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

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... ew 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. 5 INTRODUCTION The 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 irradiation testing strategy for new coated particle fuels. Preliminary research has indicated that high-temperature gas reactor technology has significant potential to satisfy the safety, economic, proliferation, and waste disposal concerns that face nuclear electric generating technologies. However, numerous technical issues must be addressed before this technology becomes commercially viable. The work in this project will be performed by the Idaho National Engineering and Environmental Laboratory (INEEL) in collaboration with the French CEA and the Massachusetts Institute of Technology. The project has been organized into five tasks to accomplish the objectives above: in Task 1, information will be exchanged relative to material property databases and existing fuel models, in Task 2, an integrated fuel model will be developed that includes the effects of multi-dimensional failure mechanisms and phenomena not currently in the models, in Task 3, deterministic fuel performance calculations will be performed to evaluate the capacity of classical TRISO fuel to reach extended burnups, and thereby establish requirements for fuel materials, in Task 4, the feasibility of using particle fuel in a fast neutron environment will be investigated, and in Task 5, an irradiation testing strategy for prototype fuel particles will be developed. 7 TASK 1: INFORMATION EXCHANGE ON EXISTING PARTICLE FUEL DATA, MODELS, AND COMPREHENSION Responsible Leads: INEEL, CEA Brief Description of Objectives: The CEA and INEEL will exchange their current databases on coated particle fuel performance during irradiation and the computer models and material property correlations that have been developed to describe that performance. This information will include fission gas release data (e.g., release to birth ratios for various fission gases) from irradiation experiments, and postirradiation examination results documenting the physical state of the TRISO coatings and kernels after irradiation. In addition, information on the pedigree of the fuel including fabrication conditions will be included when available. (Much of the previous U.S. database will be obtained from the open literature or from GA.) Included with this information, the INEEL will provide detailed results from the NPR-1, NPR-1A and NPR-2 experiments that were conducted at the INEEL as part of the New Production Reactor program. Once received, the particle fuel databases will be reviewed and critically assessed. This assessment will identify data needs in regard to implementation and further development of fuel behavior models. Attempts will be made to fulfill those data needs. Task Technical Status Overview: INEEL: Work has begun on reviewing and assessing particle fuel material property correlations. These correlations are used in model predictions of fuel performance during irradiation. Such predictions are useful to understand the interplay of important phenomena that could occur outside of the existing irradiation envelope of temperature, burnup and fast neutron fluence. It has been observed that property data are generally lacking for materials exposed to high fuel burnups and neutron fluences. This current lack of data will introduce uncertainty into model 8 predictions of fuel performance. Several key material properties that affect fuel performance are briefly discussed below. PyC Shrinkage and Swelling Under irradiation, the PyC layers of the fuel particle experience either shrinkage or swelling which affects the amount of stress experienced by the SiC layer. The correlations currently used to represent shrinkage, or swelling, of the PyC layers were obtained from empirical fits to data as compiled by the CEGA Corporation (CEGA 1993). These correlations are good up to fluences of 3.7 x 10 25 n/m 2 and are functions of temperature, anisotropy, density and fast neutron fluence. Figure 1-1 displays several fits and the underlying data at various temperatures. The amount of data is rather sparse which introduces significant uncertainty when the correlations are extrapolated beyond neutron fluences of 5 x 10 25 n/m 2 .