Publications

For the WIPP, chemical and physical characteristics of MgO suggest it to be the most beneficial backfill choice, particularly because it has the ability to buffer the aqueous chemical conditions to control actinide volubility. In the current experimental program, the authors are developing a technical basis for taking credit for the complete set of attributes of MgO in geochemical, hydrogeological, and geomechanical technical areas, resulting in an improved conceptual model for the WIPP such as the following. Water uptake by MgO will delay the development of mobile actinides and gas generation by microbes and corrosion. Reduced gas generation will reduce or even eliminate spallings releases. As MgO hydrates, it swells, reducing porosity and permeability, which will inhibit gas flow in the repository, in turn reducing spallings releases. Hydration will also result in a self-sealing mechanism by which water uptake and swelling of MgO adjacent to a groundwater seep cuts off further seepage. Reaction with some groundwaters will produce cementitious materials, which will help to cement waste particles or produce a cohesive solid mass. Larger particles are less likely to be entrained in a spallings release. If sufficient water eventually accumulates in a repository to support microbial gas generation, magnesium carbonate cements will form; also producing good cohesion and strength.

During inspections of the Waste Isolation Pilot Plant Waste Shaft in May 1990, patchy areas of apparently degraded concrete were observed on the inner surface of the shaft liner between approximately 810 feet and 900 feet below the surface. The apparent cause of this degradation is chemical reaction of the concrete with magnesium-bearing brine in the annulus between the concrete liner and the host rock. The greater thickness of the degraded layer below the joint may be related to the different chemical compositions that were determined by analyses of the paste portions of concrete samples from above and below the joint. The analytical results support a complex mechanistic explanation of concrete degradation observed behind the liner and in the joint: chemical weakening of the concrete paste; cracking by precipitation of solids in pores; and increased permeability due to calcium chloroaluminate formation. Additional sampling, analyses, and regular monitoring are worth considering to bound the vertical extent of Waste Shaft liner degradation, detect concrete liner degradation in other shafts, and measure any ongoing degradation that may be occurring.

Waste Isolation Pilot Plant (WIPP) sealing program results are embodied in the initial seal system strategy and reference design. The design provides a common basis for calculations and analyses so that results can be compared directly. The sealing strategy combines both long- and short-term seal components. Crushed salt is the principal long-term barrier to fluid flow. Short-term seal components are used until creep consolidation is sufficient. Concretes developed specifically for WIPP seals and a swelling clay material that exhibits low permeability to WIPP groundwater and brine have been chosen for the short-term components. A body of evidence exists showing the stability of these materials for the length of time they are required to function. Reference designs are described and drawings are shown for each of the principal multi-component seals. Confidence in the sealing strategy and the reference designs resulted from a combination of laboratory tests, numerical modeling, and in situ demonstrations. The sealing strategy, materials, and designs for the WIPP repository are consistent with the concepts and designs proposed previously for other national and international waste management programs. Past accomplishments and planned activities in the sealing program will produce a detailed conceptual design for the seal system and a seal system performance model. 48 refs., 11 figs., 6 tabs.

The purpose of this report is to summarize for the record the objectives, planning, progress, and documentation of excavating Brine Inflow Room Q and implementing an initial set of Room Q tests. The Room Q tests were designed primarily to test the scale-up accuracy of the current brine inflow model by providing data on brine flow from the host rock salt to a large-scale excavation in the Waste Isolation Pilot Plant (WIPP). Data from these tests will also be used to reduce uncertainties in flow parameters and evaluate proposed mechanistic models. Room Q was excavated with a tunnel boring machine. The initial tests were implemented to measure brine flow parameters and room closure. Pore pressure, permeability, and brine inflow were measured with 15 tools emplaced in boreholes above, below, and on the north side of the room at a station 75 feet into the host rock from the entrance to the room. These measurements were made before, during, and after Room Q was bored to obtain data on the responses in the host rock to the boring process. Closure measurements were started almost immediately after excavation progressed past each measurement station. The designs, timing, sequence, procedures, and as-built records for these tests were documented in Sandia National Laboratories WIPP Quality Assurance files and Westinghouse WIPP Engineering records. 21 refs., 5 figs., 5 tabs.

Several issues have been identified as pertinent to sealing of radioactive waste repositories in bedded salt formations. These issues include: overall seal system functions and strategy for waste isolation; performance requirements for the seal system; need for redundancy; locations of long-term seals in excavations and boreholes; requirements for sealing interbeds and the disturbed rock zone (DRZ) seal stability for the required lifetime; and need for short-term seals in addition to long-term seals. These issues are defined in general terms, and some principles that may be useful in addressing them are presented. Although this presentation derives from experience with bedded salt, it has applicability to domal salt as well.