Finite element calculations have been performed to determine the structural response of waste-filled disposal rooms at the WIPP for a period of 10,000 years after emplacement of the waste. The calculations were performed to generate the porosity surface data for the final set of compliance calculations. The most recent reference data for the stratigraphy, waste characterization, gas generation potential, and nonlinear material response have been brought together for this final set of calculations.

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Final disposal room structural response calculations

A long-term assessment of the Waste Isolation Pilot Plant (WIPP) repository performance must consider the impact of gas generation resulting from the corrosion and microbial degradation of the emplaced waste. A multiphase fluid flow code, TOUGH2/EOS8, was adapted to model the processes of gas generation, disposal room creep closure, and multiphase (brine and gas) fluid flow, as well as the coupling between the three processes. System response to gas generation was simulated with a single, isolated disposal room surrounded by homogeneous halite containing two anhydrite interbeds, one above and one below the room. The interbeds were assumed to have flow connections to the room through high-permeability, excavation-induced fractures. System behavior was evaluated by tracking four performance measures: (1) peak room pressure; (2) maximum brine volume in the room; (3) total mass of gas expelled from the room; and (4) the maximum gas migration distance in an interbed. Baseline simulations used current best estimates of system parameters, selected through an evaluation of available data, to predict system response to gas generation under best-estimate conditions. Sensitivity simulations quantified the effects of parameter uncertainty by evaluating the change in the performance measures in response to parameter variations. In the sensitivity simulations, a single parameter value was varied to its minimum and maximum values, representative of the extreme expected values, with all other parameters held at best-estimate values. Sensitivity simulations identified the following parameters as important to gas expulsion and migration away from a disposal room: interbed porosity; interbed permeability; gas-generation potential; halite permeability; and interbed threshold pressure. Simulations also showed that the inclusion of interbed fracturing and a disturbed rock zone had a significant impact on system performance.

/pdf/coupled-multiphase-flow-and-closure-analysis-of-repository-1aez59zjtw.pdf

Coupled multiphase flow and closure analysis of repository response to waste-generated gas at the Waste Isolation Pilot Plant (WIPP)

This paper concludes that a 70/30 wt % salt/bentonite mixture is preferable to pure crushed salt as backfill for disposal rooms in the Waste Isolation Pilot Plant. The Waste Isolation Pilot Plant, near Carlsbad, NM, is designed to be the first mined geologic repository for the safe disposal of transuranic (TRU) radioactive waste generated by DOE defense programs since 1970. The repository is located about 655 m below the land surface in an extensive bedded salt formation. This report examines the performance of two backfill materials with regard to various selection criteria, such as the need for low permeability after closure, chemical stability, strength, ease of emplacement, and sorption potential for brine and radionuclides. Both salt and salt/bentonite are expected to consolidate to a state of permeability {le} 10{sup {minus}18} m{sup 2} that is adequate for satisfying regulations for nuclear repositories. The results of finite-element calculations that were used to arrive at this conclusion will be described. The real advantage of the salt/bentonite. backfill depends, therefore, on bentonite`s potential for sorbing brine and radionuclides. Estimates of the impact of these properties on backfill performance are presented.

/pdf/the-advantages-of-a-salt-bentonite-backfill-for-waste-4zaor1hl0g.pdf

The advantages of a salt/bentonite backfill for Waste Isolation Pilot Plant disposal rooms

An error was discovered in the ALGEBBRA script used to calculate the disturbed rock zone around the disposal room and the shear failure zone in the anhydrite layers in the original version. To correct the error, a memorandum of correction was submitted according to the Waste Isolation Pilot Plant (WIPP) Quality Assurance program. The recommended course of action was to correct the error, to repeat the post-process, and to rewrite Section 7.4, 7.5, 8, and Appendix B in the original report. The sections and appendix revised by the post-process using the corrected ALGEBRA scripts are provided in this revision. The original report summarizes a series of structural calculations that examine effects of raising the WIPP repository horizon from the original design level upward 2.43 meters. Calculations were then repeated for grid changes appropriate for the new horizon raised to Clay Seam G. Results are presented in three main areas: (1) Disposal room porosity, (2) Disturbed rock zone characteristics, and (3) Anhydrite marker bed failure. No change to the porosity surface for the compliance re-certification application is necessary to account for raising the repository horizon, because the new porosity surface is essentially identical. The disturbed rock zone evolution and devolution aremore » charted in terms of a stress invariant criterion over the regulatory period. This model shows that the propagation of the DRZ into the surrounding rock salt does not penetrate through MB 139 in the case of both the original horizon and the raised room. Damaged salt would be expected to heal in nominally 150 years. The shear failure does not occur in either the upper or lower anhydrite layers at the moment of excavation, but appears above and below the middle of the pillar one day after the excavation. The damaged anhydrite is not expected to heal as the salt in the DRZ is expected to.« less

/pdf/structural-evaluation-of-wipp-disposal-room-raised-to-clay-4kayo4x0ne.pdf

Structural Evaluation of WIPP Disposal Room Raised to Clay Seam G

This paper describes development of the conceptual and mathematical models for the part of the Waste Isolation Pilot Plant (WIPP) repository performance assessment that is concerned with what happens to the waste over long times after the repository is decommissioned. These models, collectively referred to as the {open_quotes}Disposal Room Model,{close_quotes} describe the repository closure process during which deformation of the surrounding salt consolidates the waste. First, the relationship of repository closure to demonstration of compliance with the Environmental Protection Agency (EPA) standard (40 CFR 191 Appendix C) and how sensitive performance results are to it are examined. Next, a detailed description is provided of the elements of the disposal region, and properties selected for the salt, waste, and other potential disposal features such as backfill. Included in the discussion is an explanation of how the various models were developed over time. Other aspects of closure analysis, such as the waste flow model and method of analysis, are also described. Finally, the closure predictions used in the final performance assessment analysis for the WIPP Compliance Certification Application are summarized.

Waste isolation pilot plant disposal room model

/pdf/a-summary-of-the-models-used-for-the-mechanical-response-of-2e1ekhbcli.pdf

A summary of the models used for the mechanical response of disposal rooms in the Waste Isolation Pilot Plant with regard to compliance with 40 CFR 191, Subpart B

F.T. Mendenhall

Papers

A summary of the models used for the mechanical response of disposal rooms in the Waste Isolation Pilot Plant with regard to compliance with 40 CFR 191, Subpart B