JRP CALL information
Supported By

European Commission

Short description of the work
The aim of the project was to investigate solubility of uranium in lanthanum zirconate (La2Zr2O7), which has been previously studied as a model system for the immobilisation of actinides. Zirconate pyrochlores are highly radiation tolerant and extremely durable making them ideal candidates for the disposal of high activity long half life wastes, including plutonium, uranium and minor actinides. This work looked at the effect of substituting uranium (as a plutonium analogue) into lanthanum zirconate, La2Zr2-xUxO7+ɣ, under different processing conditions. Oxidising and reducing atmospheres, air and H2/N2 respectively, have been used to investigate the speciation of uranium in different valence states which have been determined using X-ray absorption specrocopy (XAS).
Structural analysis has been performed using X-ray diffraction but due to the small scattering factor of oxygen the technique is in-sensitivity to oxygen vacancy ordering in these materials and hence it is difficult to observed subtle structural changes. Transmission electron microscopy (TEM) is more sensitive to cation and anion ordering and it has been demonstrated that this ordering can be determined from analysis of electron diffraction patterns. The presence / absence of sharp or diffuse scattering at key super-lattice positions enables differentiation of different structure types which appear identical by X-ray powder diffraction. It was therefore possible to determine the solubility limit of uranium and the structural modulations that occurred as a function of uranium content.

 

Short description of the workNeptunium is a minor actinide that is produced in a nuclear power reactor. The neptunium recovery, through the PUREX process, is currently difficult. Indeed, the PUREX raffinate is characterized by a very complex mixture of several elements in a high concentration of nitric acid and a high ionic strength. In this environment, neptunium can be found in several oxidation states, making it more difficult to separate with other actinides. In the 60s, Sullivan et al showed that the neptunium, in its form neptunyl Np(V), could form complexes with many other elements such as actinides or metals present in the PUREX media. This interaction was shown by UV-Vis spectrophotometry, monitoring the absorption band of the f-f transition of Np(V) at 980nm. During the complexation, this band decreases strongly in favor of a new bands corresponding to the cation-cation interaction. This band appears separate from the main one by a more or less significant shift depending on the complexing element.
This study aimed to revive the work of Sullivan on spectrophotometry and Guillaume’s work on Raman. As originally planned in the project, the elements Fe, Cr and Rh were studied. Two other elements were also added to this list: dysprosium and ruthenium. Dysprosium was investigated to demonstrate an interaction between Np(V) and a characteristic element of the lanthanide group. Ruthenium was studied for its interest in the PUREX process, an interaction between Np(V) and this element was observed by Ch. Maurice during her PhD. The manipulations were used to conduct a thorough study on the effect that Np(V) has on Fe(III) and Dy(III) and continuing qualitatively the study of elements Cr(III), Rh(III) and Ru(III). Complexation constant was calculated in two different environments for Fe and Dy.

 

Short description of the work
The goal of this study was to improve the scientific knowledge of the catalytic phenomenon due to the redox properties of Tc. The role of hydrazinium, hydroxylammonium, U(IV) and Pu(III) were evaluated in both 1.0 M nitric and 1.0 M perchloric acids at room temperature by Uv-Vis spectroscopy for determination of reduced technetium species from pertechnetic acid, Tc(VII). The information obtained from the spectra will be used to choose candidates for XAFS measurements and potential speciation of Tc within these systems.
The reactions performed in 1.0 M perchloric acid initially showed no reduction of pertechnetic acid, Tc(VII) in hydrazinium and hydroxylammonium solutions. The solutions were revisited after 20 days with a noticeable change in absorbance for hydranizium perchlorate solution and an indication of no Tc(VII) remaining in solution with new absorbance peaks at 257 nm and 336 nm. As for the hydroxylammonium perchlorate solution, a small increase in absorbance in the 450-470 nm range(broad) similar to reduced species of Tc within literature but no true species has been assigned.

 

Short description of the work
The corrosion and contamination of stainless steels in conditions similar to those found in a reprocessing plant has been studied. Different surface finishes of stainless steel were used in the project; the as-received material, ground and polished, and electrolytically etched. The 304L steel was held in vials of nitric acid (4 or 12 M) containing a transuranic contaminant. These were 1 g/L solutions of americium and plutonium and 0.5 g/L of neptunium. The solutions were held at 50°C for a month.
Subsequently, autoradiography was undertaken in order to determine the location and extent of any radionuclide contamination on the stainless steel coupon. All samples showed transuranic uptake with dependence on acid molarity and the surface morphology. ICP-MS of the solutions has been conducted in order to accurately describe the level of uptake. XPS and SEM/EDX were also performed on the steel samples to get a clearer understanding of the contaminant speciation and corrosion behaviour of the steel. It was seen that the former was unsuitable due to insufficient contaminant concentration on the immediate steel surface. However, the latter has elucidated the extent of pitting and intergranular corrosion of the material.

 
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