Study of the dissolution of uranium nitrides in nitric acid by measuring the isotope ratios, ¹⁵N/¹⁴N, of the formed products

The aim of this study was to investigate the dissolution behavior of nitride fuels in nitric acid. The use of nitride fuels in nuclear reactor has many advantages compared with the oxide fuels. One problem in employing nitrides as fuels is the formation of radio-toxic ¹⁴C upon irradiation of natural nitrogen (¹⁴N:99.64 pc, ¹⁵N:0.36 pc) in a nuclear reactor (¹⁴N (n,p) ¹⁴C reaction). The use of ¹⁵N-enriched fuels avoids these drawbacks. This study was undertaken so as to better understand the mechanisms of the dissolution process and also to follow the distribution of the expensive nitrogen isotope ¹⁵N from the point of view of its behaviour during the recycling process. This study is based on previous work, where the evolution of the nitrogen compounds formed during the dissolution was measured as a function of time for different dissolution parameters. Using ¹⁵N-enriched uranium nitrides or ¹⁵N-enriched nitric acid, two methods were developed to study the influence of the dissolution parameters, nitric acid temperature and concentration, on the ¹⁵N/¹⁴N ratios of the nitrogen, nitrogen oxides and ammonium ions utilising a coupled gas-chromatograph/mass spectrometer. The main results are: - similar isotopic composition for NH₄⁺ and UN; - mixed ¹⁴N/¹⁵N composition for N₂ and N₂O; - similar isotopic composition for NO, NO₂ and HNO₃; - no influence of the dissolution parameters on the isotopic composition of the products; an exception maybe made for the N₂ case, which contains more ¹⁵N with increasing acidity and temperature. This work confirms that the first dissolution step is the oxidation of UN with HNO₃ to form NH₄⁺ and HNO₂ and that HNO₂ has a catalytic role in the dissolution to form other products. And we can conclude that to recycle ¹⁵N, the ammonium ions must be recycled, at least for the case where nitrides are dissolved directly in HNO₃. (author)