Three Dimensional Optical Imaging of Neptunium Redox Speciation-A Feasibility Study

One of the most pressing problems facing society today is the management of existing and future waste forms arising from nuclear energy production. Here, the redox chemistry of the actinide elements plays a crucial role in many aspects of nuclear fission including safe disposal strategies and new recovery and recycling routes. Understanding the chemistry of actinides in engineered environments is imperative for the management of existing and future fission products (nuclear waste) arising from nuclear power production, particularly for underground geological disposal. In particular, the redox chemistry of neptunium, a key radionuclide found in appreciable quantities in high level waste is complex, changeable and currently not well understood.

Over the lifespan of the proposed geological disposal facility, one of the principal hazards is a change in chemistry of neptunium that may result in leaching from the repository, breaching primary containment and entering the engineered environment. Due to the particular complex redox and chemical speciation of neptunium, crucial mechanistic information on redox chemistry and speciation that affects its interactions with engineered and natural encapsulating materials including the host rock and backfill material is lacking and remains one of the principal chemical challenges facing this field. In this feasibility study, we will address the prospect of using one and two photon fluorescence and phosphorescence spectroscopy and microscopy as a non-destructive technique to address this problem. We aim to visualise, locate and spatially map the different oxidation states of neptunyl that can co-exist in solution in model conditions using well defined complexes and aqua ions in with the ubiquitous geologically relevant minerals silica, alumina and calcite at previously unseen levels of detail (sub micrometer resolution). We have recently demonstrated that neptunyl(V) and (VI) emission occurs in the green and blue regions of the electromagnetic spectrum and are equally as intense as the uranyl(VI) ion, whose optical properties are well known and have been used by us for fluorescence and phosphorescence microscopy imaging. This means that both oxidation states can be detected simultaneously so that highly sensitive, informative three-dimensional imaging can be used to understand neptunyl geochemistry below the micron scale. This will add much needed important information to the safety case for nuclear waste disposal in a range of heterogeneous systems.

This proposal is firmly grounded in the "nuclear fission" area, but will also make important contributions across a range of other areas including photon science and nanotechnology, environmental science, analytical science and surface science, specifically of actinide containing materials for optical sensing. This is a new and rapidly emerging area of rapid sensing, identification and characterisation of radioactive metal containing materials in line with the need for a sustainable future nuclear fuel cycle involving legacy clean-up operations, decommissioning and new ways to deal with future nuclear wastes.

Our primary dissemination routes will be through high impact peer reviewed journal articles and conference/workshop presentations that will introduce this optical imaging technique as part of a tool box of techniques to the nuclear community. Here, our project partners (Small, Smith and Woodall) from the National Nuclear Laboratories will be the source of primary industrial advice and they will advise on the best routes to follow and help the team disseminate their results and seek further advice/consultations from other nuclear companies that are potential users of this technology. These include the Nuclear Decommissioning Authority (NDA), Radioactive Waste Management Ltd. (RWM), Sellafield Ltd., Areva Mining and the Atomic Weapons Establishment (AWE), who have been previous past project partners on consortia grants. These are all potential end users of the proposed technology and we will ensure that we seek advice and showcase our results as appropriate.

Work will be disseminated at major international and national conferences, through online press releases and promotional material by the University of Manchester's Press Office when appropriate. We will aim to publish in leading journals (e.g. Science, Nature Publishing Group, Journal of the American Chemical Society, Angewandte Chemie, Chemical Science), as well as in specialist journals, to maximise impact. Future collaborations will be facilitated by links established through the PIs active involvement with the EPSRCs Next Generation Nuclear Centre for Doctoral Training (NGN CDT) and the STFC Environmental Radiation Network in addition to the COST-CM1006 (European f-Element Network), the EU-Actinide NMR network and the EU Actinide TALISMAN network that the PI was heavily involved in. These networks give the PI and Co-Is opportunities to build collaborations with spectroscopists, nuclear scientists engineers and theoreticians. Mechanisms for communicating this work to the public and exploiting potential commercialisation opportunities and industrial applications are in place.