Work Package 1 of the MIND project addresses remaining key issues for the geological disposal of ILW concerning the long-term behaviour, fate and consequences of organic materials in the waste along with H2 generated by corrosion and radiolysis. This review begins this work by collating information concerning the inventory and nature of organic materials present in ILW and some LLW that requires geological disposal. The review has then considered the physical and chemical conditions that the organic materials will be subjected to during storage and geological disposal. The review then provides a summary description of the physical and chemical nature of the main organic materials present in ILW and their known chemical and radiolytic degradation behaviour. The effect of the organic degradation products on radionuclide speciation, which could affect how some radionuclides are transported in groundwater, is then considered to focus subsequent work in this field. Inventory information concerning the distribution of the different types of organic material present in ILW and some LLW is collated from published national sources. This confirms that the materials selected for study in the MIND project; bitumen, organic ion exchange resins and halogenated polymers (PVC), are present in significant amounts in the national inventories. Cellulose is present in generally lower amounts in most inventories, but it is already established that it is of significance with regard to the strong complexation effect of its alkaline hydrolysis products. Cellulose is included in the Work Package 1 studies, but is not considered in detail by this review because comprehensive reviews exist in peer reviewed literature. The review of organic ILW also collates information relevant to the radiation dose, timescale and chemical conditions that the waste and encapsulant materials will be subjected to during storage and geological disposal. This information, together with information regarding the physical and chemical nature of the organic materials and current understanding of the effects of chemical and radiolytic degradation is used to refine the design of radiolytic and biodegradation experiments that are being undertaken in MIND Work Package 1. In the next phase of the MIND project ion exchange resin, PVC, bitumen and cellulose materials will be irradiated using 60Co ϒ radiation at a range of dose rates. The first irradiation experiments will be undertaken under a range of relevant pH conditions with air and inert gas headspace. The first experiments will be to ensure that sufficient soluble organic material is produced in order to characterise the compounds present and to use the leachate in subsequent biodegradation experiments. Further experiments will then be undertaken at lower doses. The biodegradation experiments will be inoculated by various cultures, including consortia adapted to high pH conditions and indigenous microbes sampled from underground rock laboratories. Biodegradation studies will utilise a range of electron acceptors, including nitrate and sulfate identified by the review as being of most significance. The process of methane generation under cement buffered ILW conditions will also be explored. The soluble organic species will be characterised before and after biodegradation to identify compounds for complexation studies to draw conclusions regarding the degradation of known complexants such as the cellulose degradation product ISA. The solid residues and biomass materials will also be utilised for microscopy studies including interactions with radionuclides. The information collated in this review is of prime importance to the design of experimental work undertaken in Task 1.2 of Work Package 1 of the MIND project. The review will also provide a basis of Task 1.3 that examines processes under in situ wasteform and repository conditions and modelling and integration in Task 1.4 and wider integration and dissemination through MIND Work Package 3.
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|Published - 14 Dec 2015