Dissolution kinetics and rate-controlling mechanism of sintered uranium nitride pellets in nitric acid

Abstract Uranium nitride (UN) is a promising advanced nuclear fuel material because of its high thermal conductivity and high melting point. Efficient reprocessing of UN is essential for minimizing uranium consumption and reducing the volume and radiotoxicity of nuclear waste. Dissolution is a key step in the reprocessing process. This work investigates the dissolution kinetics of sintered UN pellets in nitric acid. UN powder was synthesized from UO 2 powder by carbothermic nitridation and characterized by X-ray diffraction (XRD). Pellets with densities of 90–92.5% of the theoretical density were produced by spark plasma sintering (SPS) at 1923 K and 75 MPa. Dissolution experiments were conducted in 4, 6, and 8 M nitric acid at 323, 343, and 363 K. A dissolution extent of 100% was achieved under several of the investigated conditions. Increasing the nitric acid concentration from 4 to 8 M and the temperature from 323 to 363 K resulted in an increased dissolution rate. Kinetic analysis using the shrinking-core model showed that surface chemical reaction control described the dissolution behavior better than diffusional transport. The apparent activation energies obtained from Arrhenius plots ranged from 22.9 to 35.3 kJ/mol, further indicating that the dissolution process is predominantly controlled by a surface chemical reaction.