ABSTRACT
Nuclear reactor fuel is usually based on uranium, present as the metal or the dioxide, UO2, or as uranium-plutonium mixed oxide ((U, Pu)O2 or MOX). Opening or removal of the fuel cladding during industrial reprocessing of irradiated uranium-bearing fuel is followed by dissolution in nitric acid, HNO3, solution. The technical literature was surveyed to identify means for dealing with the cladding and determine the effects of process variables (e.g., temperature, acid concentration, off-gas control, fuel condition) on the rates and characteristics of uranium-based fuel dissolution in HNO3 and the corresponding behaviors of plutonium and fission product release from the fuel during dissolution. Findings from this survey and information on the processes and equipment used to address the cladding and dissolve the fuel are presented.
The reactions of irradiated uranium metal and dioxide with nitric acid are highly exothermic (heat-producing) and produce oxides of nitrogen (NO x ) gases plus minor amounts of other gases of chemical reaction origin, uranium in the form of dissolved uranyl nitrate, UO2(NO3)2, and plutonium in the form of dissolved plutonium (IV) or (VI) nitrate, Pu(NO3)4 or PuO2(NO3)2. These dissolution reactions release fission products from the fuel in the form of gases (krypton, xenon, and iodine), dissolved fission product nitrates, insoluble fission product residues (such as epsilon phase metals and zirconium/niobium precipitates), and, for MOX fuels with high plutonium contents, undissolved PuO2. In the interest of high process throughput, simplicity in process control, and because of high reaction heat, most fuel dissolution is conducted in boiling or near-boiling solution. Because of autocatalysis mediated by nitrous acid, HNO2, batch dissolutions start slowly but accelerate as the autocatalyst builds in concentration. The reaction rates are linearly dependent on geometric surface area. The dissolution generally is isotropic but will proceed more rapidly at the ends than along the walls of extruded cylindrical uranium fuel and be sensitive to surface roughening or etching for both metal and oxide. With these limitations, the dissolution rate can be expressed as the rate of linear penetration (e.g., μm/hour).
Results of numerous studies of uranium and uranium dioxide dissolution in nitric acid show remarkable consistency, with the dissolution rate at boiling dependent on the nitrate concentration raised to approximately the 2.5–2.9 power. Although the dissolution reaction is mediated by nitrous acid, other catalysts or additives have been tested in efforts to increase the uranium metal dissolution rate. None, however, was found to produce acceptable reprocessing plant feed solutions. The uranium metal and dioxide dissolution rates increase with decreasing metal grain size, increase with developing surface roughness caused by corrosion, and decrease with irradiation. Plutonium and fission product release during dissolution depend on the bulk fuel dissolution rate and are higher earlier in the dissolution processing due to their higher relative concentrations near the fuel surface.
The mechanical and chemical means to achieve cladding removal and uranium-based fuel dissolution in nitric acid are addressed in this chapter.
