Gadolinium Oxalate Solubility Measurements In Nitric Acid Solutions

HB-Line will begin processing Pu solutions during FY2012 that will involve the recovery of Pu using oxalate precipitation and filtration. After the precipitation and filtration processes, the filtrate solution will be transferred from HB-Line to H-Canyon. The presence of excess oxalate and unfiltered Pu oxalate solids in these solutions create a criticality safety issue if they are sent to H-Canyon without controls in H-Canyon. One approach involves H-Canyon receiving the filtrate solution into a tank that is poisoned with soluble gadolinium (Gd). Decomposition of the oxalate will occur within a subsequent H-Canyon vessel. The receipt of excess oxalate into the H-Canyon receipt tanks has the potential to precipitate a portion of the Gd poison in the receipt tanks. Because the amount of Gd in solution determines the maximum amount of Pu solids that H-Canyon can receive, H-Canyon Engineering requested that SRNL determine the solubility of Gd in aqueous solutions of 4-10 M nitric acid (HNO3), 4-12 g/L Gd, and 0.15-0.25 M oxalic acid (H2C2O4) at 25 C. The target soluble Gd concentration is 6 g/L. The data indicate that the target can be achieved above 6 M HNO3 and below 0.25 M H2C2O4. For 6 M HNO3, 10.5 g/L and 7 g/L Gd are soluble in 0.15 M and 0.25 M H2C2O4, respectively. In 4 M HNO3, the Gd solubility drops significantly to 2 g/L and 0.25 g/L in 0.15 M and 0.25 M H2C2O4, respectively. The solubility of Gd at 8-10 M HNO3 exceeds the solubility at 6 M HNO3. The data for 4 M HNO3 showed good agreement with data in the literature. To achieve a target of 6 g/L soluble Gd in solution in the presence of 0.15-0.25 M oxalate, the HNO3 concentration must be maintained at or above 6 M HNO3.