Thermal Conductivity of Rocksalt and Other Geologic Materials from the Site of the Proposed Waste Isolation Pilot Plant
The measurements first reported by Acton on the thermal conductivity of samples taken from a borehole at the site of the proposed nuclear waste isolation pilot plant (WIPP) near Carlsbad, NM, have been extended to include additional samples and higher temperature measurements. Samples for our measurements were taken from several depths of three wells, including the well AEC 8 from which Acton obtained his samples. These samples ranged from relatively pure rocksalt (NaCl) with small amounts of interstitial anhydrite to essentially nonsalt samples composed of gypsum or clay. The measurements in this latest series were conducted at Sandia using the longitudinal heat flow apparatus described by Acton, at the Los Alamos Scientific Laboratory (LASL) using a transient line source technique and at Dynatech Corp., Cambridge, MA, using a linear heat flow comparative technique.
In general, the data for similar coarse grained translucent rock salt samples from the three laboratories agreed reasonably well with the LASL and Sandia results, typically being about 20% higher than those of Dynatech. The measured rocksalt conductivities in the temperature range 300–700 K are described relatively accurately by an expression of the form λ = λo (300/T)γ, where λo= conductivity at 300 K, γ = 1.14, and T is the temperature in K. The Sandia and LASL data are best described by λo = 6 W/m-K, while for the Dynatech data, λo = 5 W/m-K yields the best fit. Slack has proposed that a two-parameter expression of the form given above for λ is generally applicable to a wide variety of nonmetallic solids, with the deviation of γ from one resulting from both thermal expansion effects and optic-acoustic phonon interactions, which are not included in the standard analysis of thermal conductivity caused by phonon transport. The data in the range T > 500 K frequently deviated by ~±15% from the predicted behavior. This is not believed to be the result of the onset of radiative thermal transport because the deviations are negative as well as positive. Infrared transmission measurements on rock-salt samples from the proposed WIPP site show no transmission in the 3–10 µm wavelength range for samples > 5 cm thick. Use of the estimated infrared absorption coefficient in the Rosseland radiative conductivity relation also leads to the conclusion that there is little radiative conduction for T < 800 K.
For nonsalt samples from the proposed WIPP site, values of λo fall in the range 0.5–3 W/m-K, and frequently γ values are in the range γ < 1. All samples were dense with little or no porosity evident. On the basis of these experiments, we have concluded that the thermal conductivity of materials found at the site can be predicted to an accuracy ~±30% from knowledge of the composition and grain size of these materials.
KeywordsWaste Isolation Pilot Plant Thermal Expansion Effect Waste Canister Idaho National Engineer Laboratory Savannah River Plant
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