Alain Chartier

Modeling and Simulation of Nuclear Fuel Materials

We review the state of modeling and simulation of nuclear fuels with emphasis on the most widely used nuclear fuel, UO2. The hierarchical scheme presented represents a science-based approach to modeling nuclear fuels by progressively passing information in several stages from electronic structure calculations to continuum level simulations. Such an approach is essential to overcome the challenges posed by radioactive materials handling, experimental limitations in modeling extreme conditions and accident scenarios, and the small time and distance scales of fundamental processes. When used in conjunction with experimental validation, this multiscale modeling scheme can provide valuable guidance to development of fuel for advanced reactors to meet rising global energy demand.

Molecular dynamics simulation of energetic uranium recoil damage in zircon

Defect production and amorphisation due to energetic uranium recoils in zircon (ZrSiO4), which is a promising ceramic nuclear waste form, is studied using molecular dynamics (MD) simulations with a partial charge model. An algorithm that distinguishes between undamaged crystal, crystalline defects and amorphous regions is used to develop a fundamental understanding of the primary damage state. The amorphous cascade core is separated from the surrounding crystal by a defect-rich region. Small, chemically inhomogeneous amorphous clusters are also produced around the core. The amorphous regions consist of under-coordinated Zr and polymerised Si leading to amorphisation and phase separation on a nanometer scale into Zr- and Si-rich regions. This separation could play an important role in the experimentally observed formation of nanoscale ZrO2 in ZrSiO4 irradiated at elevated temperatures.

Atomistic Simulation of Amorphization Thermokinetics in Lanthanum Pyrozirconate

The kinetics of amorphization in La2Zr2O7 pyrochlore is investigated using molecular dynamics simulations. Irradiation damage is simulated by continuous accumulation of cation Frenkel pairs at various temperatures. As observed experimentally, La2Zr2O7 first transitions to the fluorite structure, independent of the temperature, and amorphization occurs at low temperatures. A model fit of the simulated dose-temperature curve reproduces experimental results in the literature, with a low temperature amorphization dose D0=1.1 displacement per cation and an activation energy Eact=0.036eV. Present simulations indicate that point defect recombination can control the temperature dependence of amorphization driven by point defect accumulation.

Comment on: Large swelling and percolation in irradiated zircon

A recent model for the large radiation-induced swelling exhibited by irradiated zircon (ZrSiO4) is partially based on results of molecular dynamics (MD) simulations of the partial overlap of two collision cascades that predict a densified boundary of polymerized silica and the scattering of the second cascade away from the densified boundary (Trachenko et al 2003 J. Phys.: Condens. Matter 15 L1). These MD simulations are based on an atomic interaction potential for zircon (Trachenko et al 2001 J. Phys.: Condens. Matter 13 1947), which, according to our analysis, only reproduces some of the crystallographic properties at equilibrium and does not adequately describe the atomic scattering physics for zircon, and on simulation methodologies for energetic events that are ill defined. In fact, the interatomic potential model used by Trachenko et al yields a significantly more rigid structure, with very high Frenkel defect formation energies and extremely low entropy and specific heat capacity. Consequently, the reported results of the cascade simulations, which are events far from equilibrium, may be artifacts of both the potential model and simulation methodologies employed. Thus, the structural changes predicted by the simulations must be viewed cautiously, as these simulation results cannot be taken as confirmation of a new scattering physics process that is the basis for the proposed swelling model. In this comment, the deficiencies in the atomic interaction potential and methodologies employed by these authors are critically reviewed, and the validity of the cascade overlap simulations and proposed physics is discussed.

Disordering and grain boundaries of (Ni,Fe)Cr2O4 spinels from atomistic calculations

A novel empirical potential has been developed to evaluate the thermodynamic stability of Ni(1-x)Fe(x)Cr2O4 spinels. The simulations confirm the hypothesis that the NiCr2O4-FeCr2O4 pseudo-binary has normal structure spinel up to 1000 K and stabilizes as a solid solution. However, the disordering energy (normal to inverse spinel) is found higher for FeCr2O4 than for NiCr2O4 spinel. The formation energies of tilt, twist, and random grain boundaries have been calculated in pure NiCr2O4 and FeCr2O4. The same behavior has been found for both spinels. Detail analysis of the grain boundaries structure shows that the cation coordination number is a key parameter for the stability of the grain boundaries. With this criterion, we evidenced that the structural and energetic differences are caused only by nickel and iron cations.