b'A combinatorial modelingExisting experimental data on metallic fuel coupled with and simulation and separatecomputational science elucidated the mechanism of the effect test approach toporous structure formation at the top of fuel pins.investigate unknownT his project analyzed the formation and characteristics of an unknown porous microstructure known as fluff found at the top of metallic fuel pins. microstructural evolutionFluff could influence reactivity during reactor operation or unexpected transients. This work analyzed neutron radiographs in the available database to in metallic fuel pin develop correlations between the fluff, the fuel characteristics, and irradiation parameters. It was found that fluff is heavily dependent on fuel composition and burnup. Operating temperature influences fluff formation for uranium containing 10% weight of zirconium, but not as strongly compared to burnup and fuel composition. An image analysis technique was developed to avoid subjectivity in fluff measurements, which had up to 15% variation when measurements were PROJECT NUMBER: performed manually. This automated method provided the most comprehensive21A1050-006FP statistical study of fluff (1,097 pins) to date and validated the results obtained manually. Furthermore, BISON modeling and simulation applied in this study TOTAL APPROVED AMOUNT: suggested a possible mechanism for fluff formation. Fluff may result as a byproduct$660,000 over 3 years of stresses between the fuel and cladding and may be related to fission gas and PRINCIPAL INVESTIGATOR: porosity formation. Finally, a series of separate effect tests have been performed inFidelma Di Lemma collaboration with two universities. These focused on evaluating creep behavior for porous materials and simulating the thermal gradients experienced by the slug at CO-INVESTIGATORS: the top of the fuel pin to evaluate its deformation and resulting stresses. Such data Andrei V Gribok, INL will provide important insight to further BISON modeling of this phenomenon.Luca Capriotti, INLPavel G Medvedev, INL Jie Lian, Rensselaer Polytechnic InstituteTravis Knight, University of South Carolina Example of fluff and its composition.20'