Neptunium content

Such amount allowed to replace some part of depleted uranium dioxide by neptunium dioxide in the central zone of 30 1 volume, neptunium content being 13.5 % heavy nuclei. Deliberately increased neptunium content allowed to sharpen its effect upon the parameters under investigation. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

In research at the Institute of Radiochemistry, Karlsruhe, West Germany, during the early 1970s, investigators prepared alloys of neptunium with indium, palladium, platinum, and rhodium. These alloys were prepared by hydrogen reduction of the neptunium oxide in (he presence of finely divided noble metals. The reaction is called a coupled reaction because the reduction of the metal oxide can be done only m the presence of noble metals. The hydrogen must be extremely pure, with an oxygen content of less than 10 25 torr. 

Tin and americium were so extensively sorbed under all conditions that isotherm data could not be obtained. These elements are not significantly mobile in the Mabton Interbed aquifer. Values of Freundlich constants for technetium, radium, uranium, neptunium, and plutonium are given in Table IV. The Freundlich equation did not fit the selenium sorption data very well probably because of slow sorption kinetics or precipitation. Precipitation was also observed for technetium at 23°C for concentrations above 10 7M. This is about the same solubility observed for technetium in the sandstone isotherm measurements. Linear isotherms were observed only in the case of radium sorption. In general, sorption on the Mabton Interbed was greater than on the Rattlesnake Ridge sandstone. This is probably due to the greater clay content of the Mabton standard. 

The most significant difference between Dowex 1 and Dowex 2 resin is in the order of selectivity of the hydroxide ion. All other mechanical variables between the two resins, such as pore size, shrinkage, capacity and water content, are negligible dif-erences. We conclude that these differences are insufficient to explain the disparity of adsorption of neptunium. 

Analysis of the nuclide content of the plume over Finland suggests that the Soviet estimate of the neptunium emission may be too big by a factor of about 3 (T.Raunemaa, private communication, 1987). There is a suggestion that Np-239 was released slowly over some days and partly in the form of one of the decay products (G.Lewis, private communication, 1988). 

The introduction of this considerable neptunium amount into the fuel makes the neutron spectrum more rigid, and the increase of spectral indices following high energy band of the neutron spectrum, is 5% to 11%, that is approximately two times less than that for the MOX fuel with traditional plutonium content. 

In the Reprocessing Fuel Cycle (RFC) option, the unused uranium and the plutonium produced in the reactor are recovered leaving the minor actinides with the fission products as HLW. (The radiotoxicity of these wastes will be significantly less than that of the spent fuel although the toxic lifetime is determined by the minor actinides - neptunium, americium, curium - and, to a lesser extent, by some of the long-lived fission products content of the waste.) As mentioned previously, this was the scenario initially envisioned by the nuclear power industry to reprocess fuel for two reasons ... 

The quite similar chemical properties of uranium and the transuranium elements are the reason for their presence in the irradiated fuel as thermodynamically stable double oxides. According to the redox potential of the fuel matrix, it has to be assumed that neptunium and plutonium will appear in their most stable oxidation state +4. Due to their ionic radii, which are very similar to that of U(IV), both NpO and Pu02 will form mixed crystals with UO2. The type and the properties of the crystal lattice remain essentially unchanged only the dimensions of the elementary cell decrease from 0.5468 nm (5.468 A) in pure UO2 to 0.5466 nm (5.466 A) at 1% plutonium content. This means that neptunium and plutonium will remain at the positions in the irradiated fuel where they were formed the same behavior is to be expected for the higher transuranium elements as well. 

Continuous plutonium recovery. Continuous removal of neptunium or plutonium is possible in a liquid-fuel reactor. This yields a product with a loiv Pu ° content and increases the value of the plutonium [23]. 

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