Characterisation of the Barrier Performance of Cements
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Characterisation of the Barrier Performance of Cements F. P. Glasser Chemistry Department, Meston Buildings, University of Aberdeen, AB24 3UE
Abstract Portland and modified Portland cements are useful matrices for solidification and storage of wastes. Concrete containing cement is likely to be introduced into the repository. The further use of cement as an engineered barrier reduces materials incompatibility problems. Cement has a high internal pH and maintains a high buffering reserve of alkalinity. This in turn, reduces the solubility of many radionuclides. The source of the immobilisation (chemical, physical) is sensitive to nuclide species: sorption and precipitation are the main binding mechanisms. The conditioning action of cement could affect the performance of other engineered barriers. The deterioration of cements is discussed. Areas of research needs are highlighted.
Introduction The 25th anniversary of this series of meetings coincides with a crucial time for the future of nuclear energy. Nuclear energy presently supplies 16% of world electricity. This production is virtually without emission of particulates, sulfur and nitrogen oxides and volatile organics. Concerning carbon, the IAEA states, “the complete nuclear power chain, from resource extraction to waste disposal, including reactor and facilities construction, emits only 2 – 6 grams of carbon equivalent per kilowatt hour. This is about the same as wind and solar power” [1]. It is not my purpose to debate the future of nuclear power, but to focus on waste issues: biohazards and the long period of confinement required to ensure protection of the biosphere emerge as key issues concerning the acceptability of nuclear power. Wastes may be kept in storage for prolonged periods prior to disposal. Low-level wastes comprise by volume the bulk of the inventory. Mobile wastes - liquid, particulate, sludge, floc, etc. - are best solidified. Cements emerge as a leading matrix contender in this application: they are easily handled, by remote processing if necessary, are non-toxic and non-flammable and give good biological shielding. Moreover, they give confidence: modern Portland cements have been in use for ~150 years and Ca(OH)2-based compositions have been successfully used in civil engineering for >2000 years [2, 3]. Natural analogues of cement also exist [4]. Cement, introduced to facilitate solidification, can also be used for barrier formation with resulting simplification of potential materials incompatibility problems. Engineered geological disposal is the principal option for mixed and dilute wastes. The existence of natural analogue reactors, such as those at Oklo and Palumotto [5, 6] gives confidence that underground disposal of wastes is the safest of the currently-available long term disposal options. However, not all geologic sites offer proof of good performance: granites and JJ9.1.1
high grade metamorphic rocks are fracture-prone. Coupled with uncertainities about present and future groundwater flows regimes, it has proven difficult if not i
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