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.