Development of Ceramic Waste Forms for High-Level Nuclear Waste over the Last 30 Years
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Development of Ceramic Waste Forms for High-Level Nuclear Waste over the Last 30 Years Eric Vance Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Menai, NSW, 2234, Australia ABSTRACT Many types of ceramics have been put forward for immobilisation of high-level waste (HLW) from reprocessing of nuclear power plant fuel or weapons production. After describing some historical aspects of waste form research, the essential features of the chemical design and processing of these different ceramic types will be discussed briefly. Given acceptable laboratory and long-term predicted performance based on appropriately rigorous chemical design, the important processing parameters are mostly waste loading, waste throughput, footprint, offgas control/minimisation, and the need for secondary waste treatment. It is concluded that the ìproblem of high-level nuclear wasteî is largely solved from a technical point of view, within the current regulatory framework, and that the main remaining question is which technical disposition method is optimum for a given waste. INTRODUCTION Solids for the immobilisation of high-level waste (HLW) from nuclear fuel reprocessing or weapons production, have been under development for over 50 years. The principal aim is to prepare a nearly water-insoluble solid, with minimal process wastes being produced. This can be done by either (a) separating the radioactive ions from the bulk of the waste and then incorporating them in the waste form or (b) incorporating the waste as a whole into a solid. While method (b) involves fewer process steps in general, method (a) can lead to minimisation of the waste form volume for HLW, and hence ease pressure on limited and very expensive HLW repository space. While it has been argued that the variability and predicted long-term uncertainties of waste form performances are trivial with respect to the uncertainties in the near- and far-field predicted performance of a geological repository, the fact that waste forms can be subjected to laboratory examination remains a compelling driving force to persist with waste form performance studies. Clearly improving waste form performance will decrease environmental risk irrespective of the precise source term used in modelling. As a rough guide, a waste form is considered as a serious candidate material if the normalised leach rate of the most intrinsically soluble species (typically alkalis) in a comparatively large volume (V) of hot deionised water is < 1 g/m2/day, based on geometrical surface area (SA) and with SA/V being < 0.1 cm-1. The normalised leach rate pertains to the fraction of the inventory leached out per unit time of a given species and not the absolute quantity. Immobilisation of HLW was seen as a problem as early as 1953 and a paper [1] dealt with the use of fired clay for this purpose. Many types of waste forms-cements, glasses, glassceramics, ceramics and metal alloys- have been subsequently proposed and researched. An
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