Engineered barriers

Site characterization studies include a surface-based testing program, potential environmental impact, and societal aspects of the repository. Performance assessment considers both the engineered barriers and the geologic environment. Among features being studied are the normal water flow, some release of carbon-14, and abnormal events such as volcanic activity and human intmsion. The expected date for operation of the repository is 2013. 

I mentioned that new computer codes - RAFSCATT 1 and RAF-SCATT 2 have been very recently formulated by Dr. Gary Jacobs and Mr. William Anderson of Rockwell Hanford s Basalt Waste Isolation Project. These codes relate required engineered barrier (i.e., waste packages and seals performance to draft NRC and EPA criteria). A key part of these codes is that they partition... 

The near field of the repository includes the engineered barrier system (EBS, i.e., canister and buffer) and the waste form. Also included in the near field is the interface between the buffer and the host rock, denoted as excavation disturbed zone (EDZ). In terms of waste/water interactions, the geochemical evolution of the near field is essential as it controls the composition of the fluids that will eventually contact the waste. 

Fig. 1. Potential colloid generation processes in a nuclear waste repository near field (for explanation, see text) (HLW, high-level waste EBS, engineered barrier system).

The proposed Swiss repository for SF, HLW, and ILW is situated in the Opalinus Clay of the Zurcher Weinland in northern Switzerland, where an exploratory borehole was drilled near the village of Benken (Nagra 2002a). The Opalinus Clay formation consists of a well-consolidated clay shale, which is suitable for the construction of small, unlined tunnels and larger, lined tunnels at depths of several hundred metres. The engineered barrier system includes the waste containers and the backfill of construction, operation, and emplacement... 

FAA FA FBC FC FEBEX FFFF FGD FP FSU FT FTIR FUETAP Flame atomic absorption Fly ash Fluidized bed combustion Filter cake Full-scale engineered barriers experiment (in crystalline host rock) Flow-field flow fractionation Flue gas desulphurization Fission products Former Soviet Union Fourier transforms Fourier transformed infrared spectroscopy Formed under elevated temperature and pressure... 

In extrapolating laboratory results to installed slurry wall of thickness L, the commonly applied idealized BCs may not provide an accurate representation of the transitions between an engineered barrier and native aquifer material in the field. To provide conservative predictions for design, Rabideau and Khandelwal (1998b) recommend the combination of a constant concentration entrance (x = 0) BC with a zero-concentration exit (x = L) BC. In particular, several commonly used BCs should be avoided because they distort the nature of the diffusive flux at the boundaries (e.g., the Danckwerts constant-flux entrance BC and the zero-gradient exit BC). These issues are discussed in greater detail by Rabideau and Khandelwal (1998b). 

Disposal facilities for nonhazardous waste (e.g., municipal/indus-trial landfills) normally are constructed without substantial engineered barriers, such as a rock cover or cement waste forms, that would deter inadvertent intrusion into waste, and the waste itself often is in a readily accessible physical form. Therefore, in determining exempt waste, scenarios for inadvertent intrusion involving permanent occupancy of disposal sites and normal human activities that could access waste would be appropriate. Examples include excavation in the construction of homes and permanent residence on... 

The assumed disposal systems for exempt waste and low-hazard waste both involve near-surface disposal, and either type of waste often would be emplaced sufficiently close to the surface that inadvertent intrusion into the waste could occur as a result of normal human activities. However, there are differences in the two types of disposal systems that should be taken into account in developing appropriate scenarios for inadvertent intrusion. Disposal facilities for low-hazard waste frequently include engineered barriers to deter inadvertent intrusion, impenetrable waste forms, or deliberate emplacement of more hazardous wastes at locations where access to the waste during normal human activities would be less likely. Most importantly, as noted previously, current plans call for institutional control to be maintained over hazardous waste disposal sites for a considerable period of time after facility closure, which allows for substantial... 

ENGINEERED BARRIER BREACH (DEGRADATION AND FAILURE OF ENGINEERED BARRIERS)... 

Intermediate-level waste is waste with high concentrations of intermediate-level, short-lived waste or intermediate-level, long-lived waste (IAEA, 1981). Such waste would be suitable for disposal in a near-surface facility incorporating engineered barriers and would include higher-activity Class-B and Class-C waste, as defined in NRC s 10 CFR Part 61 (NRC, 1982a). 

This proposal differs from the others discussed previously in that the first three waste classes all would include waste that is generally acceptable for disposal in a near-surface facility. However, these three classes differ in the extent to which engineered barriers would be relied upon to inhibit migration of radionuclides and exposures of inadvertent intruders. 

Fernandez, A. M., B. Baeyens, M. Bradbury, and P. Rivas. 2004. Analysis of the porewater chemical composition of a Spanish compacted bentonite used in an engineered barrier. Phys. Chem. Earth 29 105-118. 

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