Bentonite clay backfill

The disposal concept is based on cooling of spent fuel bundles for 30-40 years whereafter they are encapsulated into iron-copper canisters. The canisters would be deposited into a network of tunnels, in crystalline bedrock at the depth of 400-700 meters, and isolated from the rock by a layer of bentonite clay. After operational period, all imdergroimd spaces would be backfilled and sealed and the above groimd buildings demolished. The disposal concept is illustrated in Fig. 1. 

Figure 13.33 Schematic diagram of the engineered barrier system (BBS) showing the high-level nuclear waste in its metal container, surrounded by a buffer or backfill (usually of compacted bentonite clay), in contact with the host rock. The BBS and rock affected thermally by the waste are sometimes termed the near field, with more distant surrounding rock termed the far field. After The status of near field modeling. Proc. Technical Workshop, copyright 1995 by OECD. Used by permission.

Many national programs plan to surround containers of their nuclear waste in a geologic repository, with a backfill of compacted bentonite clay (Fig. 13.33). A chief function of the clay backfill is to adsorb radionuclides and so retard their release from the engineered barrier system. Conca (1992) measured the apparent diffusion coefficient (D ) and apparent distribution coefficient (K [ml/g]) of some radionuclides in bentonite clay as a function of clay moisture content and compaction density. Measurements were made for clay densities from 0.2 to 2.0 g/cm, which correspond to porosities of 93 to 25%, respectively. With decreasing porosity, values declined by roughly 10 to 10 -fold. However, for the same porosity reduction, values were usually lowered by 10-fold and more, indicating less adsorption with compaction (Fig. 13.38). 

Slurry walls, also known as slurry cutoff walls or vertical cutoff walls (see Fig. 7c), are vertical walls constructed by excavating a trench with a back-hoe or clamshell and simultaneously filling the trench with a stabilizing slurry which is typically prepared from a mixture of bentonite clay and water, or bentonite elay, eement, and water. The slurry forms a thin filter cake (typieally <3 mm for bentonite slurry and perhaps a few tens of millimeters for cement-bentonite slurry) with a low hydraulic conductivity (typically < 10 ° m/sec) onthe sides ofthe trench. The filter cake minimizes slurry loss from the trench, stabilizes native soil on the sidewalls of the treneh, and provides a plane for slurry stabihzation of the exeavated trench. Both bentonite and cement-bentonite slurries typieally contain 4-7% (w/w) sodium bentonite mixed with water. The hy-drauhc conductivity of slurry walls is affected significantly by the qualify of the baekfill material and typieally ranges from 10 to 10" m/sec, with lower values associated with backfill materials that contain sufficient quantities of clay. 

A typical backfill material for magnesium anodes consists of 75 percent hydrated gypsum, 20 percent bentonite clay, and 5 percent sodium sulfate. A mixture of 50 percent molding plaster (plaster of Paris) and 50 percent bentonite clay works well with zinc anodes. These backfill mixtures are available from suppliers or are provided in prepackaged containers as shown in Fig. 13.16. Figure 13.17 illustrates how these anodes are installed in their prepackaged backfill containers. 

In the Swedish KBS-project a mixture of bentonite and quartz (10 90) or compacted bentonite were proposed at an early stage as possible backfill materials. Bentonite is a rather common mont-morillonitic clay with good mechanical properties and chemical... 

The conceptual design of the repository (Figure 2) consists of a series of parallel tunnels, where the wastes would be emplaced in boreholes excavated in the floors of the gallery. The centreline distance between adjacent tunnels is 10 m and the centreline distance between adjacent inground boreholes for the wastes is 4.44 m. The depth of each borehole is 4.13 m and the diameter is 2.22 m. The overpack for vitrified wastes would be emplaced into the borehole, and a bentonite buffer material would be compacted around the overpack. The tunnels would also be backfilled with a mixture of gravel and clay. 

Sacrificial anodes used for the protection of buried metal structures are surrounded by so-called backfills, which are usually mixtures of bentonite (fine clay absorbing water) with inorganic salts (gypsum, sodium sulfate, magnesium sulfate, sodium chloride, etc.). The role of backfllls is based on ... 

Galvanic anodes are surrounded by a backfill which is usually a mixture of gypsum, bentonite, and clay. Table 5.11 shows a typical composition of a backfill. 

Backfill The special soil placed around the anodes to provide uniform resistivity. The material used as backfill in an impressed current system is generally coke breeze whereas in the galvanic system the backfill is composed of a mixture of gypsum, bentonite and clay, the composition being dependent on soil resistivity. Bond An electrical connection between two metallic structures. 

---