Characterization and Radiation Resistance of a Mixed-Alkali Borosilicate Glass for High-Level Waste Vitrification
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Mat. Res. Soc. Symp. Proc. Vol. 608 C 2000 Materials Research Society
sample. Two separate series of glasses were prepared. In the NBS series, the amount of B 2 0 3 in the current composition used by BNFL was varied whilst the ratios of the other components remained the same. In the ABS series, the SiO 2 /B 2 0 3 ratio (K) was kept at fixed values and the alkali/B 20 3 ratio (R) was varied. The NBS series of glasses were annealed at 450'C for 1 hour. The various values of K and R used are shown in Fig I. sio, NBS
0.0
0.0".1.0
0.2
0.4
0.6
0.8
1.0
B20,
Fig I Batch compositions (mol%) of samples used inthe present work. 0 represents samples of insufficient thermal stability or chemical durability. Preliminary chemical analysis of the boron content of the samples has been carried out using quantitative "IB NMR (Table 1). Boron can be present in the network in 3-fold coordination to oxygen to give trigonal planar [B0 3] units (B3) or 4-fold coordination to oxygen to give tetrahedral [B0 4] units (B4). By comparing the combined intensities of the B3 and B4 peaks with those of a reference material (Pyrex®) of known boron content, an estimate of the boron content of each sample was made. The experimental details of physical and structural characterisation using DTA, NMR and density measurement have been presented previously [8]. RESULTS The nominal and analysed boron compositions of the glass samples are listed in Table I. Most samples were formed easily and produced transparent, amorphous glasses. Approaching the glass-making limit for this system, several compositions proved to be unsuitable for use and are highlighted with circles in Fig I. The variations in calculated density of each sample, as a function of K and R, are shown in Fig.II. It can be seen that there is a general trend toward a significant increase in measured density at low R, before a levelling off, within experimental error, at larger R. This is in fairly close agreement with the results obtained by Tang & Jiang [9] for the Na 2 O-B 20 3-SiO 2 system, where the value of R at which the density begins to level off increases with K. Fig Ilb illustrates the similarity of this behaviour with that observed by Budhwani and Feller [10] for the Na- and Li-borosilicate systems. The Li-Na-borosilicate dependence on density lies between that of the individual alkali ternary systems, with a tendency to follow that of the Na-borosilicate.
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Table 1. Nominal and analysed boron composition of prepared samples. Boron content analysed via MAS NMR. Sample
K
R
Nominal B20 3 (mol %)
Analysed B20 3 (mol %) (± 5% of value)
NBS7 NBS6 MW NBS8 NSB14 NSB9 NSBIO
-
-
8.57 13.57 18.57 23.57 25.57 28.57 33.57
-
ABS01 ABS02 ABS03 ABS04 ABS05 ABS06 IH IB IF IG IC IE ID
2.41 2.41 2.41 2.41 2.41 2.41 1 1 1 i 1 I I
0.38 0.85 1.14 1.29 1.46 2.27 0.35 0.5 0.67 0.75 0.86
26.38 23.46 21.99 21.26 20.52 17.59 42.5 40 37.5 36.5 35 32.5 30
-
-
17.7 19.8 22.6 27.8 31.5
24.8 -
22.2 21.7 21.3 -
40.7 42.5 37.1 37.2 Outside glass formation region.
(a) 2.50-
Sample
K
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