Two-dimensional finite element analyses were used to investigate the closure of WIPP disposal rooms filled with backfill and rooms filled with a combination of waste and backfill. Two different backfill materials were considered. The analyses provide estimates of the porosity in the disposal room as a function of time. These results have been used to help evaluate the suitability of the backfill materials for use in the repository.
The Waste Isolation Pilot Plant (WIPP) located near Carlsbad, New Mexico is being developed by the U. S. Department of Energy for the purpose of providing a research and development facility to demonstrate the safe disposal of transuranic waste resulting from defense activities. The facility will consist of an array of rooms mined in a bedded salt formation at a depth of 650 meters below ground level. 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. Backfill will then be placed in the empty space surrounding the drums and in the access-ways to the disposal rooms. Because the salt formation is a creeping medium, each disposal room will close over time compacting the waste and backfill. As a result of this compaction, both the waste and backfill will become much less permeable to the flow of brine than they are initially. The room closure will also slow and eventually stop as the stresses increase in the material inside of the room and thus apply a back pressure to the room boundary. Placing backfill in the rooms and access-ways reduces the initial amount of void volume in the repository and thereby decreases the length of time required to sufficiently compact the waste. The use of backnll also decreases the amount of damage that will accumulate in the salt formation by reducing the amount of closure required to reach the final compacted state. Additives can be included in the backnll to absorb brine, gas, and radionuclides. One backfill material under consideration is crushed salt. Four different disposal room configurations have been analyzed using the two-dimensional finite element code SANCHO (Stone, et al. 1986). The disposal room geometry was the same in each configuration, only the contents of the room were varied. The following conditions were analyzed: (1) a room completely filled with an all-salt backfill, (2) a room completely filled with a salt-bentonite mixture containing 70% by weight crushed salt and 30% by weight bentonite, (3) a room containing waste surrounded by an all-salt backfill, and (4) a room containing waste surrounded by a 70% salt 30% bentonite backfill. The main objective behind this work was to produce estimates of the porosity in the waste and backfill as functions of time. This information can be used as a basis for selecting a suitable backfill. The results can also be used to help determine if some form of preprocessing will be necessary to reduce the initial porosity in the waste containers.
