Neptunium behaviour

Since plutonium is the actinide generating most concern at the moment this review will be concerned primarily with this element. However, in the event of the fast breeder reactors being introduced the behaviour of americium and curium will be emphasised. As neptunium is of no major concern in comparison to plutonium there has been little research conducted on its behaviour in the biosphere. This review will not discuss the behaviour of berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium which are of no concern in the nuclear power programme although some of these actinides may be used in nuclear powered pacemakers. Occasionally other actinides, and some lanthanides, are referred to but merely to illustrate a particular fact of the actinides with greater clarity. 

The remaining exceptions concern the lanthanide series, where samarium at room temperature has a particular hexagonal structure and especially the lower actinides uranium, neptunium, and plutonium. Here the departure from simple symmetry is particularly pronounced. Comparing these three elements with other metals having partly filled inner shells (transition elements and lanthanides), U, Pu, Np have the lowest symmetry at room temperature, normal pressure. This particular crystallographic character is the reason why Pearson did not succeed to fit the alpha forms of U, Pu, and Np, as well as gamma-Pu into his comprehensive classification of metallic structures and treated them as idiosyncratic structures . Recent theoretical considerations reveal that the appearance of low symmetries in the actinide series is intimately linked to the behaviour of the 5f electrons. 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

When more them one solute is involved in the consideration of the process design, the situation becomes much more complex since the extraction behaviours of the different solutes will usually be interdependent. In the case of irradiated thermal reactor fuels the solvent extraction process will be dealing with uranium containing up to ca. 4% of fission products and other actinides. These will have only a minor effect on uranium distribution so that a single-solute model may be adequate for process design. However, in some cases nitric acid extraction may compete with U02 extraction and a two-solute model may be needed. In the case of breeder reactor fuels the uranium may contain perhaps 20% of plutonium or thorium. Neptunium or protactinium levels in such fuels may also not be negligible and, under these circumstances, the single-solute... 

The behaviour of neptunium has been studied in the same contactor in quite a number of experiments, resulting in residual Np concentrations in the U product of about 1 %. Nearly 99 % of the Np went with the Pu product stream under optimized process conditions, e.g. 0,2 M HNO in the BXS flow and a maximum phase... 

A laboratory study was undertaken to determine the behaviour of neptunium in the WAK flowsheet, and to devise a procedure for its recovery. Based on static ( ) and counter-current experiments (J5), the conclusion was reached that about half of the Np is co-extracted with the U and Pu in the HA-HS mixer-settlers of WAK while the other half is rejected to the HAW, see Fig.1. It could also be shown that an increase of the aqueous acidity, or the addition of pentavalent vanadium as an oxidant into the lower stages of the HA mixer-settler (6), would increase the Np yield in the organic solvent. In the 1BX-1BS mixer-settlers where the partitioning of U and Pu is carried out by use of uranium (IV)nitrate - hydrazine nitrate, a splitting of the coextracted Np between the two product streams was observed the proportions of the (co-extracted) Np which ended up in the 1CU (uranium product) stream fluctuated from 30 to 93 % while the difference amount (from 7 to 70 %) ended up in the 1 BP (plutonium product) stream. 

Although identical in their chemical behaviour these isotopes respond differently towards neutron bombardment. At speeds between those of fast and thermal (relatively slow), neutrons are captured by U(2 3 8) without fission, producing a very active isotope U(239), which loses an electron producing a new element neptunium, Np, which in turn loses an electron yielding plutonium,... 

Neptunium, like uranium, can occur in aqueous solution in four oxidation states trivalent, tetravalent, pentavalent and hexavalent. The most studies have been conducted on the hydrolysis and solubility of neptunium(V) species and phases. There are much fewer studies on the hydrolysis and solubility of neptu-nium(IV). Not many studies are available that have determined the solubility or stability constants of neptuniumfVI). However, it is clear from the studies that have been undertaken that the hydrolytic behaviour of neptunium(VI) is quite similar to that of uraniumfVI). There is only a single study that has determined stability constants for the hydrolytic species of neptunium(III). 

Also, no stability constants are available for polymeric species of neptu-nium(IV). It is expected that neptunium(IV) should display similar behaviour to uranium(IV). 

The longest-lived isotope of americium is Am with a half-life of about 7370 years. Other relatively long-lived isotopes are Am and Am. Americium can also form four oxidation states in aqueous solution trivalent, tetravalent, pentavalent and hexavalent. No data are available for the tetravalent and hex-avalent oxidation states and only a relatively small amount for both the trivalent and pentavalent states. The behaviour of all oxidation states should be reasonably similar to those of uranium, neptunium and plutonium. 

Electromigration of carrier-free radionuclides. 5. Ion mobilities and hydrolysis of Np(V) in aqueous perchlorate solutions. Radiochim. Acta, 42, 43-46. Runde, W. and Kim, J.L (1994) Chemical Behaviour of Trivalent and Pentavalent Americium in Saline NaCl-Solutions. Studies of Transferability of Laboratory Data to Natural Conditions. Report RCM-01094, Technische Universitat Miinchen, 236 pp. Runde, W., Neu, M.P., and Clark, D.L. (1996) Neptunium(V) hydrolysis and carbonate complexation experimental and predicted neptunyl solubility in concentrated NaCl using Pitzer approach. Geochim. Cosmochim. Acta, 60, 2065-2073. 

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