Solidification in composites (chemical incorporation and encapsulation)

Composites can be thought of as multi-barrier waste forms. Usually a com­posite waste form is required to meet a specific waste form criterion, e. g. heat loading, respirable fines, compressive strength, etc. A single-phase or multiphase crystalline ceramic or even a glass can further be encapsulated in a metal, a glass, or an encapsulant waste form such as cement, geopoly­mers, hydroceramics, bitumen, etc. The encapsulant phase offers a second level of protection to the release of radionuclides or hazardous components in the waste form as shown in Table 6.10. Composites include many GCMs such as glass bonded sodalites that have already been discussed (on page 214). Composites can also include deteriorated cement waste forms that are remediated by encapsulation (see Table 6.10). A few examples are given below and others are shown in Fig. 6.3.

Metal matrix

In metal matrix waste forms a metal is used as the encapsulant for either glass or crystalline materials in which the radionuclides or waste species are already atomically bonded. The advantages of this type of encapsulation include (1) improved thermal conductivity of the waste package, (2) poten­tially decreased leach rates of radionuclides because of the metal matrix encapsulation, (3) improved mechanical strength and decreased dispersa — bility on impact, and (4) improved radiation protection during handling [152, 153]. The encapsulation of waste forms in metal matrices was pursued in the US and developed full scale in PAMELA, which was a joint Belgian — German project located in Belgium.

Vitromelt is a composite waste form in which glass beads (0.5 cm) are embedded in a metal matrix (usually a Pb alloy) [154-156] . For example, waste immobilized in calcium silicate pellets was encapsulated in a lead matrix. In one variation of the commercial PAMELA vitromelt process, phosphate glass beads containing HLW were produced by passing molten glass through nozzles. The beads were subsequently fed into a container and infiltrated with molten lead alloy to produce a composite waste form (‘vit — romet’). The beads, with a diameter of 0.5 cm, occupy up to 66% of the total volume. Increased thermal conductivity of the waste form leading to lower waste temperatures is one of the most important advantages of this product.

I n studies related to vitromelts, immobilized waste pellets have been coated with pyrolytic graphite, before encapsulating in a metal matrix, in order to improve the leach resistance. Application of other coatings has also been reported, including alumina, titania, silica, silicon carbide, chromium silicide, and chromium oxide, together with a variety of metals including Ni, Fe, and Mo. Dual coatings of pyrolytic graphite and alumina have also been reported. Metal matrices have included Pb-based alloys (e. g., Pb-Sb, Al, Sn), Al-based alloys (e. g., Al-Si, Cu, Ti), and Cu. Particles can be coated by conventional ceramics (e. g., Al2O3, TiO2 . or SiO2) or by carbon products (e. g., PyC, Cr7C3 . or SiC), glass (borosilicate or aluminosilicate), or metals (e. g., Ni, Si, or Fe) before being encapsulated). Uncoated, sintered super — calcine pellets have been encapsulated in vacuum-cast Al-12Si and glass-coated, sintered supercalcine pellets encapsulated in vacuum-cast Al-12Si. Supercalcine pellets have also been PyC/Al2O3 coated before encapsulation in gravity-sintered Cu.

Cermets are related composite waste forms in which radionuclides in the form of small oxide or silicate particles +1 mm in size are dispersed in a metal matrix [152]. The unique aspects of the waste form are the very fine scale on which the radionuclide-containing phases are dispersed, the fact that the alloy is primarily composed of hydrogen reducible metals which are already in the waste, the high thermal conductivity, and reduced leach rates due to the alloy encapsulation. Developmental work on cermet was performed using simulated wastes, radionuclide-containing simulated wastes, West Valley acid THOREX wastes, and SRS HLW sludge and un­neutralized SRS wastes. Waste loadings of up to around 30% have been reported. The addition of elements in excess of stoichiometric requirements is used to guarantee the formation of specific ceramic phases, e. g. excess Al and Si to ensure the formation of pollucite.

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