Gas barrier design for the WIPP

Abstract

The Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico, is planned as the first mined repository for transuranic (TRU) wastes generated by defense programs of the United States Department of Energy (DOE). In coming years, tests with radioactive wastes are planned to be conducted at the WIPP. Potential tests include evaluation of gases generated by wastes emplaced in mined alcoves. Barriers must be installed in the entries to the test alcoves to limit gas release during testing. This paper discusses several rock mechanics issues involved in the design of an Alcove Gas Barrier (AGB) for use in these potential tests. The unique requirements placed on a gas barrier, when coupled with the geologic setting and strict regulations, make the AGB design challenging from several perspectives of rock mechanics. The AGB structure will be placed in the WIPP underground, which comprises a layered evaporite sequence of rock. A schematic of the design as it might appear in the WIPP underground is shown in Figure 1. The underlying requirement is that the AGB reduce gas leakage from a test alcove to an acceptable limit. The most likely route for gas leak-age is through a disturbed rock zone (DRZ), which develops in response to the excavation. Among other effects, the loading on the structure is a function of the geometrical arrangement and time, including considerations of installation and service life. Resolution of design issues also requires a defensible measure of conservatism. These considerations give rise to several issues in rock mechanics which are the emphases of this paper