Hot Isostatic Pressing of Cs-exchanged IONSIV IE-911 samples is shown to produce a mixture of ceramic phases, the nature and mass fractions of these have been determined by Rietveld analysis of powder X-ray diffraction data. The main Cs phase that forms is Cs2TiNb6O18, after this reaches approximately 30% of the total crystalline content the remaining Cs is partitioned into Cs2ZrSi6O15. Durability tests using the PCT-B method for 7 days at 90 °C with deionised water lead to Cs leach rates of 0.032 and 0.038 g∙m−2∙day−1 for samples exchanged to 6 and 12 wt% Cs, respectively, indicating a durable wasteform is produced.

An understanding of the effect of cumulative radiation damage on the integrity of ceramic wasteforms for plutonium and minor actinide disposition is key to the scientific case for safe disposal. Alpha recoil due to the decay of actinide species leads to the amorphisation of the initially crystalline host matrix, with potentially deleterious consequences such as macroscopic volume swelling and reduced resistance to aqueous dissolution. For the purpose of laboratory studies the effect of radiation damage can be simulated by various accelerated methodologies. The incorporation of short-lived actinide isotopes accurately reproduces damage arising from both alpha-particle and the heavy recoil nucleus, but requires access to specialist facilities. In contrast, fast ion implantation of inactive model ceramics effectively simulates the heavy recoil nucleus, leading to amorphisation of the host crystal lattice over very short time-scales. Although the resulting materials are easily handled, quantitative analysis of the resulting damaged surface layer has proved challenging.

In this investigation, we have developed an experimental methodology for characterisation of radiation damaged structures in candidate ceramics for actinide disposition. Our approach involves implantation of bulk ceramic samples with 2 MeV Kr+ ions, to simulate heavy atom recoil; combined with grazing incidence X-ray absorption spectroscopy (GI-XAS) to characterise only the damaged surface layer. Here we present experimental GI-XAS data acquired at the Ti and Zr K-edges of ion implanted zirconolite, as a function of grazing angle, demonstrating that this technique can be successfully applied to characterise only the amorphised surface layer. Comparison of our findings with data from metamict natural analogues provide evidence that heavy ion implantation reproduces the amorphous structure arising from naturally accumulated radiation damage.

The focus of this research programme is to develop a single phase ceramic wasteform for waste PuO2 that is unsuitable for fuel manufacture. A suite of synthetic mineral systems have been considered including titanate, zirconate, phosphate and silicate based matrices. Although a wealth of information on plutonium disposition in some of the systems exists in the literature, the data is not always directly comparable which hinders comparison between different ceramic hosts. The crux of this research has been to compile a database of information on the proposed hosts to allow impartial comparison of the relative merits of each system.

The UK has a considerable stockpile of separated plutonium; a legacy of over 50 years of civilian nuclear programmes. This material has been considered both as an asset for future energy generation and a liability due to the proliferation threat. A proportion of the PuO2 stocks may be consumed by nuclear fission, in mixed oxide (MOx) or inert matrix (IMF) fuels but a quantity of waste PuO2 will remain which is unsuitable for fuel manufacture and will require immobilisation. A research program is currently underway to investigate the potential of various single phase ceramic formulations for the immobilisation of this waste PuO2 fraction. In this work a number of synthetic mineral systems have been considered including titanate, zirconate, phosphate and silicate based matrices. Although a wealth of information on plutonium disposition in some of the systems exists in the literature, the data is not always directly comparable which hinders comparison between different ceramic hosts. The crux of this research has been to compile a database of information on the proposed hosts to allow impartial comparison of the relative merits and shortcomings in each system.

Silver exchanged zeolites A, X and Y were used to occlude silver iodide (AgI) at 400°C. Heating to 900°C in a hot isostatic press caused decomposition of the zeolite materials and the formation of more dense phases. Silver zeolites A and X both formed sodalite, seen as a potential 129I waste form, while silver zeolite Y formed an x-ray amorphous phase containing AgI. Silver zeolite A produced the best potential waste form, a monolithic sodalite with negligible porosity.

A number of possible options have been proposed for the encapsulation and immobilisation of long lived actinide (Act) fractions in nuclear waste. Ceramics offer superior durability against chemical migration and the ability to be tailored to accommodate a variety ofdifferent waste streams. Research on the fabrication of dense, durable crystalline matrices for the safe disposal of fissile plutonium is ongoing and this study reports quantitative chemical, structural and spectroscopic analysis on fluorite based host phases.

Ceramics based on the fluorite structure are known to be able to incorporate a variety of actinides and in this work two candidate ceramic matrices were investigated: a pyrochlore, Gd2Zr1.60Ce0.20Hf0.20O7; and a zirconolite, (Ca0.90Gd0.10)(Zr0.50Ce0.20Hf0.20Gd0.10)Ti2O7. The chemical compositions of the two major phases observed in the ‘zirconolite’ sample were consistent with the 2M and 4M zirconolite polytypes and the presence of the 4M structure was confirmed by Electron Diffraction (ED). The major phase in the ‘pyrochlore’ ceramic was confirmed by ED to have the pyrochlore structure. Electron Energy Loss Spectroscopy (EELS) data indicated the presence of both Ce3+ and Ce4+ in all the samples.

Tailored ceramics are promising potential hosts for the immobilization of actinides [1]. In this study the potential of a range of different ceramic systems were investigated for the incorporation of waste plutonium, using Ce as a Pu surrogate. Durable actinide containing minerals exist in nature and provided excellent target phases for the titanate, zirconate, silicate and phosphate based formulations examined here [2]. The Ce single phase solid solution limits for each system were established and the processing parameters required to produce high quality ceramic specimens were optimised. Importantly, this was achieved within the constraints of a generic processing route suitable for fabrication of Pu bearing samples.

A range of transition metal bearing hollandite phases, formulated Ba1.2B1.2Ti6.8O16 (B2+ = Mg, Co, Ni, Zn, Mn) and Ba1.2B2.4Ti5.6O16 (B3+ = Al, Cr, Fe) were prepared using an alkoxide - nitrate route. X-ray powder diffraction demonstrated the synthesis of single phase materials for all compositions except B = Mn. The processing conditions required to produce > 95 % dense ceramics were determined for all compositions, except B = Mg for which the maximum density obtained was > 93 %. Analysis of transition metal K-edge XANES data confirmed the presence of the targeted transition metal oxidation state for all compositions except B = Mn, where the overall oxidation state was found to be Mn3+. The K-edge EXAFS data of Ba1.2B1.2Ti6.8O16 (B = Ni and Co) were successfully analysed using a crystallographic model of the hollandite structure, with six oxygen atoms present in the first co-ordination shell at a distance of ca. 2.02Å. Analysis of Fe K-edge EXAFS data of Ba1.2B2.4Ti5.4O16 revealed a reduced co-ordination shell of five oxygens at ca. 1.99Å.

Reactions of zeolite Na-A with AgI, and the sodium, copper and lead forms of zeolites A, LTA, X and Y with NaI, have been examined as possible starting routes to the long term immobilisation of iodine-129. Heating the salts in air, at 500°C, with the sodium forms of the zeolites leads to the formation of occlusion products, where the iodide salt migrates into the zeolite pores. Detailed studies of the Na-A / 5AgI complex indicate it has a uniform distribution of Na, Si, Al, Ag and I, and is thermally stable to ca. 750°C, where there is a substantial weight loss as iodine is released. In situ powder X-ray diffraction studies have been used to monitor the occlusion reaction at 400°C, and show that the occlusion product decomposes to produce a single crystalline phase at 800°C prior to further decomposition at 850°C to a mixture of nepheline and elemental silver.

The ability of the cubic phase of zirconia to accommodate in solid solution the oxides of rare earth elements with differing cationic radii has been investigated. Mixed oxide phase assemblages were prepared by hydrolysing zirconium butoxide with solutions of rare earth element nitrates followed by drying, calcining and sintering. The resulting products were characterised by X-ray diffraction and energy dispersive spectroscopy. The cubic zirconia phase can accept into solid solution the larger, non-cubic stabilising, rare earth element ions such as lanthanum in the presence of the cubic stabilising oxides of yttrium and samarium. As the proportion of the larger rare earth element ions is increased the formation of pyrochlore type compounds is favoured.