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This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint should not be cited or reproduced without permission of the author. This document was prepared as an account of work sponsored by an agency of the United States Government.

The objective of this INERI project is to develop improved fuel behavior models for gas reactor coated particle fuels and to develop improved coated-particle fuel designs that can be used reliably at very high burnups and potentially in fast gas-cooled reactors. Thermomechanical, thermophysical, and physiochemical material properties data were compiled by both the US and the French and preliminary assessments conducted. Comparison between U.S. and European data revealed many similarities and a few important differences. In all cases, the data needed for accurate fuel performance modeling of coated particle fuel at high burnup were lacking. The development of the INEEL fuel performance model, PARFUME, continued from earlier efforts.The statistical model being used to simulate the detailed finite element calculations is being upgraded and improved to allow for changes in fuel design attributes (e.g. thickness of layers, dimensions of kernel) as well as changes in important material properties to increase the flexibility of the code. In addition, modeling of other potentially important failure modes such as debonding and asphericity was started. A paper on the status of the model was presented at the HTR-2002 meeting in Petten, Netherlands in April 2002, and a paper on the statistical method was submitted to the Journal of Nuclear Material in September 2002. Benchmarking of the model against Japanese and an older DRAGON irradiation are planned. Preliminary calculations of the stresses in a coated particle have been calculated by the CEA using the ATLAS finite element model. This model and the material properties and constitutive relationships will be incorporated into a more general software platform termed Pleiades. Pleiades will be able to analyze different fuel forms at different scales (from particle to fuel body) and also handle the statistical variability in coated particle fuel. Diffusion couple experiments to study Ag and Pd transport through SiC were conducted. Analysis and characterization of the samples continues. Two active transport mechanisms are proposed: diffusion in SiC and release through SiC cracks or another, as yet undetermined, path. Silver concentration profiles determined by XPS analysis suggest diffusion within the SiC layer, most likely dominated by grain boundary diffusion. However, diffusion coefficients calculated from mass loss measurements suggest a much faster release path, postulated as small cracks or flaws that provide open paths with little resistance to silver migration. Work is ongoing to identify and characterize this path. Work on Pd behavior has begun and will continue next year. INTRODUCTIONThe objectives of this INERI project are to:develop improved fuel behavior models for gas cooled reactor particle fuels (initially for TRISO-coated fuels), develop improved gas cooled reactor particle fuel coating materials and designs that will reliably reach very high burnups, assess the extension of the particle fuel concept to hard spectrum gas-cooled cores, and develop an irradiati...