Joshua Taylor White

Prospective advanced accident tolerant UN-USi fuel forms for light-water reactors are being studied at Los Alamos National Laboratory. In this paper we present a recent effort to fabricate and characterize U{sub 3}Si{sub 5} and UN-U{sub 3}Si{sub 5} fuel. The U{sub 3}Si{sub 5} is prepared by arc-melting stoichiometric amounts of uranium and silicon metals. The arc melting process consists of cutting U metal plate into small pieces that can be placed in a tri-arc furnace for melting. The material is slowly heated until a uniform melt of the U and Si is achieved. The fabrication of the U{sub 3}Si{sub 5} pellets followed a conventional cold-press and sinter process. As for the fabrication of UN-U{sub 3}Si{sub 5}, the uranium nitride (UN) feedstock powder was synthesized using carbothermic reduction nitridation. The UN is milled and sieved to sub-74 μm. The UN was mixed with EBS (ethylene bis-stearamide) and milled for 45 minutes. The UN-EBS was combined with 10 wt.% U{sub 3}Si{sub 5} powder (equivalent to 15 vol.% in UN) and additional EBS. The fabrication of the UN-U{sub 3}Si{sub 5} pellets followed a conventional cold-press and sinter process similar to that used to fabricate the U{sub 3}Si{sub 5} pellets. A sample UN-U{sub 3}Si{sub 5} pellet was cross-sectioned, ground, and polished for optical microscopy. The UN and U{sub 3}Si{sub 5} phases are apparent as well as pores and micro-cracks. The affect of micro-cracking on fuel performance is unknown and should be assessed during post-irradiation examination.

Characterization of the properties of U-Si compounds first required synthesis of high purity material for each of the compositions. The four compounds targeted for investigation in this work were U{sub 3}Si, U{sub 3}Si{sub 2}, USi, and U{sub 3}Si{sub 5}. Specimens of U{sub 3}Si{sub 2}, U{sub 3}Si{sub 5}, and USi with specific geometries for individual testing procedures were prepared using a powder metallurgical method. Thermophysical properties of the U-Si compounds were determined within 100 K of their respective melt point or decomposition temperature. Thermal expansion was determined with a dilatometer, specific heat capacity by the ratio method in a differential scanning calorimeter using a sapphire standard, while thermal diffusivity was determined with a laser flash analyzer. Oxidation performance of the U-Si compounds was determined in a thermogravimetric analyzer (TGA), which measures the weight change of a sample as a function of gas composition and temperature. The results show that each of the U-Si compounds is electronically conductive leading to higher thermal conductivity values than the reference fuel UO{sub 2}. Higher thermal conductivity values will lead to enhanced safety margins within a reactor core and provide better economics over the lifetime of a fuel cycle. However, no improvements have been realized over the reference UO{sub 2} case with regards to oxidation performance in air although more testing is required to substantiate the differences especially in steam and corrosive aqueous LWR environments. Finally, there exists a lack of knowledge in the literature with regards to low burnup irradiation testing on all of the U-Si binary compounds, which would be necessary to simulate their performance in a reactor setting.