Leaching Studies of Crystalline Sodium Phases in Nuclear Waste Forms
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Stephen V.
Topp,
Inc.
FOR NUCLEAR WASTE MANAGEMENT
editor
163
LEACHING STUDIES OF CRYSTALLINE SODIUM PHASES IN NUCLEAR WASTE FORMS E.R. VANCE AND T. ADL Materials Research Laboratory, Park, PA 16802
The Pennsylvania State University, University
INTRODUCTION 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 sodaliteOther Na phases which might occur in titanate-tailored type materials [4]. [2,5] or titanate-encapsulated [6,7] forms are perovskite [8,9], Na2 Ti3O7 (especially in titanate-encapsulated materials) and Na2 TiSiO5 or Na2 Ti 2 Si2O9, Sphene is of interest etc., if the waste contained sand or zeolites [10]. since it should be resistant to silica-rich ground water [11,12] and it also can contain Na [11]. Acmite is an alteration product of hydrothermallytreated PNL-76-68 glass [12]. Na-bearing phosphates may occur in phosphatetailored ceramics designed to immobilize transuranics in monazite [13,14]. The present work concerns the reaction of the above materials with aqueous media in the temperature range 25-3000C. The phosphates studied were NaTi 2 (P0 4 )3 and NaZr 2 (P04) 3 which are relatively insoluble in boiling water [15]. Other materials studied were NaFe02, Na2ZrSi05 and Na2ZrSi 2O7 . EXPERIMENTAL Materials were mostly made by combination of solutions to assure atomicscale mixing, but Nb and Ti were added as oxides. Silica was in the form of a sol and P was added as (NH4)H 2 P0 4 solution. After mixing, drying at "I00°C and calcination at 't600°C, the calcines were pulverized, pelletized at a 0 pressure of q200 MPa, and fired at 800-1350 C, depending on the melting point and thermal stability of the compound under study. Firing was normally carried out at "50°C below the melting point or the temperature at which the phase became unstable due to Na volatility; the object was to maximize the yield of the desired phase and its crystallinity. The firing treatments of many of the phases are given in Table I and those of the others in the body of the text. Leaching studies were typically performed on v50 pm granules. These granules consisted of qi pm-sized crystallites and were porous to some extent. Reactions at temperatures higher than 100 0 C were carried out in sealed teflon 0 or gold tubes. For time-dependence studies at 25 C, sized granules were contained in sealed polypropylene flasks containing deionized water to which was The flasks were added a few drops of HCl or KOH to attain the desired pH. placed in a shaker bath. Normally 0.25 gm of solid was added to 25 ml of liquid, with 0.5 ml aliquots being withdrawn periodically for spectr
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