Howard E. Sims

Extraction of Neptunium (IV) Ions into 30% Tri‐Butyl Phosphate from Nitric Acid

The distribution of Np(IV) between 0.08–4.5 M HNO3(aq,eqm) and ∼30% tributyl phosphate has been modelled, accounting for the formation of 1:1 and 1:2 nitrate complexes and Np(IV) hydrolysis in the aqueous phase and the extraction of Np(NO3)4(TBP)2 into TBP. The potential formation and extraction of NpOH(NO3)3(TBP)2 and Np(NO3)4(TBP)2.HNO3 species, including spectroscopic evidence, and oxidations of Np(IV) to Np(V) and Np(VI) in the solvent phase have also been considered. The model highlights some key gaps in the available thermodynamic data.

Modelling Np(IV) Solvent Extraction between 30% Tri-Butyl Phosphate and Nitric Acid in the Presence of Simple Hydroxamic Acids

Abstract Formo- and aceto-hydroxamic acids are very effective reagents for stripping tetravalent actinide ions from a tri-butyl phosphate phase into nitric acid. A model describing the partitioning of actinide (IV) ions has been derived accounting for reactions in the aqueous and solvent phases, including complex formation with nitrate and hydroxamate ions. Predicted distribution ratios for Np(IV) are compared with experimental data in the presence of aceto-hydroxamate ions. Additionally, a value of 0.473 (±0.004) kgmol−1 for the Np(IV) ion interaction coefficient with nitrate ions (εNp4+-NO3−) was determined.

Magnesium hydroxide bulk and colloid-associated 152Eu in an alkaline environment: colloid characterisation and sorption properties in the presence and absence of carbonate

The distribution of 152Eu between magnesium hydroxide bulk, colloids and solution has been assessed under alkaline conditions, such as those in nuclear fuel storage ponds. The colloidal phase has been characterised by two complementary methods: coupled ultrafiltration-ICP-AES and scanning electron microscopy. The quantity and the size distribution of the colloidal phase is strongly ionic strength-dependent. A decrease of the quantity of colloids, in particular the larger size ranges, has been observed with increasing ionic strength. Small colloids (1 kDa-10 kDa fraction) are predominant at all ionic strengths. The morphology of colloids, observed by field-emission gun scanning electron microscopy, appears to change from hexagonal prismatic (characteristic to the mineral) to spherical (energetically more favourable) as size decreases. The distribution of 152Eu between the solid and liquid/colloidal phases has been investigated at carbonate concentrations ranging from 0 to 10(-2) M by coupled ultrafiltration and gamma-spectrometry. Mg(OH)2 bulk appears to be a very strong sorbent for 152Eu, since complete sorption onto the bulk happens for carbonate concentrations as high as 10(-3) M. Scavenging of 152Eu by Mg(OH)2 colloids is negligible in the presence of Mg(OH)2 bulk. The distribution of 152Eu between liquid and colloidal phases has been investigated in the absence of bulk at various carbonate concentrations. A significant uptake of 152Eu by the colloids in solution has been observed, which decreases with increasing carbonate concentration. 152Eu appears to be mainly associated to the smallest colloids (1 kDa-10 kDa fraction). There is a strong correlation between the sorption properties and the surface area of the colloids.

Some aspects of neptunium acetohydroxamic acid chemistry under acidic conditions

Abstract Hydroxamic acids are promising complexant based alternatives to the reductant (U4+ or Fe2+) based selective stripping of Np (and Pu) from a uranium loaded 30% TBP/OK solvent during the reprocessing of irradiated nuclear fuels. Acetohydroxamic acid (AHA) has the benefits of being both a reductant and complexant that efficiently strips Np (and Pu) from solvent phase without adding salt wastes to actinide separation processes. To model these processes, an understanding of Np-hydroxamate chemistry in aqueous and organic solvent is necessary. Three aspects of this system are discussed.

Plutonium and Americium Alpha Radiolysis of Nitric Acid Solutions

The yield of HNO2, as a function of absorbed dose and HNO3 concentration, from the α-radiolysis of aerated HNO3 solutions containing plutonium or americium has been investigated. There are significant differences in the yields measured from solutions of the two different radionuclides. For 0.1 mol dm-3 HNO3 solutions, the radiolytic yield of HNO2 produced by americium α-decay is below the detection limit, whereas for plutonium α-decay the yield is considerably greater than that found previously for γ-radiolysis. The differences between the solutions of the two radionuclides are a consequence of redox reactions involving plutonium and the products of aqueous HNO3 radiolysis, in particular H2O2 and HNO2 and its precursors. This radiation chemical behavior is HNO3 concentration dependent with the differences between plutonium and americium α-radiolysis decreasing with increasing HNO3 concentration. This change may be interpreted as a combination of α-radiolysis direct effects and acidity influencing the plutonium oxidation state distribution, which in turn affects the radiation chemistry of the system.

Characterisation of plutonium species in alkaline liquors sampled from a UK legacy nuclear fuel storage pond

Within the UK there are a number of nuclear legacy fuel storage ponds and silos that contain substantial volumes of corroding spent Magnox fuel pieces. The fuel and wastes within these ponds, including highly radioactive sludges, must be retrieved and processed during decommissioning. Sludges and other intermediate level wastes will then be encapsulated in a wasteform suitable for storage and disposal, whilst residual activity must be removed from pond liquors and process effluents prior to any authorised discharges. Understanding the nature and behaviour of the radionuclides in the ponds, including any potential for activity transfer from solid to solution phases, is critical in the environmental clean up of these nuclear legacy facilities. Plutonium isotopes (with 241Am) dominate the α-activity within these ponds. Herein, the Pu species in samples taken from a UK legacy fuel storage pond and downstream Holding Tank on the Sellafield site are shown to be predominantly associated with suspended solid phases. Analyses of the residual soluble Pu concentrations indicate differences in Pu solubility between different areas of the pond, which speciation studies suggest are related to differing Pu oxidation state distributions. The implication is that Pu redox chemistry varies across the pond and this controls Pu solubilities and, by implication, Pu behaviour during waste processing. Simple treatment methods to suppress soluble Pu-α are also suggested.

Molecular Hydrogen Yields from the α-Self-Radiolysis of Nitric Acid Solutions Containing Plutonium or Americium

The yield of molecular hydrogen, as a function of nitric acid concentration, from the α-radiolysis of aerated nitric acid and its mixtures with sulfuric acid containing plutonium or americium has been investigated. Comparison of experimental measurements with predictions of a Monte Carlo radiation track chemistry model shows that, in addition to scavenging of the hydrated electron, its precursor, and the hydrogen atom, the quenching of excited state water is important in controlling the yield of molecular hydrogen. In addition, increases in solution acidity cause a significant change in the track reactions, which can be explained as resulting from scavenging of eaq- by Haq+ to form H•. Although plutonium has been shown to be an effective scavenger of precursors of molecular hydrogen below 0.1 mol dm-3 nitrate, previously reported effects of plutonium on G(H2)α between 1 and 10 mol dm-3 nitric acid were not reproduced. Modeling results suggest that plutonium is unlikely to effectively compete with nitrate ions in scavenging the precursors of molecular hydrogen at higher nitric acid concentrations, and this was confirmed by comparing molecular hydrogen yields from plutonium solutions with those from americium solutions. Finally, comparison between radionuclide, ion accelerator experiments, and model predictions leads to the conclusion that the high dose rate of accelerator studies does not significantly affect the measured molecular hydrogen yield. These reactions provide insight into the important processes for liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.