Hydrogen Absorption Mechanism of Zirconium Alloys Based on Characterization of Oxide Layer

Continuum model for hydrogen pickup in zirconium alloys of LWR fuel cladding

“…In our model, hydrogen diffusion through the dense barrier oxide layer is taken as the rate-limiting step for H pickup [5][6][7] . The hydrogen diffusion follows the equation ) , (0 ≤ ≤ ( )) E represents the electric filed across the Zr oxide film and is calculated by…”

“…Many studies have focused on understanding the transport of hydrogen through the barrier oxide layer, as this transport is often regarded as the rate-limiting step for H pickup [5][6][7] . A number of factors, including oxide morphology, alloy additive elements and local stress, play important roles in the hydrogen transport process.…”

“…According to density functional theory (DFT) calculations, the hydrogen migration energy in the suboxide is higher than that in pure Zr, so the suboxide layer may also slow down the H diffusion and contribute to the diffusion barrier 10 . Using in-situ nuclear reaction analysis, Une et al measured the deuterium concentration depth profile in oxide layers of Zr alloys corroded in D 2 O steam 7 . The result shows a nearly flat concentration profile in the outside layer followed by a steeply decreasing concentration in the inner layer, which agrees well with the anticipated higher diffusivity in the porous oxide and lower diffusivity in the dense oxide.…”

“…Due to the lattice mismatch between Zr oxide and metal, high compressive stress is generated in the oxide near the interface 19,20 . Raman spectroscopy measurement revealed that the stress varies cyclically and can be as large as several GPa 7 . DFT calculations found that under 1GPa compressive stress hydrogen diffusion coefficient in tetragonal ZrO 2 is only about 60% of the coefficient without stress at 600 K 15 .…”

“…Further TEM analysis discovered that extended networks of degraded grain boundaries were formed from the oxide surface to near the metal/oxide interface, probably due to the preferential dissolution of zirconia in LiOH solution 7 . In this case the H pickup process is controlled by the dissociation reaction of H 2 O at the front of the degraded grain boundaries, rather than the hydrogen diffusion process.…”