About this Research Topic

Background

During the nuclear fuel cycle, stages of fabrication and of in-pile processing imply diffusion phenomena taking place in the mixed oxide ceramics. In the fabrication process, interdiffusion of uranium and plutonium cations lead to an homogeneous solid solution, and to an increased grain size. Diffusion of oxide anions lead to a deviation from stoichiometry of the fuel, which is a major parameter of its physical properties, such as thermal conductivity. These phenomena can also occur during the in-pile stay of the fuel with an impact upon creep properties. Gaseous fission products are preferred to be kept in the bulk of fuel pellets to avoid a bad cooling of fuel rods. The time for the fission gas to reach grain boundaries depends on grain size.

For all these reasons, it is necessary to be able to quantify these diffusion phenomena, either taking part in the bulk of grains, at their surfaces, or along grain boundaries.

Some properties of non-irradiated mixed oxide fuels (such as grain size, uranium-plutonium homogeneity, as well as deviation from stoichiometry, inherited from fuel fabrication) constitute the cornerstone of fuel in-pile behaviour. Fuel performance also relies upon gaseous fission products release kinetics. The goal here is to provide an improved diffusion coefficient data-set for enhanced numerical predictions.

Recent advances in diffusion simulation of actinides aim at seeking the most probable diffusion paths from ab initio and molecular dynamics calculations and seem to be promising. Experimental techniques are also developing in the nuclear field, such as electrical conductivity measurements under carefully fixed oxygen potential and temperature. Positron annihilation spectroscopy recently developed techniques leading to point defects determination implied in diffusion phenomena, or also direct diffusion study, which should offer fertile ground for new advances in this field and provide better data. These can serve to fit a reduced set of parameters describing species mobility by means of thermodynamics-based mobility fits. Lastly, these values are useful to feed in fuel performance codes such as Bison or Pleiades.

The scope of this Research Topic encompasses both experimental and numerical works, in the following areas of interest:
• Diffusion coefficient assessment: either tracer diffusion coefficients, self-diffusion coefficients, or chemical diffusion coefficients, with a special effort in order to determine the thermodynamic conditions for which their value is given, and an effort to reach an estimation of their uncertainty.
• All diffusion paths of interest for nuclear fuel fabrication or in-pile behaviour, e.g. surface diffusion, grain-boundary diffusion, bulk diffusion.

Experimental work may rely on direct experiments involving tracers, or diffusion couples, but also on indirect measurements, through for instance creep studies and sintering study, even if they lead to a partial knowledge of the diffusion coefficient, such as its activation energy.

Numerical work may range from direct atomic scale modelling of the diffusion phenomena, through the modelling of complex phenomena underpinned by several elemental diffusion phenomena, to the fit of mobility parameters of species.

All manuscript types are welcome in this Research Topic.

Keywords: diffusion, diffusion kinetics, mixed oxide nuclear fuel, diffusion coefficient, ionic mobility, experimental study of diffusion, modelling of diffusion cations, oxide ion, fission products, sintering, grain growth, homogenization

Important note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.

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