The role of neptunium cation-cation complexes in Np(V) disproportionation

Lead Research Organisation: University of Manchester
Department Name: Engineering and Physical Sciences

Abstract

**The project is partly funded by The National Nuclear Laboratory**
Neptunium is present in spent nuclear fuel but is not recovered through PUREX and instead
becomes split over several reprocessing streams adding cost/complexity to plant designs. This occurs because
of the rich redox chemistry of neptunium. Disproportionation of neptunium(V) to neptunium(IV/VI) is one
important contributing mechanism, particularly at high acidities, and it is thought that disproportionation is
promoted by cation-cation-interactions (CCIs) where neptunyl-oxo atoms act as Lewis bases to other metals;
this may facilitate electron/proton transfer reactions making valency-control difficult. This is problematic because
multiple interconverting species then co-exist in solution, which has major implications for routing within PUREX.
Furthermore, there is significant speculation about how these processes might operate in bioreduction pathways,
which demands a better understanding to help protect our environment. To further complicate matters, it has
been suggested that neptunyl(V)-neptunyl(V) CCIs behave differently to uranyl(V)-uranyl(V) and plutonyl(V)-
plutonyl(V) CCIs, and because all three elements are present in PUREX that neptunyl-uranyl and neptunylplutonyl
CCIs may also exist. This PhD will probe this issue by: (i) preparing well-defined model homobimetallic
di-neptunyl complexes based on path-finding uranyl analogues; (ii) prepare heterobimetallic neptunyl-uranyl/-
plutonyl complexes; (iii) develop supporting ligands to move to those that more closely mirror real scenarios; (iv)
study the mechanistic and kinetic reactivity of the CCI disproportionation reactions, thus transforming our
understanding of this complex chemistry. This chemistry will provide high-impact outputs and train the student in
Schlenk-line/glovebox methods and a wide range of world-leading structural, spectroscopic, and magnetic
characterisation techniques at UoM (Centre for Radiochemistry Research) and NNL.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R512394/1 01/10/2017 30/09/2021
1938436 Studentship EP/R512394/1 01/10/2017 31/03/2022 Jonathan David Cryer
 
Description Model compounds have been synthesised using uranium to hopefully transfer to neptunium in the future.
Also found that the subtle changes in the ligand design has a large effect on the product isolated.
Exploitation Route The system developed using uranium can hopefully be transferred to neptunium which will be able to help inform future research into neptunium disproportionation at an academic and industrial level.
Sectors Energy