Unlike irradiated U02 fuel, which dissolves readily in hot nitric acid, irradiated Th02-U02 fuel dissolves only very slowly and incompletely in this reagent. After extensive research, Oak Ridge National Laboratory concluded [B14] that the best reagent for producing a nitrate solution from Th02-U02 fuel was a mixture of nitric and hydrofluoric acids. This reagent has two serious drawbacks:

1. Mixed nitric and hydrofluoric acids react also with stainless steel and zircaloy, so that both the cladding and the stainless steel dissolver itself are attacked

2. Dissolution of Th02 is much slower than that of U02 in nitric acid.

Oak Ridge found [El] that corrosion of 304L and 309SCb stainless steel by the mixture of HN03 and HF could be reduced to an acceptable level by addition of aluminum nitrate, A1(N03)3, to the mixture of HN03 and HF without decreasing the rate of solution of Th02 by more than 20 percent. The aluminum nitrate acts by complexing the fluoride ion. The composition recommended for the solvent is 13 Af HN03, 0.05 M HF, 0.1 M A1(N03)3. Reddick [R6] has summarized the extensive research that has been conducted on dissolution of Th02 in HN03-HF-A1(N03)3 mixtures. This procedure still has drawbacks. Parts of a stainless steel dissolver exposed to hot HN03-HF vapors, which contain no complexing A1(N03)3, are not protected and will corrode. Aluminum nitrate is only partially effective in preventing reaction of HF with zircaloy cladding. Aluminum nitrate increases the volume of nonvolatile solids in the waste.

The rate of dissolution of Th02-U02 fuel is higher the lower the density of the fuel and the smaller the particle size. For example, with 200 percent stoichiometric excess of reagent, from 25 to 40 h were required to dissolve completely pellets 0.66 cm in diameter having 90 to 95 percent of theoretical density, whereas in 5 h, 99 percent of such fuel dissolved when the fuel was first crushed to under 100 mesh [ВІЗ]. Fuel with only 60 percent of theoretical density dissolved almost 10 times as fast as fuel having 90 percent density.