ABSTRACT:

The Yucca Mountain Site Characterization Project has completed two pilot-scale tests of the potential repository drip shield configuration over waste packages. One test included backfill over the drip shield and one did not. Both tests were constructed at ¬ scale of the potential repository emplacement drift dimensions, and both employed heaters to control temperatures of the simulated drift wall and simulated waste package. Test results confirmed that the drip shield would function as intended by design. No appreciable water penetrated the drip shield structures; no condensation was detected under the drip shields. Temperature differences from natural convection were measured on the simulated waste package and drip shield. Water vapor pressure throughout the test cells varied by about 10%. The calculated dew point temperatures were always below temperatures on the simulated waste package and drip shield.

INTRODUCTION

Design Basis Configuration

The US Department of Energy (DOE) has been studying a site at Yucca Mountain, Nevada to determine the site's suitability for a geologic repository for the nation's commercial and defense spent nuclear fuel and high-level radioactive waste (DOE 1998a). The DOE must demonstrate that the proposed disposal site will safely contain and isolate radioactive contaminents from the accessible environment for up to 10,000 years or longer (DOE 1998b). This demonstration is founded in large part on numerical models of the natural and man-made systems that extrapolate repository performance over the time period of regulatory interest (DOE 1998c). The simulations are built upon known physical processes and demonstrated to be valid by field, laboratory, and pilot-scale test data.

The Engineered Barrier System pilot-scale tests provide such data. They are conducted at ¬ scale and represent a section of emplacement drift 5.6 m in diameter and 16 m long. Figure 1 illustrates the Yucca Mountain Project's potential repository layout. In addition to providing data to support model development and validation, the tests can be used to demonstrate construction feasibility. The tests, because of their size and constructability, can be assembled and conducted quickly, relatively inexpensively, and iteratively so that previous test results can guide design of subsequent tests.

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