2017 | AFC ACCOMPLISHMENTS 141 viable fuel additive, the additive has to preferentially bind the lanthanides over other fuel constituents.The as-cast fuel indicates a preference to bind the lanthanides.A small amount of Zr-Sn precipitates are still present due to a small excess of Sn over the lanthanides. To better investigate and understand the interaction between the additive and the lanthanides, a diffusion couple was run between U-10Zr-4.3Sn and the lanthanides, shown in Figure 1b. In the as-cast and annealed alloys, high temperature phases generated in the arc melter are possible; phases that won’t form under reactor temperature, i.e. 650ºC. In the diffusion couple, the temperature never goes beyond 650ºC. In Figure 1b, the lanthanides are on the left, and the alloy on the right.The large, dark precipitates in the upper right hand corner are Zr5Sn3.The lanthanides are aggressively reacting with the precipitates near the interface. Ln5Sn3 is forming, releasing Zr from the precipitate. Uranium is coming into what was once the Zr5Sn3 precipitate and forming UZr2 with the excess Zr.This result shows conclusively the preference for Sn to bind the lanthanides over other fuel constituents.This result, along with the stability of Zr5Sn3 in the as-cast fuel, show tin to be an excellent additive material for controlling FCCI. To continue the investigation of tin, more diffusion couples are underway. In addition, two alloys, U-20Pu- 10Zr-4.3Sn and U-20Pu-10Zr-4.3Sn- 4.7Ln, were fabricated in the Fuel Manufacturing Facility (FMF) in FY17, to explore tin interactions in an alloy containing Pu. Characterization and diffusion couples will be run in FY18. In FY16, 2 alloys with Pd included were fabricated in FMF. During FY17, those alloys were cut for analysis, and transferred to the appropriate facility for characterization.The alloy compositions (in wt %) are U-20Pu-10Zr-3.86Pd, U-20Pu- 10Zr-3.86Pd-4.3Ln (where Ln = 53Nd-25Ce-16Pr-6La).A third alloy, U-19Pu-0.7Zr-4.3Ti-5Mo, was also fabricated and sectioned for analysis.This is follow-on work to the U-0.7Zr-4.3Ti-5Mo alloy investigated at Colorado School of Mines.All three alloys have been characterized by SEM, and the Pd alloys have also been characterized with transmission electron microscopy (TEM). Diffusion couples have been run between the 3 alloys and iron. Those samples will undergo SEM and possiblyTEM analysis in FY18. In addition to analyzing the diffusion zone, the bulk material represents the annealed microstructure, and will be investigated as well. Fuel-cladding chemical interaction due to fission product lanthanides can be prevented using additives to react with the lanthanides.