Thermophysical properties of bentonite

P/THME/19
THERMOPHYSICAL PROPERTIES
OF BENTONITE
M. Plötze1, U. Schärli 2, A. Koch 3, H. Weber 4
1. ETH Zurich, Institute for Geotechnical Engineering, CH-8093 Zurich, Switzerland
([email protected])
2. Ing.-büro Schärli, Geologie + Geophysik, Giblenstrasse, CH-8049 Zurich, Switzerland
([email protected])
3. RWTH Aachen, Applied Geophysics, Lochnerstr. 4-20, D-52056 Aachen, Germany
([email protected])
4. NAGRA - National Cooperative for the Disposal of Radioactive Waste, Hardstrasse 73,
CH-5430 Wettingen, Switzerland ([email protected])
INTRODUCTION
The heat transfer is one of the important functions of the buffer material in HLW disposal besides limiting
the entry of water and radionuclide retardation. The mean parameters describing the thermal properties of
a material are the heat conductivity λ (W/m.K), the heat capacity c (J/kg.K) and the thermal diffusivity κ
(m2/s). The heat transport is a result of different mechanisms, in minerals themselves by a phonon
mechanism. Factors of influence are thereby apart from the composition and mineral orientation in the
sample also the material density and - porosity as well as the water content and the temperature during the
measurement.
The thermal characteristics of different highly compacted bentonite blocks as well as granular material of
compacted bentonite were characterised in actual HLW disposal research projects (e.g. EURATOM HE
FIKW-CT-2001-00132).
EXPERIMENTAL CONCEPT
The investigated bentonites were a Ca,Mg-bentonite from Almeria (Spain) and a natural sodium bentonite
from Wyoming (MX80).
The heat conductivity was measured with a transient method. For the compacted bentonite blocks the
Quick Thermal conductivity Meter (QTM) was used, which is based on an impulse of thermal flow into
the analysed material with a linear surface probe (a thin heating wire) was used. In the centre of the heating
wire lies a thermocouple, which registers the temperature at the boundary surface between measuring probe
and sample. The measuring probe is pressed during the measurement on an evenly flattened surface of the
sample (size about 10x10 cm) and heated for 20 s. The heat impulse penetrates several millimetres in the
material. The temperature rise is registered with the thermistor and the heat conductivity considering the
appropriate boundary conditions is computed. The compacted blocks were measured at different water
contents (oven dry w = 0%, ρ = 2.22-2.36 g/cm3, air dry w = 12.2-12.9%, ρ = 2.12-2.25 g/cm3 and stored
at 85% relative humidity w = 19.6-21.0%, ρ = 1.83-1.89 g/cm3) at 20°C and 90°C parallel and
perpendicular to the compaction direction.
For the granular material a HLQ and VLQ needle probe (device TK04, TeKa Germany) was used. Here
the bentonite granules (w = 5.7%, ρ = 2.1 g/cm3) were measured in loose filled (bulk density γ = 1.6 g/cm3)
and in compacted state (bulk density γ = 1.8 g/cm3) with air as well as with oil as pore filling.
The measurement of the specific heat capacity is based on the principle of mixing two materials with
different temperatures (here bentonite with 0°C and water with 20°C). On the assumption that the internal
energy of the two materials before and after their mixing remains constant and one of the two materials
admits the thermal capacity is known, the thermal capacity of the other material can be computed. The
INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE
CLAYS IN NATURAL & ENGINEERED BARRIERS
FOR RADIOACTIVE WASTE CONFINEMENT
Page 579
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measurements were carried out on three samples (150 g as grains 1-4 mm) with various water contents (air
dry, oven dry and stored at 85% relative humidity). The hydration effect (heat release by water adsorption
of the dry clay) is to be taken into account.
RESULTS AND INTERPRETATION
The measured heat conductivities lie between 1.0 and 1.3 W/m.K. The heat conductivities of the air dry
samples shows as expected the lower values (< 1.15 W/m.K). These values show the influence of the water
content on the heat capacity of the bentonite. The anisotropy is very weak. The values parallel to the inaxial compression direction (perpendicular to the layering = axial) are only slightly lower. The value for
the moist sample, registered at 90°C, is approx. 20% lower than determined at ambient temperature. That
corresponds to the general trend of decreasing heat conductivity with increasing temperature. However, an
unknown quantity of humidity of the wrapped bentonite sample can be escaped during the 2 hours of the
heating phase. In addition drying cracks can have formed at the sample surface. Both factors lead likewise
to a reduction of the measured heat conductivity. After drying at 105°C the heat conductivity drops down
to 0.68 W/m.K). The measurements at 90°C and 20°C are in the same order of magnitude. These values
are more affected by the mentioned shrinkage cracking during the thermal treatment in the oven.
The strong influence of these “macropores” on the heat conductivity is obvious in measurements of
granular material. The loose filled material shows a heat conductivity of 0.34 W/m.K, the compacted
material a slightly higher value with 0.58 W/m.K.
The specific heat capacity c of the air-dry and the moist samples are in the same range (between 1.15 and
1.25 J/g.K). The oven dry samples show clearly lower specific heat capacity (between 0.6 and 0.7 J/g.K).
The hydration effect ΔTHyd is for the air-dry and moist samples as expected clearly lower (0.2-0.4 K) as for
the oven dry samples (2.5 K).
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CLAYS IN NATURAL & ENGINEERED BARRIERS
FOR RADIOACTIVE WASTE CONFINEMENT