Book Description

Injection of intermediate-level radioactive wastes (specific activity of less than 6 x 103γCi/mL, consisting mainly of radionuclides, such as strontium and cesium, having half-lives of less than 50 years) mixed with cement into a thick shale formation is a promising and feasible disposal method. Hydraulic fracturing provides openings in the shale to accommodate the wastes. Ion exchange and radionuclide-adsorption materials can be added to the grout during mixing to further increase the radionuclide-retaining capacity of the grout. After solidification of the grout, the injected wastes become an integral part of the shale formation, and therefore the wastes will remain at depth and in place as long as the injection zone is not subjected to erosion or dissolution. Problems concerning safety of the disposal method are (1) the potential for inducing vertical fractures, (2) phase separation during and after the injections, (3) the reliability of methods for determining the orientation of induced fractures, (4) the possibility of triggering earthquakes, and (5) radionuclides being leached and transported by ground water. In bedded shale, a difference between tensile strength normal to and that parallel to bedding planes favors the formation of fractures along bedding planes that are nearly horizontal. Even in areas where vertical stress is slightly greater than the horizontal stresses, nearly horizontal bedding-plane fractures can be hydraulically induced in shale at depths less than 1,000 meters. Test injections should be made during site evaluation to determine if horizontal bedding-plane fractures can be induced. The orientation of induced fractures can be indirectly monitored by recording injection pressures during injection time and by measuring the decay of water injections and the uplift of ground surface after the injections; however, it can be directly determined by gamma-ray logs made in observation wells before and after each injection, if the injected fluid or wastes contain enough gamma-ray emitting radionuclides. If waste grout is properly mixed, phase separation should be less than one percent of the total amount injected. The mobility of waste in the separated liquid is further decreased by the low permeability (less than 10−6 darcy) and the large ion-exchange and adsorption capacity of shale, which thus reduce the potential for contamination. Grout injections do not cause extensive increases in pore pressure within shale, and a disposal site should be located in a geologically stable and tectonically relaxed area, that is, an area lacking local active faults. Thus a disposal in shale in such areas can avoid the two necessary and essential conditions for triggering earthquakes by fluid injections, an increase in pore pressure and rock already stressed near its breaking strength. Waste injections are made in several stages at different levels through an injection well. After the first series of injections at the greatest depth, the well is plugged by cement at that depth. The second series of injections are made a suitable distance above the first. The repeated use of the injection well distributes the cost of constructing injection and monitoring wells over many injections, thereby making hydraulic fracturing and grout injection economically attractive as a method for the disposal of radioactive wastes. Theoretical considerations about inducing nearly horizontal beddingplane fractures in shale are discussed, as are field procedures for site selection, safety, and the monitoring and operation of radioactive waste disposal. Case histories are used as examples to demonstrate the application of the theory and techniques of field operations.