Ewan R. Maddrell

Zeolite - Salt Occlusion: A Potential Route for the Immobilisation of Iodine-129?

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.

Microchemical and crystallographic characterisation of fluorite-based ceramic wasteforms

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, Gd 2 Zr 1.60 Ce 0.20 Hf 0.20 O 7 ; and a zirconolite, (Ca 0.90 Gd 0.10 )(Zr 0.50 Ce 0.20 Hf 0.20 Gd 0.10 )Ti 2 O 7. 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 Ce 3+ and Ce 4+ in all the samples.

Single Phase Ceramic Wasteforms for Plutonium Disposition

Four single phase ceramic formulations were investigated as potential wasteforms for plutonium disposition, using Ce as a Pu surrogate: pyrochlore, Gd2(Zr1-2xCexHfx)O7; zirconolite, (Ca1-x/2Gdx/2)(Zr1-5x/2CexHfxGdx/2)Ti2O7; britholite, (Ca2+xY8-2xCex)Si6O26; and kosnarite, Na(Zr2-xCex)P3O12. The single phase solid solution limits for each formulation were established and the processing parameters required to produce high quality ceramic specimens were optimised. This was achieved within the constraints of a generic processing route suitable for fabrication of Pu bearing samples. Whereas the pyrochlore, zirconolite and britholite formulations show considerable promise as single phase ceramic wasteforms, the kosnarite formulation was found to be unsuitable.

Effect of Conjoint Additions of Rare Earth Element Oxides on the Phase Stability of Zirconia

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.

An Evaluation of Single Phase Ceramic Formulations for Plutonium Disposition

Ceramics are promising potential hosts for the immobilization of actinide containing wastes. Work has been reported in the literature on multiphase systems, such as SYNROC [1], and on single phase systems such as pyrochlores [2] and zirconia [3], but assessment of the different waste-forms by direct comparison of literature data is not always easy due to the different processing and fabrication routes employed. In this study a potential range of different ceramic systems were investigated for plutonium disposition using the same processing scheme. Durable actinide containing minerals exist in nature and provided excellent target phases for the titanate, zirconate, silicate and phosphate based formulations examined here [4]. The Ce solid solution limits for each particular substitution mechanism 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. (authors)

Synthesis and characterisation of transition metal substituted barium hollandite ceramics

A range of transition metal bearing hollandite phases, formulated Ba 1.2 B 1.2 Ti 6.8 O 16 (B 2+ = Mg, Co, Ni, Zn, Mn) and Ba 1.2 B 2.4 Ti 5.6 O 16 (B 3+ = 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 Mn 3+ . The K-edge EXAFS data of Ba 1.2 B 1.2 Ti 6.8 O 16 (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. 2.02A. Analysis of Fe K-edge EXAFS data of Ba 1.2 B 2.4 Ti 5.4 O 16 revealed a reduced co-ordination shell of five oxygens at ca. 1.99A.

The Development of a Ceramic Waste Form for Immobilisation of Highly Active Wastes from Radical Purex Reprocessing Operations

The development of novel, Radical Purex, reprocessing technologies, leading to fission product waste streams with high levels of inert constituents, may mean that vitrification is no longer the optimum solution for the immobilisation of highly active wastes at the back end of the nuclear fuel cycle. A ceramic phase assemblage is described which uses the inert constituents of the waste stream as a functional component of the waste form, permitting high waste loadings to be achieved and thus enabling waste minimisation considerations to be satisfied. The initial development of this phase assemblage is presented.