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[en] The Savannah River Site's HB-Line Facility completed a second neptunium oxide production campaign in which nine (9) additional cans of neptunium oxide were produced and shipped to the Idaho National Laboratory and Oak Ridge National Laboratory in the 9975 shipping container. These additional cans were from a different feed solution than the first fifty (50) cans of neptunium oxide that were previously produced and shipped via a Letter of Amendment to the 9975 Safety Analysis Report for Packaging (SARP) content table. This paper will address the challenges associated with demonstrating the neptunium oxide produced from the additional feed solution was equivalent to the original neptunium oxide and within the content description of the Letter of Amendment.
[en] We explored the electrocatalytic activity of PEDOT:PSS modified GC (PEDOT:PSS/GC) electrode for electrocatalysis of NpO_2"2"+/NpO_2"+ redox couple in nitric acid and its performance is compared with conventionally used electrodes (GC, Au and Pt electrodes). (author)
[en] The solubilities of NpO2(s) in the KURT (KAERI Underground Research Tunnel) granitic groundwater with low ionic strength were measured experimentally and calculated by a geochemical code. Then these results were compared with each other as well as with foreign results. The concentrations of neptunium were measured as 6 x 10-8 - 2 x 10-8 mol/L at a pH 9.5-11.1 and Eh = -0.2 V, and less than 5 x 10-9 mol/L at a pH = 11.8-13.0 and Eh -0.3-0.44 V. The dominant aqueous species were presumed as Np(OH)x(CO3)y4-x-2y complexes and Np(OH)4(aq) at pH = 9.5-13 under the Eh<-0.2 V reducing condition. (author)
[en] Knowledge about the behavior of the actinides in aqueous media of compositions similar to those of groundwaters is required in order to model the transport characteristics of these elements in the geosphere. Neptunium is an element of particular concern because of its relatively long decay period and relative abundance in nuclear waste. The most probable oxidation state for this element in a natural environment is V, which in this case takes the form of the singly charged neptunyl ion, NpO2+. The present work was aimed at obtaining data on the hydrolysis and carbonate complexation of NpO2+ which in turn could be used to establish speciation schemes of this ion under conditions similar to those found in nature. The experimental approach chosen was based on the determination of neptunium solubility of a well-defined solid phase as a function of carbonate concentration in a medium of constant ionic strength
[en] Highlights: • We synthesize and characterize Np2O5 and NpO2. • Enthalpy of decomposition of Np2O5 to NpO2 agrees with existing thermodynamic data. • The calorimetric methodology is straightforward and reliable.
[en] Small-scale experiments were performed to demonstrate the feasibility of fusing refractory actinide oxides with a series of materials commonly used to decompose minerals, glasses, and other refractories as a pretreatment to dissolution and subsequent recovery operations. In these experiments, 1-2 g of plutonium or neptunium oxide (PuO2 or NpO2) were calcined at 900 degrees Celsius, mixed and heated with the fusing reagent(s), and dissolved. For refractory PuO2, the most effective material tested was a lithium carbonate (Li2CO3)/sodium tetraborate (Na2B4O7) mixture which aided in the recovery of 90 percent of the plutonium. The fused product was identified as a lithium plutonate (Li3PuO4) by x-ray diffraction. The use of a Li2CO3/Na2B4O7 mixture to solubilize high-fired NpO2 was not as effective as demonstrated for refractory PuO2. In a small-scale experiment, 25 percent of the NpO2 was oxidized to a neptunium (VI) species that dissolved in nitric acid. The remaining neptunium was then easily recovered from the residue by fusing with sodium peroxide (Na2O2). Approximately 70 percent of the neptunium dissolved in water to yield a basic solution of neptunium (VII). The remainder was recovered as a neptunium (VI) solution by dissolving the residue in 8M nitric acid. In subsequent experiments with Na2O2, the ratio of neptunium (VII) to (VI) was shown to be a function of the fusion temperature, with higher temperatures (greater than approximately 400 degrees C) favoring the formation of neptunium (VII). The fusion of an actual plutonium-containing residue with Na2O2 and subsequent dissolution was performed to demonstrate the feasibility of a pretreatment process on a larger scale. Sodium peroxide was chosen due to the potential of achieving higher actinide recoveries from refractory materials. In this experiment, nominally 10 g of a graphite-containing residue generated during plutonium casting operations was initially calcined to remove the graphite. Removal of combustible material prior to a large-scale fusion with Na2O2 is needed due to the large amount of heat liberated during oxidation. Two successive fusions using the residue from the calcination and the residue generated from the initial dissolution allowed recovery of 98 percent of the plutonium. The fusion of the residue following the first dissolution was performed at a higher temperature (600 degrees Celsius versus 450 degrees Celsius during the first fusion). The ability to recover most of the remaining plutonium from the residue suggest the oxidation efficiency of the Na2O2 fusion improves with higher temperatures similar to results observed with NpO2 fusion
[en] Thermodynamic stability of NpO3 and PuO3 has been evaluated in order to study the possibility of preparing these oxides. The proposed scheme of calculating free energy is based on the postulated similarity of Np(6) and Pu(6) properties with those of hexavalent uranium. The reaction Msup((6))sub(gas)+30sub(gas)2-=MO3, where M=Np or Pu, is considered which describes the formation of the crystal of trioxide of the corresponding metal from ionic gas being in the state of limiting dilution (no interaction between the ions). It has been shown that the formation of neptunium and plutonium trioxides in the systems Np-O2 and Pu-O2 is thermodynamically unfavourable at all practically achievable oxygen pressures
[en] Solubilities of NpO2 were measured in 0.1 M Na2CO3 (pH 11.25) and 0.1 M Na2CO3-0.5M H2O2 (pH 11.25), respectively, for two weeks. Three detection methods such as gas proportional counting (GPC), liquid scintillation counting (LSC) and ICP-MS were used for the measurement of dissolved NpO2 in the solutions and the results by different methods were compared with each other. The solubility of NpO2 increased as the contact time increased and those after 2 weeks showed 4.4 x 10-9 M in 0.10 M Na2CO3 (pH 11.25) and 2.4 x 10-8 M in 0.10 M Na2CO3-0.5M H2O2 (pH 11.25), respectively