A structural finite element analysis is presented that examines the effect of internal gas generation on the response of a perfectly sealed disposal room mined from a bedded salt formation. The gas pressure build-up inside of the room, the stress field in the salt formation, and the consolidation of the room contents are examined. Numerical results indicate that, under perfectly sealed conditions, the gas pressure will exceed the lithostatic stress. Even at the highest internal pressure, all principal stresses in the salt formation remain compressive.
The Waste Isolation Pilot, Plant (WIPP) in Carlsbad, New Mexico will serve as a research and development facility to demonstrate the safe disposal of transuranic waste resulting from defense activities. The waste storage area will consist of an array of rooms mined in a bedded salt formation and will be located approximately 650 meters underground. Under the present design, the majority of the contact-handled transuranic (CH-TRU) waste will be stored in a loosely-packed state inside of 55-gallon drums, and crushed salt will be used as backfill around the drums. Compaction of the waste and backfill will occur as salt in the surrounding formation flows inward. Gases will be generated as a by-product of the corrosion and decomposition of CH- TRU waste and waste containers that will be stored in the WIPP repository. Microbial degradation of the waste will produce carbon dioxide as well as potentially significant quantities of nitrogen, hydrogen sulfi.de, and methane. Anoxic corrosion of iron and iron-based alloys in the waste will produce hydrogen gas. A recent study (Lappin, et al. 1989) suggests that virtually all of the gas will be generated during the first 700 years following emplacement of the waste; however, both the total amount of gas formed and the duration of gas generation remain uncertain. The possibility exists that the salt formation will be impermeable to the gases that are produced. If this is the case, gas generation will lead to a slow build-up of pressure inside the disposal rooms. Disposal room pressurization is of concern for two reasons. First, pressurization would retard closure of the room and limit the final degree of compaction of the waste and backfill. Second, the pressurization could create fractures in the host salt formation, extend preexisting fractures, or cause separation along clay seams or fractured anhydrite layers that exist near the repository horizon. A large fracture zone might provide a path for radionuclide migration. The room was assumed to be filled and perfectly sealed immediately after excavation. The ideal gas law was used to calculate the gas pressure in the disposal room based on the void volume in the disposal room and the assumed amount of gas generated by the waste. For the anticipated mixture of gases in the disposal room, the ideal gas law predicts pressures that are within 5% of the pressures predicted with the Redlich-Kwong equation of state (Lappin, et al. 1989) for the amounts of gas and storage volumes that evolved during the course of the analysis.
