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The microstructures of rapidly solidified U-6wt%Nb powder particles synthesized by centrifugal atomization were characterized using scanning electron microscopy and transmission electron microscopy. Observed variations in microstructure are related to particle sizes. All of the powder particles exhibited a two-zone microstructure. The formation of this two-zone microstructure is described by a transition from solidification controlled by internal heat flow and high solidification rate during recalescence (micro-segregation-free or partitionless growth) to solidification controlled by external heat flow with slower solidification rates (dendritic growth with solute redistribution). The extent of partitionless solidification increased with decreasing particle size due to larger undercoolings in smaller particles prior to solidification. The metastable phases that formed are related to variations in Nb concentration across the particles. The microstructures of the powders were heavily twinned.

U-silicide or U-nitride coated U-Mo particle dispersion fuel in Al (U-Mo/Al) was in-pile tested to examine the effectiveness of the coating as a diffusion barrier between the U-7Mo fuel kernels and Al matrix. This paper reports the PIE data and analyses focusing on the effectiveness of the coating in terms of interaction layer (IL) growth and general fuel performance. The U-silicide coating showed considerable success, but it also provided evidence for additional improvement for coating process. The U-nitride coated specimen showed largely inefficient results in reducing IL growth. From the test, important observations were also made that can be utilized to improve U-Mo/Al fuel performance. The heating process for coating turned out to be beneficial to suppress fuel swelling. The use of larger fuel particles confirmed favorable effects on fuel performance.

Performance of U–7 wt.%Mo with 1 wt.%Ti, 1 wt.%Zr or 2 wt.%Zr, dispersed in an Al–5 wt.%Si alloy matrix, was investigated through irradiation tests in the ATR at INL and HANARO at KAERI. Post-irradiation metallographic features show that the addition of Ti or Zr suppresses interaction layer growth between the U–Mo and the Al–5 wt.%Si matrix. However, higher fission gas swelling was observed in the fuel with Zr addition, while no discernable effect was found in the fuel with Ti addition as compared to U–Mo without the addition. Known to have a destabilizing effect on the γ-phase U–Mo, Zr, either as alloy addition or fission product, is ascribed for the disadvantageous result. Considering its benign effect on fuel swelling, with slight disadvantage from neutron economy point of view, Ti may be a better choice for this purpose.

As the fuel-cladding chemical interaction (FCCI) between metallic fuel and cladding material can occur at a relatively low temperature, electroplating of Cr as an FCCI barrier on the inner wall of the cladding was proposed. U–10 wt.%Zr and HT9 disks were used as fuel and cladding material to conduct diffusion couple annealing tests at 740 and 800 °C. Two kinds of Cr layers dependent on the temperature of plating bath were electroplated on the surface of the HT9 disks. The Cr layer of 20 μm in thickness, after being electroplated at a bath temperature of 50 °C and a current density of 35 A/dm2 for 1 h, had some cracks and a small-grained structure. However, the Cr layer plated at a bath temperature of 80 °C and a current density of 56 A/dm2 for 2 h showed a dense structure. Both Cr layers inhibited the interaction between U–10 wt.%Zr and HT9 at a relatively high annealing temperature. U penetrated into the Cr layer plated at a bath temperature of 50 °C due to its penetrable structure. The Cr layer plated at a bath temperature of 80 °C showed a better performance than the Cr layer plated at a bath temperature of 50 °C as a diffusion barrier material with less U penetration.

The growth behavior of reaction layers and heat generation during the reaction between U–Mo powders and the Al matrix in U–Mo/Al dispersion fuels were investigated. Annealing of 10 vol.% U–10Mo/Al dispersion fuels at temperatures from 500 to 550 °C was carried out for 10 min to 36 h to measure the growth rate and the activation energy for the growth of reaction layers. The concentration profiles of reaction layers between the U–10Mo vs. Al diffusion couples were measured and the integrated interdiffusion coefficients were calculated for the U and Al in the reaction layers. Heat generation of U–Mo/Al dispersion fuels with 10–50 vol.% of U–Mo fuel during the thermal cycle from room temperature to 700 °C was measured employing the differential scanning calorimetry. Exothermic heat from the reaction between U–Mo and the Al matrix is the largest when the volume fraction of U–Mo fuel is about 30 vol.%. The unreacted fraction in the U–Mo powders increases as the volume fraction of U–Mo fuel increases from 30 to 50 vol.%.

The lattice parameters of simulated DUPIC fuel and UO2 were measured from room temperature to 1273 K using neutron diffraction to investigate the thermal expansion and density variation with temperature. The lattice parameter of simulated DUPIC fuel is lower than that of UO2, and the linear thermal expansion of simulated DUPIC fuel is higher than that of UO2. For the temperature range from 298 to 1273 K, the average linear thermal expansion coefficients for UO2 and simulated DUPIC fuel are 10.471×10−6 and 10.751×10−6 K−1, respectively.