The immobilization of non-radioactive Na in nuclear waste has not been given much attention. However since Na compounds are normally very soluble, Nabearing phases may render a waste form more susceptiible to leaching of radionuclides. The Na-phases may incorporate radionuclides and also, leaching of soluble Na phases will increase the surface area of the waste form. The crystalline Na-bearing phases which might occur in tailored ceramic formulations for nuclear waste are nepheline [1,2], magnetoplumbite [3] and sodalitetype materials [4]. Other Na phases which might occur in titanate-tailored [2,5] or titanate-encapsulated [6,7] forms are perovskite [8,9], Na2Ti3O7 (especially in titanate-encapsulated materials) and Na2TiSiO5 or Na2Ti2Si2O9, etc., if the waste contained sand or zeolites [10].
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