount at the upper end of the cladding due to hydrogen migration after ∼20 years of dry storage was limited to ∼50 wt.ppm with the initial Peak Cladding Temperature (PCT) of 394 ℃. The aspirational increase in discharge burnup to 70 MWd/kgU results in the upper end hydrogen increase to ∼80 wt.ppm with the same initial PCT. This study demonstrates that hydrogen migration along the axial direction during dry storage practically poses no safety challenge as its amount is limited and can be further alleviated by extending the wet storage period.