Multi-scale calculation of carbon diffusion behavior in zirconium and zirconia - 15174

In the fuel rod cladding of pressurized water reactors (PWR) which are mainly composed of zirconium (98.4%), the oxidation reaction with water leads to the formation of an external zirconia layer (ZrO₂). Both Zr and ZrO₂ contain activation products as ¹⁴C. The goal of this study is to characterize, by the mean of a theoretical multi-scale approach, the carbon diffusion behavior in the Zr hexagonal close packed bulk and in the ZrO₂ monoclinic one to estimate the waste safety of the fuel rod claddings in the storage condition. In the hexagonal close packed zirconium bulk, it was found that there were 2 possible carbon interstitial sites: basal tetragonal site and octahedral site (noted as BT site and O site respectively). There are 3 types of possible atomic jumps: O site to BT site; O site to O site; and BT site to O site. After calculating the associated migration energies, attempt frequencies and jump probabilities, kinetic Monte Carlo (KMC) simulations were performed to simulate carbon diffusion at the macroscopic scale. The results show that the carbon diffusion in Zr bulk is extremely limited in the storage condition (50 C. degrees). For a carbon atom in the monoclinic zirconia bulk, there are 8 types of carbon interstitial sites since its more complex structure. For each site, there are 4 symmetrical positions (same structure but different orientations). After testing all the migration possibilities between each two interstitial sites, we found as many as 28 possible jumps between 2 neighbor interstitial sites. As for C in Zr, it was shown by KMC simulations that it is also very difficult for carbon to diffuse in a monoclinic zirconia bulk by interstitial mechanism at the storage condition. This study shows that the carbon diffusion in Zr and monoclinic zirconia bulk is extremely low in the storage condition. (authors)