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Allen R. Lappin Geological Projects Division 4537Sandia Laboratories Albuquerque, NM 87185 ABSTRACT Performance of both near-and far-field thermomechanical calculations to assess the feasibility of waste disposal in silicic tuffs requires a formalism for predicting thermal conductivity of a broad range of tuffs. This report summarizes the available thermal conductivity data for silicate phases that occur in tuffs and describes several grain-density and conductivity trends which may be expected to resul e from post-emplaceweat alteration. A bounding curve is drawn that predicts the minimum theoretical matrix (zero-porosity) conductivity for most tuffs as a function of grain density. Comparison of experimental results with this curve shows that experimental conductivities are consistently lower at any given grain density. Use of the lowered bounding curve and an effective gas conductivity of 0.12 W/m°C allows conservative prediction of conductivity for a broad range of tuff types. For the samples measured here, use of the predictive curve allows estimation of conductivity to within 15% or better, with one exception. Application and ~ossible improvement of the formalism are also discussed. Introduction and Objectives

One-dimensional calculations of neai-field temperatures resulting from waste emplacement in a multiple-layered tuff stratigraphy are presented.Results indicate a marked sensitivity of peak temper atures tu assignment of in-situ fluid pressure, geothermal-heat flux, waste type, and location of waste relative to a specific stratigraphic discontinuity.Under the criterion that allowable initialpower densities are limited by the occurrence of boiling at a distance of 10 m from '.unplaced wsste, allowable power densities are calculated to range up to 150 kW/acre or more, depending upon geotherrnal heat flux and waste type. CONTENTS

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