Growth Rate of Alteration Layer and Elemental Mass Losses During Leaching of Borosilicate Nuclear Waste Glass
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GROWTH RATE OF ALTERATION LAYER AND ELEMENTAL MASS LOSSES DURING LEACHING OF BOROSILICATE NUCLEAR WASTE GLASS
T. BANBA, T. MURAKAMI, AND H. ISOBE Japan Atomic Energy Research Institute, Tokai,
Ibaraki 319-11,
JAPAN
ABSTRACT
MCC-1 static leaching experiments were carried out for a simulated waste glass in deionized water for up to 364 days at 900 C in order to examine the relationship between alteration layer thickness and elemental mass losses. The thickness of the layer increased linearly at a rate of 0.63,um/day for the first 91 days, and thereafter it remained constant (about 60 Jim) independent of time. The change in normal ized elemental mass losses of B, Na, and Si with time shows the same trend as that of the change in the layer thickness. It is also demonstrated that the growth rate of the layer can be calculated from the released boron in the leachates. The shrinkage of the alteration layer during air-drying is not significant in spite of the porous structure of the layer, which results from the depletion of cations and the recrystall ization reported previously. Although the apparent release of elements almost ceases after a 91-day leaching experiment, reactions in the layer such as crystal growth continue. INTRODUCTION The long-term corrosion of nuclear waste glasses is a key factor for the safety assessment of nucýlear waste disposal. However, as the time required for the assessment (10 -106 years) cannot be demonstrated in laboratory, predictions of long-term corrosion are being obtained by using geochemical models to extrapolate short-term laboratory data. The geochemical models must then be validated by other methods such as natural analogues [1]. The use of basaltic glasses as natural analogues of nuclear waste glasses was first proposed by Ewing [2]. The problem is, however, how to util ize information of the natural analogues for modelling. One way to provide confidence in the theoretical models is to correctly predict the phases formed in the layers of naturally and experimentally altered glasses. In natural environments, the alteration layers are the most important records of the long-term corrosion of basaltic glasses and investigations to provide a detailed understanding of the mineralogy and geochemistry of the alteration layers have been reported [3-10]. More recently, studies of alteration layers have focused more on fresh water than seawater alteration because of the greater similarity of fresh water environment to that around a granitic disposal site [7-9]. The growth rate of the alteration layers of basaltic glasses in natural environments is not expressed as a function of time, because it is influenced by differences in solution composition and differences in true exposure age of the glasses to solution [11]. The relationship between the growth rate of the alteration layer of a borosil icate nuclear waste glass and elemental mass losses, which may be understood under well-controlled experimental conditions in laboratory, can give an insight into the environment around naturally alt
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