Computational studies of incorporation of Pu and Ce into ceramic matrices such as zirconolite

Lead Research Organisation: University of Manchester
Department Name: Chemistry

Abstract

Solutions to the long term storage of highly radioactive waste products, formed through the nuclear cycle are required to allow for the continued safe use of nuclear energy. The long-term solution for the storage of highly radioactive waste must be stable and be able to withstand radiation damage, without destructive changes occurring in structural composition. Various forms of solid storage devices have been considered - such as glasses and ceramics which allow for the immobilisation of radioactive materials.

The ceramic zirconolite (CaZrTi2O7) has been the focus of experimental research for several decades and its candidacy as a potential nuclear waste-form has developed over time, with experimental evidence of the system's long-term damage response to radioactive decay considered. Naturally occurring zirconolite has been found to contain actinides, such as uranium and plutonium, substituted onto the cationic sites. Methods involving the use of DFT and hybrid DFT, will be employed to determine the changes to the zirconolite system upon substitution of plutonium and cerium onto different cationic sites of the zirconolite lattice.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509565/1 01/10/2016 30/09/2021
1906051 Studentship EP/N509565/1 18/09/2017 31/03/2021 Jonathan Daffyd Tanti
 
Description Computational techniques have been used to analyse heavy element substitution into zirconolite - a ceramic material which has been identified as a potential waste form for high level nuclear waste. Zirconolite is able to immobilse nuclear waste by accommodating radionuclides onto specific atomic sites within its lattice. This research utilises the Periodic Electrostatic Embedded Cluster Method (PEECM). The PEECM separates the modelling of a solid bulk system into 3 regions. The first region is a small region of the bulk which is analysed using Density Functional Theory (DFT) - a quantum mechanical modelling method. This small region is then surrounded by an intermediate region used to prevent overpolarisation by the infinite region of point charges which surrounds both in infinite directions.

Modelling of the substitution of Cerium and early actinides (Thorium to Americium) into the zirconolite lattice has been performed and the positions of these sites and surrounding ions have been optimised. Minimal displacement was seen for all systems. Redox behaviour was seen (alteration of the charge of ions through electron transfer) for substituted ions within the system and strong correlations were found between the energy required to substitute ions into the system and the size of said species.

The choice charge balancer (charged ions incorporated to balance isovalent substitutions) is currently being studied and may reveal futher insights.

Further, this work serves as further evidence of the efficacy of this method, specifically the use of it for bulk systems, with previous research using the PEECM focussing mainly on surface chemistry.
Exploitation Route The energetic and electronic data produced by this research may inform the production of plutonium or cerium doped zirconolite for the use of research into the efficacy of the ceramic for nuclear waste immobilisation. Specific charge balancing work may in turn help provide options for further tailoring of this material.

Use of the PEECM and demonstrations of its efficacy will provide another potential tool for future computational chemists to consider especially when modelling specific defects or substitutions within a bulk structure.
Sectors Energy,Environment,Manufacturing, including Industrial Biotechology,Other