JRP CALL information
Supported By

European Commission

Short description of the work
Storage of Spent Nuclear Fuel (SNF) in an underground repository is the favoured method for long term stewardship in the EU and US. Oxidation of UO2 in the moist oxidising environment has been shown to form uranyl phase transitions that have been identified on the surface of UO2, SNF and in the oxidative weathering of pitchblende in nature. Thus an understanding of how these phases can retard or enhance the migration of the highly radioactive trans-uranium elements is important for modelling of the environmental impact of repositories. Some work has shown that the structure of the mineral is important in neptunium migration, but the oxidation state of Np is not known with certainty. Np(V) and Np(VI) are soluble in water so of the most concern. In this project we have prepared a series of uranyl minerals in the presence of neptunyl(V) and used EXAFS and HR-XANES (High Energy Resolution X-ray Absorption Near Edge Spectroscopy) to confirm the oxidation state and give information on both the structure and electronic structure of the complexes formed. Thus, a co-precipitation reaction of Np(V), uranyl nitrate potassium carbonate and sodium carbonate affords Np incorporated grimselite {grimselite = K3Na[UO2(CO3)3]} that is rich in Np (by ICP-OES). Carbonate in the form of rock structures and the ions in solution are an important environmental ly. Whilst not fully analysed at this time, the EXAFS data at U and Np L3-edge suggests Np is in its +5 or +6 oxidation state (i.e. the presence of a Np=O bond) and the local environment around uranium is similar to the parent grimselite. Confirmation of the +5 oxidation state of Np was given by examination of the HR-XANES spectrum at the M edge, with comparison of known reference standards. Further analysis of this data will yield detailed information on the electronic structure of both the Np and U in the Np incorporated grimselite and U in grimselite. M-edge spectra directly probe the f-orbitals, compared to L-edge that give information on d-orbital populations. Further to the Np incorporation experiments a series of U minerals were also examined using M-edge HR-XANES. We have recently used L-edge HR-XANES to explore the differences in studtite [UO2(O2)(H2O)2].2H2O and meta-studtite [UO2(O2)(H2O)] and we have extended this to M-edge; in combination with XPS measurements also conducted at INE this will give a good description of the differences in these minerals. My lab has also synthesised bassetite, Fe[(UO2)2(PO4)2].7H2O and found that under the conditions of our experiment U(VI) is partially reduced to U(IV) – this has important implications as this can be used as a molecular model for the interactions of uranyl with Fe minerals such as goethite and haematite.

Main visitor contact data
Name: Dr Robert Baker
Organisation: Trinity College, Dublin

JRP Identification
JRP nr: TALI-C03-02
JRP title: Neptunium Incorporation into Minerals: A Synthetic and X-ray Spectroscopic Study
JRP scope: Scope 2: Actinide in the geological environment

Visited Associated Pooled Facility
Visited APF during the stay: KIT-INE - Laboratories
Name of the APF Contact Person: Dr Tonya Vitova

Other APF and organisation involved in the JRP
Other organisations involved:
Other APF involved in the project: KIT-INE - Beamline

Description of the work done at the associated pooled facility
Start date of the stay: 1/6/2015
End date of the stay: 3/8/2015
Quantity of access: 36
Access Unit: Days
Misc.: Work during week-ends