The thermal expansion of uranium mononitride (UN) is calculated using in situ neutron powder diffraction data between 673 – 1273 K. The true, or instantaneous, thermal expansion describes the instantaneous rate of dimensional change at a given temperature. The true thermal expansion coefficient of UN calculated from this work and all available literature values are within error. The true linear thermal expansion coefficient (α_t) of UN between 250 K and 2523 K is described by: α_t=6.5(2)×10⁻⁶+3.0(2)×10⁻⁹T This can be converted to the mean linear thermal expansion coefficient (α_m), for any T₀ and T value in the temperature range, using the equation: α_m=exp[6.5×10⁻⁶(T−T₀)+3.0×10⁻⁹2(T²−T₀²)]−1T−T₀

U(X)N-based SIMFUEL samples, where X represents Zr, Nb, Mo, and Ru, were fabricated using spark plasma sintering. These samples were characterized by neutron diffraction and scanning electron microscopy to gain insights into fission product solubility and speciation at high burnup levels. The fabricated samples included pseudo-binary and higher-order compositions, allowing for the decomposition of individual fission product effects. The characterization revealed the presence of U1-xZrxN, Zr1-xUxN, ZrN, Nb1-xUx, UxNb1-x, Nb2N, URu3, Mo, and (U,Mo)Ru3 as distinct fission-product-containing phases. Notably, only Zr was found to be soluble within the primary UN fuel matrix. Significant agglomeration and formation of a (Nb-rich core)–(Nb-poor shell) microstructure was observed for the Nb-containing samples. Mo was the only fission product to form metallic inclusions and the presence of Ru led to the formation of URu3 in the pseudo-binary system (UN-10at.%Ru), or (U,Mo)Ru3 in the higher-order samples containing 1, 1.5, and 2 at.% each of all of fission product elements i.e. UN-1at.%(ZrN, Nb, Mo, Ru). No complex nitride precipitates were found to form. The phases identified in the pseudo-binary compositions were analyzed using the Thermodynamics of Advanced Fuels-International Database (TAF-ID) and showed good agreement to experimental data, except for a possible miscibility gap in the UN-ZrN tie line and absence of the (U,Mo)Ru3 phase.

The temperature-dependent effective thermal conductivity of UN-X-UO2 (X = Mo, W) nuclear fuel composite was estimated. Following the experimental design, the thermal conductivity was calculated using Finite Element Modeling (FEM), and compared with analytical models for 10%, 30%, 50%, and 70% (in mass) uncoated/coated UN microspheres in a UO2 matrix. The FEM results show an increase in the fuel thermal conductivity as the mass fraction of the UN microspheres increases from 1.2 to 4.6 times the UO2 reference at 2,000 K. The results from analytical models agree with the thermal conductivity estimated by FEM. The results also show that Mo and W coatings have similar thermal behaviors, and the coating thickness influences the thermal conductivity of the composite. At higher weight fractions, the thermal conductivity of the fuel composite at room temperature is substantially influenced by the high thermal conductivity coatings approaching that of UN. Thereafter, the thermal conductivity from FEM was used in the fuel thermal performance evaluation during LWR normal operation to calculate the maximum centerline temperature. The results show a significant decrease in the fuel maximum centerline temperature ranging from -94 K for 10% UN to -414 K for 70% (in mass) UN compared to UO2 under the same operating conditions.

In this study, the temperature-dependent effective thermal conductivity of the innovative UN-X-UO₂ (X=Mo, W) nuclear fuel composite has been estimated in the temperature range from room temperature to 2000 K. This composite fuel concept is considered as a promising accident tolerant fuel for light water reactors (LWRs). Following the previously reported experimental composite design, the composite fuel thermal conductivity was calculated using Finite Element modeling (FEM), and it is compared with analytical models of thermal conductivity for 10, 30, 50, and 70 wt.% uncoated/coated UN microspheres in a UO₂ matrix. The FEM results show an expected increase in the fuel thermal conductivity as the wt.% of the coated/uncoated UN microspheres increases – from 1.5 to 5.7 times the UO₂ reference at 2000 K. However, the analytical models show an overestimation of the fuel thermal conductivity as the wt.% increases. The results also show that Mo and W coatings have similar thermal behaviors and the coating thickness varying from 1-5 μm has an insignificant effect on the thermal behavior of the composite. However, at higher weight fractions, the thermal conductivity of the fuel composite at room temperature is substantially influenced by the high thermal conductivity coatings exceeding that of UN. Thereafter, the thermal conductivity profiles from FEM were used in the fuel thermal performance evaluation during LWR normal operation to calculate the maximum centerline temperature of the fuel composites. The results show a significant decrease in the fuel maximum centerline temperature ranging from −72 K for 10 wt.% UN to −438 K for 70 wt.% UN compared to the UO₂ under the same irradiation conditions, providing an enhanced safety margin and thermal and neutronic advantages.