An Attempt to Assess The Long-Term Crystallization Rate of Nuclear Waste Glasses
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W&SrWOMEr - V FMRADIOSCIYIVHE SIECIFIC BASIS Werner. Wtze, editor
AN ATTEMPT TO ASSESS THE LONG-TERM CRYSTALLIZATION RATE OF NUCLEAR WASTE GLASSES NOEL JACQUET-FRANCILLON, FRANCOIS PACAUD and PIERRE QUEILLE D&partment de Genie Radioactif, C.E.A. Valrho, Site de Marcoule BP 171, 30200 Bagnols sur Ceze, France
INTRODUCTION The crystallization of fission product glasses is generally caused by maintaining these glasses at high temperatures (600
to 900
C) for several days or
weeks in order to evaluate the effects of crystallization on leach resistance and mechanical properties. The examination of glasses subjected to such thermal treatment involves the identification of the crystalline phases obtainedI and the determination of T.T.T. diagrams 2 . Over the past few years it has appeared possible to measure the crystalline portion of these glasses by x-ray diffraction 3,4 , by image analysis4 or by measuring specific heat6. An exhaustive paper dealing with measurement problems has appeared recently6 At the C.E.A Marcoule,
isothermal calorimetry has been used to measure the thermal release which accompanies crystallization in order to attempt to quantify the devitrification capacity of the different glasses tested and to establish the time-temperature relationships. Such data could allow for long-term, low temperature extrapolation of the behaviour of the glass during geological storage. The present work describes the results obtained using three non-radioactive glasses of increasing chemical complexity. EXPERIMENTAL The following three glasses were chosen for their varied but well known devitrification yields: •2lithium disilicate 2 SiO2-Li20; 100 % volume in crystals. SAN 38 33 16 F' 1001 glass (for vitrification of MTR solution) 56 % vol. in crystals. SON 58 30 24 U2 glass (for vitrification of LWR solution) 5 to 6 % vol.
in
crystals. Preparation. The three glasses were melted at 13000 C in a platinum crucible; the composition of the mixed constituents is given in Table 1.
250 TABLE I COMPOSITION (wt.%) OF THE GLASSES
SiO2
Reference
2 SiO2 * Li20 SAN 383316 F'
Al 203
B203
Na 20
25.86
15.15
23.88
2.37
2.39
23.16
9.17
22.00
3.50
80.0 1001
SON 583024 U2
29.85
Li 20
Ox.FP.
Diverse
20.0
42.17
The fused glasses are cast directly in the measuring crucibles (sintered alumina for the fission product glasses, ring a sample is
graphite for the 2 SiO2
Li
2 0).
Before measu-
extracted of each type of glass and observed through an optical
microscope by reflected and transmitted light.
Apparatus.
A high temperature model SETARAM microcalorimeter was used. The
apparatus essentially consists of two fluxmeters (measure and reference) placed in a thermostatically controlled bloc that can be heated and held constant at any temperature between ambient and 10000 C. The limit of detection for thermal -1 effects is in the order of 0.85 joule h . The samples are first positioned slightly above the fluxmeters at a temperature around the glass transition region, which makes it
possible to trigger off th
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