Analysis of the Off-Gas from Dissolution of Nuclear Oxideand Carbide Fuels in Nitric Acid

Abstract

The off-gas composition upon dissolution of different types of non-irradiated nuclear fuels (UC, U 2 C 3 , U0 2 , (U,Pu)C and (U,Pu)0 2 ) in HNOj has been measured and the influence of dissolution parameters such as dissolution temperature, nitric acid molarity, fuel density and composition has been studied.For fast dissolutions less off-gas, especially N0 2 , is produced than for slow ones. No CO could be measured for the carbide dissolutions and NO is the most important constituent in the off-gas. For oxides and sesquicarbides, the N0 2 is the predominant species. Carbides are dissolved much faster than oxides and they don't leave any residue, but the solution stays dark and a slight N0 2 and C0 2 evolution continues after the dissolution is complete. This can be ascribed to the destruction of the organic compounds (mellitic acid, oxalic acid etc.) formed during dissolution.A nitric acid concentration of 6 Μ and temperatures of 80 -90 °C are optimum conditions to dissolve fuel materials in a reasonable time with a low nitrogen oxide content in the ofT-gas. The activation energy calculated from the Arrhenius plot was found to be 43.5 kJ/mol for UC. For U0 2 two values for activation energy were found, 64.1 kJ/mol if the fuel pellets are pressed from U 3 0 8 powder and 54.8 kJ/mol if the pellets are made from U0 2 powder.

Dissolution of uranium dioxide in nitric acid media: what do we know?

“…Glatz et al [8] have used a similar experimental system to that of Hermann, but they endeavored minimizing the contact duration between the off-gases and the dissolution medium by depositing the uranium dioxide pellet on a glass frit. A helium flow is applied, which dilutes and removes quickly the off-gases from the dissolution medium, thus avoiding any chemical reaction between the off-gases and the medium.…”

“…It can be seen on Sakurai's spectra that this absorption band is absent. Hermann [5] and Glatz et al [8] do not give the spectra they obtained, making the same comparison impossible. Given the strong oxidizing property of the nitric acid media in these studies, it is unlikely that N 2 O results of a reaction between the dissolution medium and the off-gases.…”

“…Thus, the detection by Hermann [5] of N 2 O in the off-gases may find its origin in the difference in the experimental procedures: Herrmann sent the off-gases through a condenser before analyzing it, while Sakurai et al [9] used a liquid-nitrogen trap to recover it before analysis. Glatz et al [8] also placed a condenser between the IR analyser and the dissolution reactor, but the fact that the off-gases are diluted with helium before flowing through it could result in the absence of recombination reactions. Thus, it seems that N 2 O is not produced during dissolution of uranium dioxide in nitric acid media, and that the reason of its detection by Hermann [5] remains unclear and could be attributed to the experimental procedure she used.…”

“…Many authors have pointed out an evolution of the NO/ NO 2 ratio depending on the concentration in nitric acid of the dissolution solution [5,8,9,12].…”

“…Glatz et al [8] and Sakurai et al [9] have measured the quantity of degased NO x during the dissolution of UO 2 pellets under various nitric acid concentrations. Table 2 presents the results they obtained, and It must also be pointed out that in the studies of Glatz et al [8], two types of pellets were dissolved.…”