Solidification by chemical incorporation

Vitrification

Vitrification is currently the most widely used technology for the treatment of high level radioactive wastes (HLW) throughout the world (Tables 6.1, 6.3 and 6.4). Development of glasses for the solidification of HLW began at different times in the US, Canada, Europe, and the USSR [17]. Different glass formulations (borosilicate, aluminosilicate, and phosphate glasses) and processing strategies were developed [18] . Currently, most of the nations that have generated HLW are immobilizing in either borosilicate glass or aluminophosphate glass. One of the primary reasons that glass has become the most widely used immobilization media is the relative simplicity of the vitrification process, e. g. melt waste plus glass forming additives and cast. There is >50 years processing experience[17] with commercial borosilicate glasses and borosilicate glasses have favorable systems evaluations in terms of both melting and product behavior.

Melting homogenizes the mixture and so this process is easier to perform remotely than a ceramic waste form process that requires powder handling, e. g. mechanical mixing of waste and ceramic additives and grinding for particle size control, followed by cold pressing and sintering or hot pressing at elevated temperatures. A second reason that glass has become widely used for HLW is that the amorphous and less rigid structure of glasses (SRO and MRO) compared to ceramics (SRO, MRO, and LRO) enables the incorporation of a very large range of elements that are atomically bonded in the flexible glass structure (see Table 6.4). Thus glasses can accommodate larger waste composition fluctuations than most ceramics.

The glass forming SRO structural groups are usually tetrahedral Si, B, Al, Fe, P surrounded by four oxygen atoms (tetrahedral coordination) or B surrounded by three oxygen atoms (trigonal coordination) and glasses are named after the predominant tetrahedral species, e. g. borosilicates have primarily B and Si with some Al, Fe, and P and aluminophosphates would have primarily Al, P, and Si. See Table 6.4 for the attributes of various types of glasses that have been used for a variety of HLW wastes and pertinent references that can be consulted.

The tetrahedra and trigonal species in glass link to each other via bridging oxygen bonds (BO). The remaining non-bridging (NBO) atoms carry a negative charge and, in turn, ionically bond to positively charged cations like Cs+, Sr+2, Ca+2 and positively charged waste species. These linkages create the MRO structural groups such as (Cs, K,Na, Li)AlO2, (Cs, K,Na, Li) FeO2, (Cs, K,Na, Li)BO2, and (Cs, K,Na, Li)SiO4 [19] or (Cs, K,Na)AlSiO4 [20] which form sheet-like units, chain-like units, and monomers [21] that further bond the waste species ionically.

The tetrahedra define the network regions, while NBO define percolation channels or depolymerized regions (DR) shown in Table 6.4 that can act as ion-exchange paths for elements that are less well bonded to the NBO. Such percolation channels are also found in rare-earth (lanthanide) alumino — borosilicate (LaBS) glasses as well (see Table 6.4) . Thus, the molecular structure of glass controls radionuclide/contaminant release by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sites through the percolation channels, and the mechanisms are similar to those observed in natural analog glasses (basalts) and in mineral analogs.

Moreover, HLW glasses melt at lower temperatures (1050-1150°C) than higher ceramic waste forms, which minimizes the volatility of radioactive components such as 99Tc, 137Cs, and 129I. While ceramics are often credited with having higher chemical durability than glasses, if the radionuclides are incorporated in an intergranular glassy phase during processing (see discus­sion in the next section), they leach at about the same rates as those from glassy wasteforms [22] .

Lastly, nuclear waste glasses have good long-term stability including irra­diation resistance and excellent chemical durability. In addition, the ease of modeling the durability of a homogeneous rather than a heterogeneous material in terms of having only one radionuclide source term is also an advantage.

A basic assumption in all glass dissolution models is that the solid being modeled is made up of a single phase and so the durability response has only one source term (see Table 6.3). Therefore, phase separated glasses (with two source terms) with two distinct glass compositions are avoided as their durability cannot currently be modeled. Often the two immiscible glass phases have different compositions, e. g. one phase is often boron-rich and has a poorer durability than the bulk and/or the matrix phase. Having a poorly soluble second phase is not desirable for HLW glasses where the distribution of the radionuclides in the two glassy phases would have to be known for every waste glass fabricated. Since the volume fraction of each phase is also related to the thermal history of each canister of glass, each canister would be different and this complicates durability modeling to the point that it is virtually impossible.

To ensure that HLW borosilicate glasses are homogeneous (not phase separated), a minimum Al2O3 limit is applied in the US. The effect of insuf­ficient Al2O3 was first reported by French researchers [23] who determined that many glass durability models were non-linear, e. g., glasses had release rates far in excess of those predicted by most models, in regions correspond­ing to low Al2O3 and in excess of 15 wt% B2O3. The phenomenon was independently discovered by US researchers and found to exist in natural basalt glass systems as well [24-26].

Additional durability source terms can occur if crystals are present in a glass because crystals create grain boundaries that can (1) selectively undergo accelerated dissolution while the crystals themselves may have a different dissolution response [27], or (2) have compositions not representa­tive of the bulk glass [28]. This will be discussed further in the next section on glass ceramics.

Glass formulations are generally homogeneous, allowing only a few weight percent crystals to form on cooling in the canister. Certain crystals such as iron spinels have little impact on glass durability as they are themselves very durable and cause minimal grain boundary dissolution [27, 29]. However, for other phases such as nepheline, acmite, and lithium silicates that are less durable than iron spinels and not isotropic, the impact on glass durability from the crystal and the grain boundaries can be pronounced. This is especially true if the crystal sequesters radionuclides as this gives a secondary source term for radionuclide release. Therefore, dura­bility testing must be performed to confirm that any crystallization that might occur during canister cooling has minimal impact [30-33]. This ensures that the last three terms in Eq. [6.1] approximate zero and that glass dissolution has a single source for radionuclide and hazardous species release

^ Durability=durability (homogeneous) + durability (amorphous

lstterm 2nd term

+ durability(crystallization) +durability (accelerated [6.1]

3rd term 4th term

This durability equation will be discussed in more detail in reference to other waste forms where the third and fourth terms in Eq. [6.1] may become important.

Glasses can be made by JHM, AJHM, induction melting, CCIM, and HIPing (see Table 6.3). Extensive reviews on vitrified waste forms can be found in the references cited in Section 6.8 and in Table 6.4.

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