Neptunium Oxide Precipitation Kinetics in Aqueous Systems at Elevated Temperatures, Part I: Colloidal, Temperature, Inert Atmosphere, and Ionic Strength Measurements

Technical Report · Thu Jun 21 00:00:00 EDT 2012

DOI:https://doi.org/10.2172/1046105· OSTI ID:1046105

Johnsen, Amanda M. ^[1]; Roberts, Kevin E. ^[2]; Prussin, Stanley G. ^[3]

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1. Univ. of California, Berkeley (United States; Dept. of Nuclear Engineering; Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States). Chemical Sciences Div.
2. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States). Chemical Sciences Div.
3. Univ. of California, Berkeley, CA (United States). Dept. of Nuclear Engineering

We evaluate the proposed NpO₂⁺(aq)-NpO₂(cr) reduction-precipitation system at elevated temperatures to obtain primary information on the effects of temperature, ionic strength, O₂ and CO₂. Experiments conducted on unfiltered solutions at 10^-4 M NpO₂⁺(aq), neutral pH, and 200°C indicated that solution colloids strongly affect precipitation kinetics. Subsequent experiments on filtered solutions at 200, 212, and 225°C showed consistent and distinctive temperature-dependent behavior at reaction times ≤ 800 hours. At longer times, the 200°C experiments showed unexpected dissolution of neptunium solids, but experiments at 212°C and 225°C demonstrated quasi steady-state neptunium concentrations of 3 x 10^-6 M and 6 x 10^-6 M, respectively. Solids from a representative experiment analyzed by X-ray diffraction were consistent with NpO₂(cr). A 200°C experiment with a NaCl concentration of 0.05 M showed a dramatic increase in the rate of neptunium loss. A 200°C experiment in an argon atmosphere resulted in nearly complete loss of aqueous neptunium. Previously proposed NpO₂⁺(aq)-NpO₂(cr) reduction-precipitation mechanisms in the literature specified a 1:1 ratio of neptunium loss and H⁺ production in solution over time. However, all experiments demonstrated ratios of approximately 0.4 to 0.5. Carbonate equilibria can account for only about 40% of this discrepancy, leaving an unexpected deficit in H⁺ production that suggests that additional chemical processes are occurring.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE; National Science Foundation
• DOE Contract Number: W-7405-ENG-48; AC52-07NA27344
• OSTI ID: 1046105
• Report Number(s): LLNL-TR--562123
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
ARGON
CARBONATES
COLLOIDS
DISSOLUTION
INERT ATMOSPHERE
KINETICS
NEPTUNIUM
NEPTUNIUM OXIDES
Neptunium
Neptunium oxide
Nuclear Repository
Nuclear Waste
PRECIPITATION
PRODUCTION
Precipitation
RADIOACTIVE WASTES
SOLUBILITY
Solubility
X-RAY DIFFRACTION