DISSOLUTION OF PLUTONIUM METAL IN 8-10 M NITRIC ACID

Technical Report · Sun Jul 01 04:00:00 EDT 2012

DOI:https://doi.org/10.2172/1045615· OSTI ID:1045615

Rudisill, T. S. ^[1]; Pierce, R. A. ^[1]

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1. Savannah River National Laboratory (SRNL), Aiken, SC (United States)

The H-Canyon facility will be used to dissolve Pu metal for subsequent purification and conversion to plutonium dioxide (PuO₂) using Phase II of HB-Line. To support the new mission, the development of a Pu metal dissolution flowsheet which utilizes concentrated (8-10 M) nitric acid (HNO₃) solutions containing potassium fluoride (KF) is required. Dissolution of Pu metal in concentrated HNO₃ is desired to eliminate the need to adjust the solution acidity prior to purification by anion exchange. The preferred flowsheet would use 8-10 M HNO₃, 0.015-0.07 M KF, and 0.5-1.0 g/L Gd to dissolve the Pu up to 6.75 g/L. An alternate flowsheet would use 8-10 M HNO₃, 0.05-0.2 M KF, and 1-2 g/L B to dissolve the Pu. The targeted average Pu metal dissolution rate is 20 mg/min-cm², which is sufficient to dissolve a “standard” 2250-g Pu metal button in 24 h. Plutonium metal dissolution rate measurements showed that if Gd is used as the nuclear poison, the optimum dissolution conditions occur in 10 M HNO₃, 0.04-0.05 M KF, and 0.5-1.0 g/L Gd at 112 to 116 °C (boiling). These conditions will result in an estimated Pu metal dissolution rate of ~11-15 mg/min-cm² and will result in dissolution times of 36-48 h for standard buttons. The recommended minimum and maximum KF concentrations are 0.03 M and 0.07 M, respectively. The data also indicate that lower KF concentrations would yield dissolution rates for B comparable to those observed with Gd at the same HNO₃ concentration and dissolution temperature. To confirm that the optimal conditions identified by the dissolution rate measurements can be used to dissolve Pu metal up to 6.75 g/L in the presence of representative concentrations of Fe and Gd or B, a series of experiments was performed to demonstrate the flowsheets. In three of the five experiments, the offgas generation rate during the dissolution was measured and samples were analyzed for hydrogen gas (H₂). The use of 10 M HNO₃ containing 0.03-0.05 M KF, 0.5-1.0 g/L Gd, and 1.9 g/L Fe resulted in complete dissolution of the metal in 2.0-3.5 h. When B was used as the neutron poison, 10 M HNO₃ solutions containing 0.05-0.1 M KF, 1.9 g/L Fe, and 1 g/L B resulted in complete dissolution of the metal in 0.75-2.0 h. Dissolution rates estimated using data from the flowsheets demonstrations agreed reasonably well with the measured rates; although, a discrepancy was observed in the Gd system. The presence of 1 g/L Gd or B in the dissolving solution had about the same effect on the dissolution rate. The predominant Pu valence in the dissolving solution was Pu(IV). The concentration of Pu(VI) was evaluated by UV-visible spectroscopy and was estimated to be significantly less than 1 wt %. The offgas generation rates and H₂ concentrations measured in the offgas from experiments performed using 10 M HNO₃ containing 0.05 M KF, 1.9 g/L Fe and either 1 g/L Gd or B were approximately the same. These data support the conclusion that the presence of either 1 g/L Gd or B had the same general effect on the dissolution rate. The calculated offgas generation during the dissolutions was 0.6 mol offgas/mol of Pu. The H₂ concentration measured in the offgas from the dissolution using Gd as the neutron poison was approximately 0.5 vol %. In the B system, the H₂ ranged from nominally 0.8 to 1 vol % which is about the same as measured in the Gd system within the uncertainty of the analysis. The offgas generation rate for the dissolution performed using 10 M HNO₃ containing 0.03 M KF, 0.5 g/L Gd, and 1.9 g/L Fe was approximately a factor of two less than produced in the other dissolutions; however, the concentration of H₂ measured in the offgas was higher. The adjusted concentration ranged from 2.7 to 8.8 vol % as the dissolution proceeded. Higher concentrations of H₂ occur when the Pu dissolution proceeds by a metal/acid reaction rather than nitrate oxidation. The higher H₂ concentration could be attributed to the reduced activity of the fluoride due to complexation with Pu as the dissolution progressed. Dissolution of Pu metal at 20 °C in 10 M HNO₃ containing 0.05 M KF showed that the Pu metal dissolves slowly without any visible gas generation. As the Pu metal dissolves, it forms a more-dense Pu-bearing solution which sank to the bottom of the dissolution vessel. The dissolved Pu did not form a boundary layer around the sample and failed to distribute homogeneously due to minimal (thermally-induced) mixing. This indicates that in the H-Canyon dissolver insert, the Pu will diffuse out of the insert into the bulk dissolver solution where it will disperse. At 35 °C, the Pu metal dissolved without visible gas generation. However, due to thermal currents caused by maintaining the solution at 35 °C, the dissolved Pu distributed evenly throughout the dissolver solution. It did not form a boundary layer around the sample.

• Research Organization: Savannah River Site (SRS), Aiken, SC (United States); Savannah River National Laboratory (SRNL), Aiken, SC (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC09-08SR22470
• OSTI ID: 1045615
• Report Number(s): SRNL-STI--2012-00043-Rev.1
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
Dissolution
H-Canyon
Plutonium Metal