PuO₂ and Fuel Residues

Building on research conducted in DIAMOND, the central hypothesis of this project is that sensitivity of actinide wasteform ceramics to radiation damage induced amorphisation is governed by the topological relationship between pristine and amorphised structures; topologically disordered (i.e. amorphous) structures are stabilised by polyhedra of low co-ordination number (forming ring structures.)  We further hypothesise that radiation induced amorphisation has significant impact on the inherent dissolution kinetics of wasteform ceramics.  We propose to test these hypotheses at several time and length scales of damage, combining synchrotron X-ray and neutron scattering studies of ion beam irradiated materials with state of the art MD simulations, and investigation of legacy Pu 238 doped SYNROC ceramics.  Research at Sheffield will focus on three key challenges:

1.      We will combine energetic heavy ion beams available at GSI Darmstadt with synchrotron and neutron PDF techniques, to make the first total structure determination of macroscopic (ca. 500 mg) ion beam amorphised materials.  We will supplement this with element specific XAS studies to develop an unprecedented description of the structure of radiation amorphised materials, making a direct link to MD simulations.  MD simulations of multiple damage cascades in the same suite of materials, combined with topological analysis, will be utilised to extract partial PDFs from the amorphised structures, for comparison with the products of ion beam irradiation.  We will also continue our study of pressure induced amorphisation of actinide wasteform ceramics, initiated in DIAMOND, to understand to what extent pressure amorphised structures act as representative analogues of the radiation amorphised structures (using PDF, XAS and MD techniques).

2.      We will use medium energy ion beam irradiation to amorphise the surface of polycrystalline ceramic or thin film specimens of candidate wasteform ceramics, to a depth of ca. 1-2 microns, using the Surrey Ion Beam and EU SPIRIT network facilities.  The dissolution kinetics of irradiated / unirradiated specimens will be investigated in both batch and dynamic alteration experiments and surface retreat studies using AFM. We will carefully select the experimental conditions such that the response of the material arises only from the surface amorphised layer.

We will characterise 30 year old legacy Pu-238 doped SYNROC ceramics available at NNL, to understand the extent to which ion beam, MD and pressure amorphised structures are representative of true alpha recoil induced amorphisation.  We will use FIB methodology to extract lift outs from different phases (perovskite, zirconolite, pyrochlore) and use TEM / ED to investigate the nature of the damaged microstructure, e.g. size and extent of residual crystalline domains and gas bubbles; the nature of the interface between damaged and crystalline relic zones; and the speciation constituent elements using EELS and XAS at the Swiss or Diamond Light Sources.