The SYNROC process proposes to immobilize high-level wastes as dilute solid solutions (i.e. as integral parts of crystal lattices) in the constituent minerals of a synthetic rock formed from a mixture of oxides, principally, TiO₂, BaO, ZrO₂, Al₂O₃ and CaO. A new modification of SYNROC, comprising the titanate mineral assemblage Ba-hollandite (BaAl₂Ti₆O₁₆), perovskite (CaTiO₃) and zirconolite (CaZrTi₂O₇) has been developed. Experiments show that the entire spectrum of high-level waste elements can be incorporated in the crystal lattices of these 3 phases and in a few minor accessory phases. This titanate assemblage has proved to be exceptionally resistant to hydrothermal leaching and in this respect, amongst others, is demonstrably superior to alternative ceramic waste forms and to borosilicate glasses. The relative stabilities of various waste forms were compared in hydrothermal leaching experiments using both pure water and lOwt.% NaCl solutions (temperature range 300-1, 000°C; pressure range 300-5, 000 bars). Borosilicate glasses are almost completely decomposed and disintegrated after only 24 hours at 350°C and 1, 000 bars, and extensive losses of hazardous high-level waste elements occurred. The phase pollucite (CsAlSi₂O₆), which provides the site for Cs-fixation in alternative ceramic waste forms, likewise begins to decompose at 400°C, with total loss of Cs by 600°C. On the other hand, the hollandite-perovskite-zirconolite SYNROC assemblage proved to be exceptionally resistant to leaching, surviving unaltered extreme conditions up to 900°C and 5, 000 bars. Hazardous species e.g. Cs, U and Sr, were quantitatively retained by the hollandite, zirconolite and perovskite phases respectively. Geochemical studies of naturally-occurring minerals containing radwaste elements are relevant to the problem of radiation damage to SYNROC phases. These imply that the a-particle flux in SYNROC is unlikely to be enough to impair the ability to immobilize radwaste for the required period. SYNROC can be produced by mixing about 10wt.% radwaste calcine with appropriate amounts of inert oxides. Subsolidus hot-pressing at 1, 200-1, 300°C in sealed Ni containers results in a dense, compact, mechanically-strong material. The production of SYNROC in its terminal state and final encapsulation and sealing are accomplished in a single step, and moreover, volatile species e.g. Cs, Ru, are quantitatively retained during hot-pressing. In contrast to borosilicate glass technology, expensive equipment for volatile recovery and recycling is not required. These advantages are believed to make SYNROC economically competitive with borosilicate glass. Moreover, the simplicity of the hot-pressing process makes it very suitable for remote operation in hot cells. SYNROC phases have structures analogous to natural minerals which have survived a variety of geological conditions for millions of years while retaining certain high-level waste elements in their crystal lattices. This fact, coupled with the exceptional resistance exhibited by SYNROC in accelerated leaching tests, leads to considerable confidence in the long-term stability of SYNROC, and in its capacity to isolate high-level wastes from the biosphere for periods sufficiently long to permit their safe decay.

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Formulae are derived for the correlation of errors in the most used lead-lead and uranium-lead diagrams. It is emphasized that with the usual measurement schemes, error correlation is high in the conventional diagrams. An inverse lead-lead diagram is described in which, by proper choice of the measurement scheme, there is no correlation between errors. This advantage is gained whilst maintaining the use of a measurement scheme which optimises the precision of the lead isotope measurements.

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¹⁸O/¹⁶O and D/H ratios, together with ¹³C/¹²C ratio for one carbonate sample, were measured for 9 whole-rock samples and 4 clay samples from the Amakusa pottery stone deposits distributed in Tertiary rhyolite dikes. The δ¹⁸O values of whole-rocks range from +13.1 to +15.8‰ relative to SMOW. Extrapolation of the δ¹⁸O values to fresh rhyolite suggests that an ¹⁸O-enrichment up to 8‰ was attained in the alteration processes. The δ¹⁸O values of altered rocks are higher than the value of sericite (+14.5‰), which could be caused by re-deposition of ¹⁸O-rich silica. The Amakusa clays are depleted in ¹⁸O by about 5‰, and enriched in D by about 10‰ relative to kaolinites of weathering origin. This may eliminate the possibility of weathering origin for the Amakusa clays. The hydrothermal water equilibrated with the Amakusa clays is estimated to have been enriched in both ¹⁸O and D as compared with present-day local meteoric waters. This hydrothermal water could be magmatic in origin, and have undergone mixing with meteoric waters and kinetic fractionation due to liquid-vapor separation.

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The present authors have studied the influence of barium and magnesium ions, which are contained together in a calcium bicarbonate parent solution, on the crystal form of carbonate formed from the parent solution. Magnesium ions in a parent solution inhibit calcite formation and favor aragonite formation, whereas barium ions inhibit aragonite formation and favor calcite formation. The degree of these influences is affected by the concentrations of these ions in the parent solution. When the influence of barium ions is stronger than that of magnesium ions, magnesian calcite is formed and the MgCO₃ content of the magnesian calcite increases with increasing concentration of magnesium ions in a parent solution. From the calcium bicarbonate solutions containing both more than 35ppm of barium ions and 1, 200ppm of magnesium ions monohydrocalcite is formed. Monohydrocalcite does not incorporate both MgCO₃ and BaCO₃ in its crystal lattice. The distribution coefficients of magnesium and barium have been measured when magnesian calcite is precipitated. The distribution coefficient of magnesium or barium for magnesian calcite tends to increase with increasing barium carbonate content or magnesium carbonate content of the formed magnesian calcite, respectively.

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