This report describes the development of a dissolution flowsheet for neptunium (Np) oxide (NpO(sub 2)) residues (i.e., various NpO(sub 2) sources, HB-Line glovebox sweepings, and Savannah River National Laboratory (SRNL) thermogravimetric analysis samples). Samples of each type of materials proposed for processing were dissolved in a closed laboratory apparatus and the rate and total quantity of off-gas were measured. Samples of the off-gas were also analyzed. The quantity and type of solids remaining (when visible) were determined after post-dissolution filtration of the solution. Recommended conditions for dissolution of the NpO(sub 2) residues are: Solution Matrix and Loading: (approx)50 g Np/L (750 g Np in 15 L of dissolver solution), using 8 M nitric acid (HNO(sub 3)), 0.025 M potassium fluoride (KF) at greater than 100 C for at least 3 hours. Off-gas: Analysis of the off-gas indicated nitric oxide (NO), nitrogen dioxide (NO(sub 2)) and nitrous oxide (N(sub 2)O) as the only identified components. No hydrogen (H(sub 2)) was detected. The molar ratio of off-gas produced per mole of Np dissolved ranged from 0.25 to 0.4 moles of gas per mole of Np dissolved. A peak off-gas rate of (approx)0.1 scfm/kg bulk oxide was observed. Residual Solids: Pure NpO(sub 2) dissolved with little or no residue with the proposed flowsheet but the NpCo and both sweepings samples left visible solid residue after dissolution. For the NpCo and Part II Sweepings samples the residue amounted to (approx)1% of the initial material, but for the Part I Sweepings sample, the residue amounted to (approx)8 % of the initial material. These residues contained primarily aluminum (Al) and silicon (Si) compounds that did not completely dissolve under the flowsheet conditions. The residues from both sweepings samples contained minor amounts of plutonium (Pu) particles. Overall, the undissolved Np and Pu particles in the residues were a very small fraction of the total solids.
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