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Atomistic modeling of nuclear cladding embrittlement by hydride: characterizing interstitial sublattice order of zirconium hydrides using Tight-Binding Ising Model​​

​Paul Eyméoud has defended his thesis on 17th December 2018 at CINAM, in Marseille (France).

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PSN-RES/SEMIA/LPTM

Authors > EYMÉOUD Paul

Publication Date > 17/12/2018

Summary

In Pressurized Water Reactors, nuclear claddings, based on zirconium alloy,
are embrittled by hydrides. In order to better understand and anticipate this
degradation of the first safety barrier, one needs to analyze zirconium hydride
precipitation mechanisms. The thesis addresses this nuclear safety issue, by modeling, at atomistic scale, chemical ordering processes between hydrogen and atomic vacancies on tetrahedral interstitial sublattice of face-centered cubic zirconium hydrides. This has been achieved into two steps : first the development of an atomistic energetic model sufficiently precise and not too much CPU time consuming, and secondly its implementation in Monte-Carlo thermostatistical simulations. Starting from a Tight-Binding (TB) Hamiltonian, the energetic model has been derived from the calculation of multiatomic interactions between hydrogen atoms, using the Generalized Perturbation Method (GPM) applied for an interstitial disorder described within the Coherent Potential Approximation (CPA). The results put in evidence that pairwise interactions prevail on higher order ones, and become negligible beyond fourth hydrogen neigborhoods. Thus multiatomic development of the ordering energy has been reduced to a Tight-Binding Ising Model (TBIM), based on effective pairwise interactions between first, second, third and fourth neighbor hydrogen atoms, with a predominance of those between third neighbours. Then, the TBIM has been validated, by checking its ability to account for H – atomic vacancy chemical order on interstitial sublattice, by comparing ordering energies of ordered structures either reconstructed using TBIM, or directly obtained from total energy calculations performed both within TB and ab initio (DFT) methods. In particular, these comparisons reveal the precision of the TBIM to reproduce energy sequences obtained from DFT, which have been also interpreted through an orbital symmetry analysis. In addition, an analysis of the impact of mechanical strains on TBIM has been performed, introducing isotropic and axial distortion on the Bravais lattice. Finally, we have introduce temperature effects through thermostatistical Monte-Carlo simulations in canonical ensemble grounded on TBIM, taking into account the variation of the effective interactions with local hydrogen concentration. This allowed us to characterize the various order-disorder transitions and to build a phase diagram of hydrogen – atomic vacancy interstitial ordering on tetrahedral interstitial sublattice of face-centered cubic zirconium hydrides.

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