Neptunium_Oxide_Precipitation_Kinetics_AJohnsen

Johnsen, A. M. ; Roberts, K. E. ; Prussin, S. G.

Lawrence Livermore National Laboratory

关键词: X-Ray Diffraction;    Neptunium;    37 Inorganic, Organic, Physical And Analytical Chemistry;    Radioactive Wastes;    Dissolution;   

DOI  :  10.2172/1046105
RP-ID  :  LLNL-TR-562123
RP-ID  :  W-7405-ENG-48
RP-ID  :  1046105

美国|英语

来源: UNT Digital Library

【 摘 要 】

We evaluate the proposed NpO{sub 2}{sup +}(aq)-NpO{sub 2}(cr) reduction-precipitation system at elevated temperatures to obtain primary information on the effects of temperature, ionic strength, O{sub 2} and CO{sub 2}. Experiments conducted on unfiltered solutions at 10{sup -4} M NpO{sub 2}{sup +}(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 {le} 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{sup -6} M and 6 x 10{sup -6} M, respectively. Solids from a representative experiment analyzed by X-ray diffraction were consistent with NpO{sub 2}(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 NpO{sub 2}{sup +}(aq)-NpO{sub 2}(cr) reduction-precipitation mechanisms in the literature specified a 1:1 ratio of neptunium loss and H{sup +} 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.