GREENCDT The kinetics of new reprocessing routes for nuclear fuel

Supervisors

1. Professor Colin Boxall, email: c.boxall@lancaster.ac.uk
2. Professor Bruce Hanson, email : b.c.hanson@leeds.ac.uk
3. Professor Robin Taylor (The National Nuclear Laboratory)
4. Dr Dave Woodhead (The National Nuclear Laboratory)

To maximise the benefits of near-zero carbon nuclear energy, closed nuclear fuel cycles may need to be implemented in the UK or elsewhere later this century. Closed fuel cycles are those in which actinides are recovered from used nuclear fuel for recycling into new fuels for advanced reactors. In the UK, this was effected by the PUREX (Plutonium-Urananium Redox EXtraction) process which is highly effective in separating and recovering plutonium and un-reacted uranium from spent fuel.

Closed cycles offer the potential advantages of minimising high level wastes and maximising use of natural uranium resources. The UK is seeking to develop advanced fuel cycles that offer further advantages in separation processes with enhanced standards in economics, safety, proliferation resistance, sustainability and flexibility. This has led to the development of the i-SANEX (innovative Selective ActiNide EXtraction) and GANEX (Grouped ActiNide Extraction) processes that also recover the minor actinides (especially neptunium and americium) for reuse in fuel.

Both i-SANEX and GANEX use novel diglycolamide (DGA) ligands for extraction. Using these ligands, this project will study the kinetics of mass transfer of nitric acid, lanthanide and actinide ions across solvent/aqueous interfaces.

The separations step of any reprocessing scheme involve a liquid/liquid (L/L) extraction process whereby the metal ions targeted for extraction – typically the actinides from the fission products in spent fuel – are transferred from a nitric acid-based aqueous phase to a receiving organic phase. Understanding the kinetics of L/L extraction plays a major role in determining key process features during scale-up (choices of contactor, extractant, reagents, flow rates, temperature etc). This understanding requires separate experimental measurement of both mass and interfacial transfer kinetics, something that is only possible if two conditions are satisfied:

1. that a defined area interface is established between the aqueous and solvent phases, so allowing for determination of the interfacial transfer kinetic parameter per unit area; and
2. that a hydrodynamically well-defined flow regime is established over / towards that interface.

This allows for the deconvolution of the contribution made to the apparent extraction rate by convective / diffusive mass transport relative to interfacial transfer kinetics.

One technique for measuring L/L transfer kinetics that satisfies both of the above conditions is the Rotating Diffusion Cell which allows for study of extraction kinetics using defined area L/L interfaces. By supporting the interface at a rotating membrane, the RDC allows extraction studies under controllable hydrodynamic flow regimes.

Thus, we propose to use the RDC to study the mass and interfacial transfer kinetics (and associated mechanisms) of non-active lanthanide ions (as trivalent actinide surrogates) in diglycolamide (DGA) based extraction systems (e.g. TEDGA, TODGA, MTTDGA) with various modifiers (e.g. octanol). Experiments will cover the full process envelope: as a function of T, pH, rotation speed/shear rate (particular attention being paid to observing the transition between thermodynamic and kinetic control of separation selectivity), [nitrate], free ligand concentration and metal ion concentration (including possible study of third phase formation at the L/L interface at high [M] and low shear rate). Kinetic models recently developed at Lancaster will be deployed to determine rate constants and whether diffusion or chemical reaction control prevail under specific conditions.

Experimental work will be conducted primarily in Lancaster’s UTGARD (Uranium-Thorium beta-Gamma Active R&D) Lab, with results informing and complementing studies on real spent fuel conducted at NNL’s Central Laboratory.