Glass ceramics and glass composite materials (GCMs)

It is sometimes difficult to distinguish between glass-ceramics (a glassy matrix which is allowed to form crystals during cooling or glassy matrices where controlled cooling is used so that certain crystalline species known to sequester radionuclides are encouraged to form) and glass composite materials (GCMs) (Tables 6.5 and 6.6) [34, 35] . GCMs are considered a composite material where the long-lived radionuclides are atomically bonded in the ceramic (mineral) phase and the glass is an encapsulating matrix phase. The glass can have little or no retention of radionuclides or act as the host for the short-lived radionuclides [ 36] . Glass-ceramics and GCMs include glass-ceramics where a glassy waste form is crys­tallized in a separate heat treatment, GCMs formed by melt crystallization (controlled or uncontrolled), and GCMs in which a refractory waste is encapsulated in glass [ 34] . Glass-ceramics and GCMs offer increased waste loadings, increased waste form density, and thus smaller disposal volumes.

One such example of a GCM in Table 6.6 is the glass bonded sodalite, as the radionuclide of concern, [ 29I, is in the ceramic phases and not in the glassy phase. Other examples of GCMs include the following [36]:

1. glass ceramics in which a glassy waste form is crystallized in a separate heat treatment [7,37];

2. GCMs in which, for example, a refractory waste is encapsulated in glass such as hot-pressed lead silicate glass matrix encapsulating up to 30 vol% La2Zr2O7 pyrochlore crystals to immobilize minor actinides [38] ;

3. GCM formed by pressure-less sintering of spent clinoptiloite from aqueous waste processing [39];

4. some difficult wastes such as the French HLW U/Mo-containing materi­als immobilized in a GCM termed U-Mo glass formed by cold crucible melting that partly crystallize on cooling [40];

5. ‘yellow phase’ containing wastes are immobilized in Russia in a ‘yellow phase GCM’ containing up to 15 vol% of sulfates, chlorides, and molyb­dates [41]; and

6. GCM that immobilizes ashes from incineration of solid radioactive wastes [42] .

Durable crystals

image97

6.1 Current homogeneous glass formulations are limited to the lower left-hand corner of this triangular diagram. If the homogeneous glasses crystallize durable crystals shown at the apex of the triangle, e. g. spinels, ZrO2, apatite, TiO2, etc., then waste loading can be increased and glass composite materials (GCMs) produced by changing the melter technology (e. g. CCIMs) or invoking a different technology such as HIPing. Ceramic waste forms are at the apex and are considered exceptionally durable waste forms but may be more appropriate as small volume waste forms as processing is more difficult. Some ceramic waste form formulations can be melted in advanced melters like CCIMs and then allowed to crystallize into GCMs. While certain species such as Mo, S, and P can create non­durable secondary phases (lower right apex of the triangle), these should be avoided or macroencapsulated, which moves their durability closer to the lower left apex of the triangle [36].

Note that alkali-rich wastes at the Hanford site that were made by in­container vitrification (ICV)[18] produced an immobilized glassy waste form with high crystal content that characterize them as GCMs [43].

Note that yellow phase is composed of species that are poorly soluble in glass such as Na2SO4 which can sequester Cs and Sr [44] , (Na, K,Cs)Cl, (Na, K,Cs)2Cr2O7, and (Na, K,Cs)2MoO4. Yellow phase is either (1) prevented from crystallizing (Fig. 6.1) or (2) the glass is heat treated to encapsulate the soluble phase (s) as GCMs. One such vitrification process given as the fifth example above produces a sulfate-chloride-molyb­date GCM by using vigorous melt agitation followed by rapid cooling of the melt to the upper annealing temperature to fix the dispersed ‘yellow phase’ into the host borosilicate or aluminosilicate glass. The sulfate- chloride-molybdate-containing GCM (see yellow phase GCM in Fig. 6.1) has only a slightly diminished chemical durability compared with sulfate- chloride-molybdate-free aluminosilicate and borosilicate glasses [36].

In many cases, until a waste form is made and analyzed for the distribu­tion of radionuclides amongst the crystalline and glassy phases, one cannot discern whether a GCM has been made (see Table 6.5). In either case, glass — ceramics and GCMs offer a useful compromise between glasses and ceram­ics, being easier and less expensive to prepare than conventional ceramics, but offering higher durability than glasses.

Depending on the intended application, the major component may be a crystalline phase with a vitreous phase acting as a bonding agent, or, alter­natively, the vitreous phase may be the major component, with particles of a crystalline phase dispersed in the glass matrix. Glass-ceramics and GCMs may be used to immobilize long-lived radionuclides (such as actinide species) by incorporating them into the more durable crystalline phases, whereas the short-lived radionuclides may be accommodated in the less durable vitreous phase [36].

Historically, crystallization of vitreous waste forms has been regarded as undesirable as the crystallization has the potential to alter the glass com­position and hence the durability of the remaining continuous glass phase could eventually be compromised when it comes into contact with water. However, there has been a recent trend towards higher crystallinity in vitre­ous waste forms so that they are more correctly termed glass-ceramics or GCMs depending on whether the glass or the crystals contain the radionu­clides (Table 6.5 ).

Table 6.5 also shows glass-ceramics where significant quantities of crystals (arising from higher waste loadings) form, such as in the Savannah River Site (SRS) high iron bearing glasses where spinel crystallizes [27] and the crystals do not incorporate the radioactive species but act as benign or inert ‘stones’ in the glass.

Historically silicate glass-ceramics were developed in the mid 1970s in Germany [45] . Silicate and phosphate glass-ceramics were also developed in the USSR [46], silicate glass-ceramics were developed in Japan [47], and titanium aluminosilicate glass-ceramics were developed in Canada [48]. GCMs represent a second generation, more sophisticated approach to the production of glass-ceramics, where the long-lived radionuclides are forced into the more durable crystalline phases by tailoring the waste-additive mixture and/or controlling the crystallization. More recently, GCMs such as the glass bonded ceramic waste forms containing sodalite and alkali halides in a borosilicate matrix have been developed for electrorefiner wastes, specifically the stabilization of 129I in sodalite and NaI. [49-51], while rare-earth oxyapatites, powellite, celsian, and pollucite [52] have been developed for rare-earth lanthanide and Cs, high Mo-containing wastes. Excellent reviews of other GCM’s, such as SYROC glass ceramics, murati — tie, and other Ti-based glass ceramics can be found in Stefanovsky et al. [3], Donald et al. [4, 8] and Lee et al. [34].

In terms of modeling the durability of glass-ceramics and GCMs, the distribution of the radionuclides amongst the crystalline and glassy phase becomes important. Referring back to Eq. [6.1] which gives the needed durability vectors for each phase, we see that the second term drops out since the glassy phase should not have glass-in-glass phase separation, leaving terms 1, 3, and 4 (Eq. [6.2]):

X Durability=durability (homogeneous) + durability(CrystamZation)

lstterm 3rd term

+ durability (accelerated [6.2]

4th term

If the glass contains no radionuclides then the first term in Eq. [6.2] also drops out. If there are multiple types of phases present and each hosts a different radionuclide, then there will be durability vectors for each phase that hosts that radionuclide as shown in the Table 6.6 example for the 129I in glass-bonded sodalite waste forms. If there are no radionuclides in the crystals then the third term drops out and it may be possible to demonstrate that the fourth term is negligible. If a given radionuclide is present in both the glassy phase and a crystalline phase, then the durability response from the glass and the crystalline phase and the grain boundary are additive as shown in Eq. [6.2] .

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