Previous work has shown that products from the chemical degradation of cellulosic matter can significantly reduce sorption of uranium(VI) and plutonium(IV) on geological materials. Uranium(IV) batch sorption experiments have now been performed to study the effect of organic degradation products in a reducing environment. Thorium(IV) sorption has also been studied since thorium is an important radioelement in its own right and has potential use as a simulant for other tetravalent actinides. Sorption onto London clay, Caithness flagstones and St. Bees sandstone was investigated.Experimental conditions were chosen to simulate both those expected close to a cementitious repository (pH ~ 11) and at the edge of the zone of migration of the alkaline plume (pH ~ 8). Work was carried out with both authentic degradation products and with gluconate, acting as a well-characterized simulant for cellulosic degradation products.The results show that the presence of organic species can cause a reduction in sorption. This is especially so in the presence of a high concentration of gluconate ions, but the reduction is significantly less with authentic degradation products.

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The solubility behaviour of uranium(VI) has been studied in a number of waters representative of the pore waters to be expected in candidate repository backfill concretes and also in solutions of calcium and sodium hydroxides. The sorption of uranium has been studied on one of these candidates, a Portland cement with a slag filler. The same cement has been hydrothermally treated to simulate the temperature conditions in the repository and the product used for comparative studies.The paper describes the experimental measurements and the results. The aqueous concentration of uranium over the uranates formed in the presence of sodium and calcium hydroxides was constant above pH 7 at 3 × 10−6 M. In the concrete pore waters the solubility of uranium was equal to or lower than this value. The distribution coefficient, Rd, for the sorption of uranium onto the slag cement was 2.5 × 104 ml g−1, and was increased by a factor two after hydrothermal treatment.

An integrated programme of batch sorption experiments and mathematical modelling has been carried out to study the sorption of plutonium and americium on a series of repository, backfill and geological materials relevant to the JNFL low-level radioactive waste repository at Rokkasho-Mura.The sorption of plutonium and americium on samples of concrete, mortar, sand/bentonite, tuff, sandstone and cover soil has been investigated. In addition, specimens of bitumen, cation and anion exchange resins, and polyester were chemically degraded. The resulting degradation product solutions, alongside solutions of humic and iso-saccharinic acids were used to study the effects on plutonium sorption onto concrete, sand/bentonite and sandstone.The sorption behaviour of plutonium and americium has been modelled using the geochemical speciation program HARPHRQ in conjunction with the HATCHES database.

A combined experimental and modelling study of the sorption of uranium and technetium on geological materials has been carried out as part of the PNC programme to increase confidence in the performance assessment for a high-level radioactive waste (HLW) repository in Japan. Batch sorption experiments have been performed in order to study the sorption of uranium and technetium onto bentonite, tuff and granodiorite from both equilibrated seawater and de-ionised water under strongly-reducing and non-reducing conditions.A preliminary study of the sorption of uranium on mineral surfaces in granodiorite has also been undertaken using a nuclear microprobe.Mathematical modelling using the geochemical speciation program HARPHRQ in conjunction with the HATCHES database has been carried out in order to interpret the results of the sorption experiments.