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Bentonite may be used, in conjunction with cementitious materials, in repository designs for TRU wastes in Japan. Therefore, possible effects of the interaction of highly alkaline water with bentonite are of interest. In this study, the sorption of plutonium and uranium has been measured on a bentonite sample that had been altered by hydrothermal exposure to a high pH solution. Samples of Kunipia-F® bentonite were exposed to a pH 14 solution of sodium, potassium and calcium hydroxides for 20 days at 130°C, 160°C and 200°C under air. X-Ray analysis showed that alteration at 160°C was similar to alteration at 200°C. Batch sorption experiments were carried out at room temperature under nitrogen by contacting 200°C-altered bentonite with synthetic equilibrated water containing both uranium and plutonium in the presence of sodium dithionite as a reducing agent. The experiments were equilibrated for three months and then sampled after centrifugation, 0.45 gm filtration, and 10,000 nominal molecular weight cut-off (NMWCO) filtration. Uranium and plutonium remaining in solution were determined. The final redox potentials were between -500 to -51OmV vs SHE and the final pH values were 12.2. Sorption of both actinides showed some dependence on the solid:liquid separation method. The average RD value for uranium sorption after centrifugation was 2 × 103cm3g−1 which increased to 3 × 104 cm3g−1 after 0.45 ώm filtration. However, the effect of a further decrease in the pore size of the filter had little additional effect; 10,000 NMWCO filtration gave an average RD of 5 × 104 cm3g−1. The sorption of plutonium after solid:liquid separation by centrifugation was very similar to uranium with an average RD of 6 × 103 cm3g−1. After 0.45 ώm filtration the increase in RD to 2 × 105 cm3g−1 was more marked than for uranium. There was no significant increase in RD value after 10,000 NMWCO filtration compared to 0.45 μm filtration.

The influence of alkaline degradation or radiolytic degradation of asphalt on plutonium solubility has been investigated. Asphalt has been contacted with water, sodium hydroxide solution or concrete leachate at 80°C for periods of up to approximately 2 years. Sodium nitrate was also present in some of the experiments. Plutonium solubilities were measured at pH 12 in the leachates and found to be less than 10-8 mol dm-3 for most degradations. Relatively low levels of Total Organic Carbon were measured in the leachates. Alpha radiolysis of asphalt in the presence of concrete and water has also been studied. Samples of asphalt were encapsulated in concrete after coating with the 238PuO2, crushed and leached at room temperature. The solubility of plutonium was measured in samples of the leachates after approximately 90 days and 180 days had elapsed. The results showed that the solubility of plutonium in the α-radiolysis leachates remained low and was in the range 2 × 10-11 to 8 × 10-9 mol dm-3. A consideration of these results, and data published elsewhere, suggests that chemical and radiolytic attack on asphalt or bitumen under anaerobic, alkaline conditions typical of a deep cementitious repository is unlikely to generate complexants for plutonium which are effective at high pH. Any enhancement of plutonium solubility is likely to be less significant than that arising from the degradation of some other organic materials.

Nirex is seeking to develop a deep underground repository for the disposal of solid intermediate-level and low-level radioactive wastes (ILW and LLW) in the UK. One possible influence on the behaviour of radionuclides is the formation of water-soluble complexants by the degradation of the solid organic polymers that will be present in the wastes. The degradation products of cellulose have been shown to increase the solubility of plutonium and other radionuclides and to reduce sorption onto near-field and far-field materials. Degradation of cellulose under anaerobic alkaline conditions produces a range of organic acids. In this paper 2-C-(hydroxymethyl)-3-deoxy-D-pentonic acid (isosaccharinic acid, ISA) is identified by High Performance Liquid Chromatography as a significant component of cellulose leachates. A combination of fractionation of cellulose leachates and plutonium solubility determinations shows that ISA is responsible for the majority of the enhancement of plutonium solubility observed in such leachates. Further degradation of ISA by chemical or microbial action may lessen the effect of degraded cellulose leachates. Experimental studies on the chemical degradation of this compound under alkaline conditions suggest that the presence of oxygen is required. Microbial degradation studies show that the plutonium solubility in solutions of ISA is reduced by their exposure to microbial action.

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.

UK Nirex Ltd is seeking to develop a deep underground repository for the disposal of solid low- and intermediate-level radioactive wastes in the UK. The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the post-closure performance assessment of the repository. One of the investigations carried out by AEA Technology under the NSARP is the investigation of the formation of water-soluble complexants from the degradation of the solid organic polymers and cellulosic materials present in such wastes. The influence of these complexants on the solubility and sorption of a number of radionuclides is also being investigated by a combination of experimental measurements and thermodynamic modelling. Under the alkaline, anaerobic conditions representative of the near field of the repository, cellulose has previously been shown to degrade to yield soluble products which enhance the solubilities and reduce the sorption of several radionuclides. As part of this investigation, cellulose was chemically degraded under conditions similar to those expected in the near field and the leachate analysed. Separation of the products by HPLC gave direct identification of 2-C-(hydroxymethyl)-3-deoxy-D-erythro-pentonic (isosaccharinic) acid as one of the degradation products. More tentative assignments were made for glycolic, formic, lactic and acetic acids. A comparison was made with the degradation products suggested in the literature. The possible mechanisms and extent of cellulose degradation are discussed, as well as the factors affecting its degradation in the near field. The sorption of plutonium on to cement from a leachate produced under alkaline, anaerobic conditions is reported.

UK Nirex Ltd are seeking to develop a deep underground repository for the disposal of solid ILW and LLW in the UK. The formation of water-soluble complexants from the degradation of the solid organic polymers and cellulosic materials present in such wastes may influence the solubility and sorption properties of a number of radioelements. A number of materials have been chemically degraded under alkaline, anaerobic conditions representative of the near field of the repository. Measurements of plutonium solubility in the leachates obtained show that the degradation of cellulose is of particular concern; the effects on the solubility can be several orders of magnitude greater than those of other organic materials. Products formed from the degradation of ion-exchange resins were found to have little effect. Solubilities of a number of other radioelements in aerobically degraded cellulose leachates are also given. The effect of reducing the ratio of celluloseto cement in the degradation experiments results in a decrease in plutonium solubility in the corresponding leachate. Several of the likely degradation products of cellulose have been individually synthesised including isosaccharinic acid, a key degradation product of cellulose. Enhanced plutonium solubilities have been measured in 10-3M solutions of these compounds. Sorption of plutonium onto cement from leachates obtained by the degradation of 10% cellulose in cement is reduced by about two orders of magnitude. Interaction with cement removes significant concentrations of complexants from solution. Thermodynamic modelling studies show that hydroxyl groups present in the compounds which result from the alkaline degradation of cellulose can strongly complex with plutonium.

The solubility and sorption of radium and tin have been studied for a cementitious near-field environment.The solubility of radium was observed to be greater than 10−7M, even with the addition of IM sulphate to the cement equilibrated water. The values of Rn for radium varied from 102 to 103 ml g−1 for the cement types and initial radium concentrations studied.The solubility of tin was observed to be about 10−4M for a range of cement equilibrated waters above tin oxides and metallic tin. The solubility of tin was observed to increase with pH. Tin solubilities were not affected by the redox conditions. Tin was present in a range of particle sizes in the sorption studies. A value of RD of 104 ml g−1 was observed for tin species less than 0.45μm.

The solubility of some of the radiologically important actinide elements have been determined in a water whose chemistry was representative of that believed to exist in potential repositories in the U.K. The solubilities of the actinides have been determined as a function of the pH of the water. The results of these experiments have been compared with the results of predictions made using the PHREEQE geochemical modelling code in order to test and validate the data base used in the model. In the light of these comparisons, suggestions are made for alternative values for the thermodynamic data and for further studies.