ABSTRACT

INTRODUCTION

The objective of the Waste Isolation Pilot Plant (WIPP) Program is to develop the technology necessary to assure the safety of a proposed repository for radioactive waste in the bedded salt deposits of southeastern New Mexico. As one of the major components of the technology, the time-dependent deformation (creep) of the salt must be well understood because consolidation of designed seals and the closure of the repository rooms, with subsequent encapsulation of the waste, are important factors in the performance of the repository. It was initially decided that a confirmative model for creep could be developed on the basis of laboratory data and a theoretical understanding of creep mechanisms, independent of in situ measurements. The constitutive model developed in this manner would then be validated, within appropriate uncertainty, against data obtained from large-scale in situ tests fielded at the WIPP facility. The validated model would provide a predictive technology to determine the structural response, or performance assessment, of the repository as extrapolated to those times in the future required by regulatory requirements.

In the early stages of the development, a simple steady state creep model, neglecting transient creep, was used to predict the closure of the in sire test rooms. When the predicted closures were compared to the first available measured closures from the South Drift, see Figure 1, the simple model was found to under predict measured closures and closure rates by a factor of three [ 1]. The steady state model, as found through trial and error, could be made to agree with measured closures by decreasing the elastic modulus by a factor of 12.5 [1]. Success of the reduced modulus approach was viewed by some as evidence of a "scale effect". This possibility remained, even though a subsequent multi-mechanism deformation (M-D) model with transient creep [2] adequately simulated the WIPP in situ tests without reducing the modulus.

Laboratory studies of the scale effect in bedded salt show little, if any, influence of specimen

size over a 1000 fold range of specimen volume [3].

Significantly, it seemed possible to address the scale effect directly through scaled in sire tests at the WIPP facility. In fact, a large scale cylindrical room had already been fielded for other purposes, so only a smaller scale cylindrical test would be required.

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