Bentonite backfill

Backfill containing a large proportion of bentonite has a tendency to change its volume with variations in water content of the surrounding soil. This can lead to formation of hollow cavities in the backfill with a considerable decrease in the current delivery. A standard backfill consists of a mixture of 75% gypsum, 20% bentonite and 5% sodium sulfate. The specific resistivity of this backfill is initially 0.5 to 0.6 m and can rise with increased leaching to 1.5 m. 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

Backfill the soil replaced over the pipe in the trench (general connotation). In cathodic protection, special backfills are packed around the anodes. These backfills are selected to lower circuit resistance of the anode for sacrificial anodes a gypsum/bentonite mixture is used, and for impressed-current anodes, coke breeze. 

Soil-bentonite backfills degrade in the presence of strong acids, bases, salt solutions, and some organic chemicals. 

The generation of colloids from bentonite discussed as a barrier and backfill material (Fig. 1,... 

Curti, E. Wersin, P. 2002. Assessment of Porewater Chemistry in the Bentonite Backfill for the Swiss SF/HLW Repository. Nagra Technical Report NTB 02-09, Nagra, Wettingen, Switzerland. 

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... 

Since granite and bentonite/quartz were decided to be the host rock and backfill material, respectively, at an early stage of the project, these two materials were studied for all of the 14 radionuclides selected and for both of the two artificial groundwaters given in Table II. The radionuclides used in the laboratory experiments are given in Table III. 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

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). 

In BMTl a typical deep geological waste repository set-up is studied. Figure 6 shows a detail of the set-up. The waste canister is surrounded by a bentonite buffer material, and the backfill for the tunnel is made of a bentonite-sand mixture. The host rock is granite. Material... 

The bentonite properties have been calibrated from various tests results during DECOVALEX II (Rutqvist et al., 2001) and have been used, together with the rock mass properties, for verification in Task A of the BMTl exercise (Chijimatsu et al., 2003).The backfill material consists of a mixture of bentonite (15%), sand and gravel. 

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. 

The experimental setup consists of a concrete container with gas vent surrounded by an engineered barrier system made of a 20/80 % bentonite/sand mixture, placed in layers, a granular backfill of the upper cavern and a concrete plug. In total, 12 different materials are considered in the numerical model (cf. Fig. 1). As the surrounding rock matrix is very impervious, only the relatively high-permeable shear zone is considered for flow outside of the EBS. Most important material parameters are given below. These parameters are obtained from independent laboratory test for the sand/bentonite (materials 6 and 8), from the literature for the other materials there is no back-estimation from the calculation results or calibration involved. 

Backfilling of deposition tunnels with bentonite-based materials... 

After the installation of each heater, the bentonite blocks, the instrumentation and backfilling of the surrounding area, the corresponding system was switched on. That is, heaters were turned on according to the installation time schedule. First heater started last September 2001 (Goudarzi Bbrgesson, 2003). Since that dale, temperature, relative humidity, total pressure and water pressure from the instrumentation have been collected. 

Unit Steel Rock Bentonite Backfill Pellets Slot... 

The height of the domain is 60 m so that the coupled behaviors in the buffer are not influenced by the boundaries. The surrounding rock is assumed linear, elastic, isotropic and homogeneous and the buffer is assumed the mixture of sand and bentonite. Material properties of the rock and the buffer are shown in Table 1. The same properties as buffer are given to the backfill. The waste canister is assumed rigid and impermeable. 

Annex A of BS7430 gives formulae for various shapes of buried conductors. See also Appendix H of Reference 1. Reference 2 shows the mathematical derivations of some basic cases. Reference 3 provides much useful information regarding buried materials. If the rod or pipe is surrounded by a casing or backfill of more conductive material such as Bentonite, then a lower resistance is obtained for the same depth, the formula is -... 

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