Solidification by encapsulation

This section primarily discusses non-thermal methods of encapsulation. The thermal encapsulation by glass is covered in Section 6.4.1 on glass ceramic materials on pages 214-217.

Cements including grouts

Stabilization and solidification with cement-based binders has been used to immobilize radioactive wastes since the beginning of the nuclear age. The process has been used to encapsulate solid waste, solidify liquid waste (including tritiated water), stabilize contaminated soils, stabilize tank-heel residues after tanks are emptied, and as low permeability barriers. Cements have also been used as binders and to encapsulate granular or cracked waste forms.

Cements microencapsulate wastes, although there is recent evidence that during hydration three binding mechanisms can also occur between the cement and metal ions in the waste [99-101]:

• precipitation of metal ions into the alkaline matrix as an oxide, mixed oxide, or as another discrete solid phase;

• adsorption or (co-)precipitation of metal ions onto the surface of cement minerals;

• incorporation of metal ions into hydrated cement minerals as they crystallize.

These mechanisms are shown as examples in Table 6.9; with the binding mechanisms (reaction of the waste with the cement or grout particles) shown as encapsulation and without the binding mechanism shown as embedding.

These processes are not mutually exclusive (so both encapsulation and embedding take place) and the above classification partly reflects slow kinetics; previously adsorbed species may be incorporated as mature cement pastes. Nevertheless, it does allow some generalized guidelines to be for­mulated. The solubility of discrete heavy metal solid phases is a limiting factor with regard to the second and third mechanisms [102], so that only ions that do not precipitate as basic oxides tend to be incorporated in, or surface adsorbed to, hydrated cement minerals to a significant degree.

The principal minerals available in the hydrated Portland cement matrix are calcium silicate hydrate (C-S-H, 50 wt%), portlandite (Ca(OH)2, 20 wt%), and Ca aluminates. The most important Ca aluminates
are ettringite (3CaO. Al2O3.3CaSO4.32H2O, 4 wt%), calcium aluminate monosulphate (3CaO. Al2O3.CaSO4.12H2O, 7 wt%) and Ca carboaluminate (3CaO. Al2O3.CaCO3.11H2O, 7 wt%) [103] . Together they make up almost 90 wt% of the mineral suite in hydrated ordinary Portland cement (OPC) paste and thus, have the greatest potential for metal(loid)-ion binding. The relative importance of the above processes for selected metals can be found in a recent review [104].

OPC is the most common type of cement used for immobilizing liquid and wet solid wastes worldwide [6]. Composite cement systems were devel­oped in the UK for ILW encapsulation using additional powders as well as OPC such as blast furnace slag (BFS) and pulverized fuel ash (PFA). These offered cost reduction, energy saving and potentially superior long-term performance. BNFL, for example, use a 9:1 ratio of BFS to OPC to reduce the heat of hydration, which for OPC cements, would otherwise limit con­tainer volumes. Large containers (see Fig. 6.3) can therefore be used safely without concern over heat from setting reactions causing water to boil off.

image102

Modeling has shown that cements can be ‘designed’ to retain radioactive and hazardous constituents [ 105] . In fact, much research has focused on improving the effectiveness of grout in adverse environments associated with the disposal of radioactive waste [106-108]. As discussed in these refer­ences, a variety of cement-polymer composites have been investigated as a means of making grouts more compatible with the radioactive and chemical constituents in waste.

For example, the addition of blast furnace slag to the Saltstone cement[20] being used to solidify Cs-decontaminated salt supernate at the Savannah River Site (SRS), provides a chemical reductant [iron(II)] and a precipitat­ing agent (sulfide) that chemically binds contaminants such as chromium and technetium as insoluble species, thus reducing their tendency to leach from the waste form. Experimentation has shown that leaching of chro­mium and technetium was effectively reduced to levels that would allow all projected future salt solution compositions to be processed into Saltstone [109] . Long-term lysimeter studies have shown that the addition of slag essentially stopped technetium-99 leaching, although it did not reduce nitrate leaching [109]. Because the SRS Saltstone admixture that is blended with 45% liquid waste is only 10 wt% OPC, 25 wt% fly ash, and 25 wt% slag, it is a geopolymeric cement as the alkali in the salt supernate reacts with the fly ash in geopolymer-like chemical reactions.

The water in the hydrated cement blends may generate H2 by radiolysis in high radiation fields and require vented canisters [110] when container­ized. While this study concentrated on transuranic (TRU) wastes containing 238Pu oxide, which is primarily alpha radiating, the other studies have dem­onstrated the radiolysis of concrete with “Co (gamma radiation) and [H (beta radiating) [111-113].

Recent comprehensive reviews of cement systems for radioactive waste disposal can be found in Pabalan et al. [114] and Glasser [115]. Long-term cement durability comparisons have been made using ancient cements, geopolymers, and mortars [116-123], some of which may also serve as natural analogues for geopolymer wasteforms [124, 125].

The cements and grout formulations are too extensive to list as examples. The durability response is complex due to the relative response of encap­sulation with some chemical reaction and embedding. Therefore, the dura­bility is usually modeled as a diffusion rate with respect to the element(s) of interest.

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