Backfill Materials

Polymer cable anodes are made of a conducting, stabilized and modified plastic in which graphite is incorporated as the conducting material. A copper cable core serves as the means of current lead. The anode formed by the cable is flexible, mechanically resistant and chemically stable. The cable anodes have an external diameter of 12.7 mm. The cross-section of the internal copper cable is 11.4 mm and its resistance per unit length R is consequently 2 mQ m l The maximum current delivery per meter of cable is about 20 mA for a service life of 10 years. This corresponds to a current density of about 0.7 A m. Using petroleum coke as a backfill material allows a higher current density of up to a factor of four. 

In terms of dead loads, the shape of the trench in which the pipe will be buried is also a factor. Generally speaking, a narrow trench with vertical sidewalls will impose less of a load on the pipe than will a wider trench with sloping side walls. It is necessary also to know the modulus of soil reaction (E), which is dependent on the type or classification of the native soil, the backfill material that is contemplated, and the desired consolidation of the backfill material. Soil consolidation is important, because it contributes to the strength of a flexible conduit in a buried pipe system. 

Construction procedures for flowable fill materials are no different than those for conventional earth backfill materials. The same methods and equipment used to mix, transport, and place flowable fill made with conventional aggregates may be used for flowable fill incorporating spent foundry... 

NS A, Blast Furnace Slag, Ideal Backfill Material for Steel Sheet Piling, 166.2, National Slag Association, Alexandria, VA, 1966. 

Visual inspection may be carried out by entering the tank if it is large enough for a person to be able to enter and walk in the tank, or by inspection of the tank s outer walls following the removal of pads or backfill material. 

The establishment of the groundwater composition by the rock and the backfill materials of the repository. 

The highly porous backfill material encourages vertical transport of the air within the trench, reducing the potential for lateral dispersion of air and contaminants into the adjacent formation. 

Our results show that coprecipitation of the REE with phosphate removed Ce, Pr, Nd, Sm, and Eu more easily from the brine than other REE. This finding might be of importance for the mobility of trivalent Am and Cm in a radioactive waste salt repository, because for these elements, owing to their almost identical ionic radii, an almost analogous geochemical behaviour is expected as for Sm and Nd (Choppin 1983 Krauskopf 1986). These radionuclides would, in the case of a leaking HLW salt repository, probably be retained when phosphate minerals are present in the backfill material. 

Poinssot, Ch., Toulhoat, P. Goffe, B. 1998. Chemical interaction between a simulated nuclear waste glass and different backfill materials under a thermal gradient. Applied Geochemistry, 13, 715-734. 

Phosphates have also been used as either impermeable or permeable reactive barriers for containment or remediation of contaminated groundwater. Fuller et al. (2002) looked at the mechanisms of U(VI) immobilization by apatites with EXAFS. Matheson et al. (2001) examined mechanisms of U(VI) removal in apatite-based permeable reactive barriers. Gauglitz et al. (1992) conducted laboratory studies on the efficacy of hydroxyapatite to remove U and Th from brine solutions. The minerals saleeite (Mg(U02)2(P04)2-9H20) and metaatunite (Ca(U02)2(P04)2-6H20) were observed. The concept of using apatite as a backfill material around repositories was explored. Apatite barriers made of Apatite II have been successfully... 

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

For both alternatives a storage in granitic bedrock at a depth of 500 m is considered. This is well below the groundwater table. The waste canisters will be placed in vertical holes in horizontal tunnels and both holes and tunnels will be filled with a backfill material (c.f. Figure 1). 

Backfill Material. The canisters are surrounded by a backfill material. This barrier will serve several different purposes, such as to... 

Thus, a proper backfill material may act as a very effective chemical and mechanical barrier, preventing free migration of radionuclides released from the waste containers. 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

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... 

With the suggested encapsulation (lead or copper) the formation of radiolysis products or changes of the properties of the backfill material due to irradiation can be neglected. 

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. 

The effects of chemical interactions on the migration of radionuclides in the backfill material and in the ground can be described by the retention factor Kj, defined as ( )... 

Retention in the Backfill Material. The radionuclide holdup time in the backfill barrier for the KBS-concepts is in the order of thousands of years or less (15). Thus, only a few of the long-lived radionuclides in HLW and SUF ( °Sr, Cs and lAm) will be able to decay within the clay barrier and the more long-lived nuclides will just be delayed in their migration out into the bedrock. In the long-term time span the backfill material is of minor importance, as far as the retaining effect is concerned. 

These results show that technetium can be contained by magnetite in the geosphere, provided reducing conditions can be maintained. This can be aided, for example, by the incorporation of iron or iron oxides in the buffer and backfill materials in the waste disposal vault. 

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