Immobilization processes and technologies

The major types of waste forms will be described in regard to the manner in which the radionuclides are immobilized and the methods by which each can be made. Different waste forms give different durability tests responses. Single-phase waste forms (glass and single-phase oxides or crystalline ceramics (minerals) have only one source of radionuclides that can leach during a durability test. In multiphase waste forms the distribution of the radionuclides amongst the phases present becomes important as each phase has its own rate of leaching for the specific elements that it sequesters. Each waste form given in Tables 6.3-6.10 will be described in terms of the radio­nuclide immobilization achieved and references given as to which condi­tioning technologies can be used to make each type of wasteform.

The immobilization of HLW is always achieved by its atomic-scale incor­poration into the structure of a suitable matrix (typically glass, a GCM, or a crystalline ceramic (also sometimes referred to as mineral analog waste forms) so that the radionuclides are incorporated into durable structures by any combination of short range order (SRO),[13] medium range order (MRO)[14] [15] or long range order (LRO).2 Glasses incorporate radionuclides and hazardous species into their atomic structure by SRO and MRO [16]. Recent experimentation has shown the existence of large cation-rich clus­ters in glass, e. g. clusters of Ca in CaSiO3 glasses and clusters of Na2MoO4

Waste form Homogeneous glass

 

Inhomogeneous glass

 

Radionuclides and hazardous species are atomically bonded in a durable glass structure usually to oxygen atoms that are also bonded to the matrix elements, Si, Al, В, P, etc., by short range order (SRO) and medium range order (MRO)

 

Some radionuclides and hazardous species are atomically bonded in a durable glass structure as with homogeneous glass, but other radionuclides reside in a very soluble immiscible glass phase (glass-in-glass phase separation)

 

Description

 

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Radionuclide

immobilization

mechanism

Key

*

+

b

Cs

U Tc

Pu

xl.

 

Chemical incorporation

 

Chemical incorporation

 

Waste loading(s)/ durability

 

(a) moderate waste loading, (b) good overall durability, (c) easy to model radionuclide release from a single phase

Joule Heated Melters (JHM), Advanced Joule Heated Melters (AJHM), Cold Crucible Induction Melters (ССІМ), Hot Isostatic Pressing (HIP), Hot Uniaxial Pressing (HUP)

 

(a) moderate waste loading, (b) poor durability for certain radionuclides, (c) impossible to model radionuclide release as the fraction of the second phase is dependent on thermal history

Joule Heated Melters (JHM), Advanced Joule Heated Melters (AJHM), Cold Crucible Induction Melters (ССІМ), Hot Isostatic Pressing (HIP), Hot Uniaxial Pressing (HUP)

 

Immobilization

technologies

 

image61"image62"

image073

Type of glass

 

Major structural components

 

Comments

 

(Si04) 4, (B04)-5, (B03)-3 and some (AI04) 5 and (Fe04)~5 structural units to which alkali, alkaline earth, and waste species bond.

 

Ease of processing, melt temperatures 1150-1200°C to minimize volatility; cold cap production if feasible minimizes volatility; most waste cations highly soluble in glass; overall waste solubility 25-40wt%; made by JHM, AJHM, induction melting, ССІМ or HIP.

 

Alkali borosilicate

[3-8, 18, 34, 61]

 

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Atomic structure of a French nuclear waste glass: unshaded region shows formation of a

(Na, Cs)2Mo04 cluster [209].

 

Lanthanide borosilicate (LaBS) [5,6, 18, 210-214]

 

(Si04) 4, (B04)-5, (B03F3and some (AI04) 5 structural units to which lanthanides, alkaline earth, and other waste species bond.

 

Higher waste loading (16-59wt%) for actinides/lanthanides than alkali borosilicates; lanthanides serve as neutron absorbers; 1300-1500°C melting causes volatilization of some radio­nuclides; corrosion similar to alkali boro-silicates; made by ССІМ, HIP, or induction melting.

 

Atomic structure of a French HLW rare earth bearing borosilicate glass. Na+, Ca2+ and Nd3^ exist in the percolation channels. PR is the polymerized region and DR is the depolymerized region [7].

 

image63"image64"

Подпись: Published by Woodhead Publishing Limited, 2013Подпись: Aluminosilicate glasses and/or alkali aluminosilicate glasses [4, 8, 61] Type of glass Major structural components

(Si04r4 and (AI04)~5 structural units to which alkali, alkaline earth, and waste species bond (similar structure to borosilicate glasses when (В04Г5 are present).

image65Melt temperature of ~1600°C causes volatilization of radionuclides; waste loading dependent on rapid cooling, e. g. 20wt% U02 if cooled rapidly while <10wt% if cooled slowly; improved durability over borosilicate glass; ССІМ, HIP.

Atomic structure of a simple generic M203(G203)2 glass (M is modifying cations, G represents tetrahedral cations). The shaded regions are the PR regions. The un-shaded regions represent the percolation channels or DR regions (from [215]).

Подпись: Published by Woodhead Publishing Limited, 2013Aluminoborate (B04)“5, (B03)“3 and some (AI04) 5.

glasses

High silicate glasses (Si04)“4

(sintered glasses)

[4, 8, 61]

Alkali alumino — (P04)-3 and (AI04)-5 structural

phosphate [3-8, units to which alkali, alkaline

34, 217-220] earth, and waste species bond.

image66Requires hot pressing and sintering at 600-800°C in order to retain volatile fission elements such as Cs, Ru, Mo and Tc; waste solubility 5-35wt%.

Atomic structure of sodium silicate glass. Glass formers are small open circles, oxygen atoms are large open circles, modifier cations are small filled circles, U atoms which form clusters are large filled circles [216].

4

Atomic structure of phosphate glass with P4O10 cage-like structures which provide the basic building block for phosphate glass formers.

Melts at lower temperatures than silicate or borosilicate systems; most cations readily incorporated; accommodates >10wt% sulfate; corrosive to materials of construction; tendency to devitrify; durability comparable to borosilicate glass if alumina content is sufficient; composition ~ 24-27 Na20, 20-24, Al203 + MemOn, 50-52 P205; JHM, AJHM, ССІМ.

Подпись: Published by Woodhead Publishing Limited, 2013Type of glass Major structural components

Подпись: (РО4Г3 and (Fe04r5 structural units to which alkali, alkaline earth, and waste species bond.Lead iron phosphate (LIP) [4, 8, 61, 221-227]

image67

Atomic structure of LIP glass. Polyphosphate chains are cross — linked by lead atoms (open circles) and iron atoms (small filled circles) which form ‘knots’ in the percolation pathways that inhibit cation diffusion [228].

40-66 PbO; 30-55 P205; 0-10 Fe203 dependent on amount of iron oxide in waste; melts 850- 1050°C; waste loading (~20wt%); abandoned due to regulatory issues with PbO component poor solubility of certain species, devitrification, poorer waste solubility than borosilicate glasses, etc.; JHM, AJHM, ССІМ.

Iron phosphate (IP) [229-247]

 

(P04)-3 and (Fe04)~5 tetrahedral structural units to which alkali, alkaline earth, and waste species bond; Fe-O-P bonds have shown to be hydration resistant whether iron is Fe2+ or Fe3" [248]

 

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Chalcogenide and chalcohalide [96, 251, 252]

 

Glasses obtained by melting chalcogen elements (S, Se, and Те) with Group V and IV elementsTe02-XCi-B203Te02- XCI-Li20Te02-XCi-Na20XCI = ‘mixed chlorides’ waste simulant at ~19wt%

 

Adapted from [11].

 

image68image69

Atomic structure of IP glasses are nano-heterogeneous, with FeP04- like regions and phosphate chains that incorporate Fe2+/Fe3+ network­modifying cations. Large atom in center of cage like structure is a waste cation [249, 250].

Good chemical durability; high solubility for many heavy metals (U, Cr, Zr, Cs, Mo, noble metals, rare earths); melts 950-1100°C; viscosity typically <1 poise; low corrosion of oxide refractories and Inconel alloys; waste loadings 25-50wt%; tendency to devitrify; JHM, AJHM, ССІМ.

S, Se, and Те glasses for radionuclides difficult to immobilize in borosilicate glass systems, i. e. 12SI. Gels such as Pt2Ge4S96 are used to immobilize actinides, noble gases, carbon dioxide, and mixed chlorides.

[from 253]

Table 6.5 Attributes of glass-ceramics and glass composite material (GCM) waste forms

Waste forms

GCM (secondary crystalline phase contains no radionuclides and/or is inert)

GCM (secondary crystalline phase contains radionuclides and should be durable)

Description

Radionuclides can be chemically incorporated in the glassy matrix (same as single phase glasses) and crystals such as spinels (Cr, Ni, and Fe species) crystallize that do not contain radionuclides and are inert.

Radionuclides can be chemically incorporated in the glass matrix and in the crystalline phases. Example shows Cs in the glass and in a secondary phase. Secondary phases need to be durable like pollucite (Cs, Na)2Al2Si4O12 and soluble phases such as (Na, Cs)2SO4 should be avoided as they are not GCMs.

Radionuclide

immobilization

mechanism

Chemical incorporation

Chemical incorporation and encapsulation

Key

* # ° — f %

Cs U Tc Pu xl.

/> V A

1 о to * o® j

V* ° +•+ 0 * 0 ° *J

+ 4* Ф

0 ftO Ж 0

v&0 * +&♦ о * ° by

Waste loading(s)/ durability

(a) higher waste loadings,

(b) secondary phases have no radionuclides,

(c) good overall durability, (d) easy to model radionuclide release from single phase glass once grain boundary dissolution is experimentally shown to be minimal

(a) higher waste loadings,

(b) secondary phases contain long-lived radionuclides, (c) glassy phase can contain the shorter lived radionuclides or no radionuclides,

(d) more complex to model radionuclide release from multiple phases and grain boundaries

Immobilization

technologies

Joule Heated Melters (JHM — crystals form on cooling), Advanced Joule Heated Melters (AJHM -1-3 vol% crystals probable), Cold Crucible Induction Melters (CCIM 10-50 wt% crystals), Hot Isostatic Pressing (HIPing >40 wt% crystals), Hot Uniaxial Pressing (HUPing >90 wt% crystals), Press and sinter (> 90 wt% crystals)

Adapted from [11].

Table 6.6 Examples of glass-ceramics and glass ceramic materials (GCM) as waste forms

 

Borosilicate based

Alkali borosilicates [44, 45, 254-257]

 

Glasses which are allowed to partially crystallize in a stirred melt pool or upon cooling; crystals are inert; crystallized glass viscosity is non- Newtonian; secondary phases must be inert; AJHM or ССІМ.

Electrorefiner wastes; radionuclide release from each phase is measured, e. g. Si, Al, Na, Li (sodalite and glass), В (glass), Cl, I (sodalite and halite); HIP or cold pressing/sintering.

Zirconolite is major crystalline phase for Pu and Gd (neutron absorber); for low purity actinide wastes; Pu partitions into crystalline phase over the glass phase by a factor of 100:1; accommodates actinides and any associated impurities; HIP.

Pyrochlore host for actinides and Sr; pollucite host for Cs and Bb; noble metal fission products form small metallic droplets. Melt temperatures from 1100-1400°C; controlled crystallization between 530 and 720°C; leaching characteristics have been noted to be comparable to the borosilicate glasses affording no significant advantages; additional work in this area has been limited, melt and control crystallization or press and sinter.

 

Borosilicate

 

NiFe204 spinels, ZrSi04, Al203

 

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Glass bonded sodalites [49-51, 258]

Synroc alumino — borosilicates [11]

 

Borosilicate 12SI in Nal, 12SI in sodalite,

Na8AI6Si6024(l)2, Cl in sodalite, Na8AI6Si6024(CI)2

Alumino — Zirconolite, CaZrTi207

borosilicate

 

Barium

aluminosilicates celsian [4, 8, 61, 259]

 

Borosilicate (sodium alumino-silicate with 2-7 wt% B203 and 3-4wt% Ti02)

 

Celsian, BaAI2Si208 pyrochlore (fttr2Ti207; RE-rare earth), Scheelite (BaMo04), Pollucite (CsAISi206) molybdenum-nosean [Na8AI6Mo04(Si04)6], Perovskite CaTi03, Diopside CaMgSi206, Eucryptite LiAISi206 spodumene LiAISi206, Nepheline, NaA1Si04

 

Waste loadings ~ 30wt% for European and Japanese commercial wastes which is usually ~16wt%; Melted at 1300°C; controlled crystallization in the range 800-1100°C; Cs was in the diopside; La, Ce, Nd, Pr in the perovskite, Sr and Sm were in the glass; noble metals were metallic.

Sphene and Synroc crystalline ceramic forms, mainly zirconolite, can also be formulated. Formation at 1300-1500°C. Actinides and BEEs, and Sr are in zirconolite; Cs and the remaining Sr into the vitreous phase; ССІМ and cool, press and sinter.

Formed by HIPing calcine (70wt%) with Si, Ті, Al metal and alkali oxides; for high Zr containing Idaho National Laboratory wastes.

Form at 1200°C. Fresnoite hosts Ba and Sr, priderite hosts Ba, pyrochlore hosts BE, actinides, BE and Sr. Cs remains in the glassy phase. Glass is 50% and crystalline phases are 50%.

Silicate based

Basalt [4, 8, 61, 261. 272, 273]

 

Complex natural oxide based on Si, Ca, Mg, Fe, Al and Ті

Alumino-silicate

glass

 

For Purex wastes: augite (Ca, Mg, Fe)2Si206 powellite (Ca, Sr) Mo04 spinel (NiFe204).

 

Glasses melt in the range 1300-1400°C. Crystallization is carried out at temperature ranges 670-700°C and 900-950°C. The chemical durability superior to that of borosilicate glasses; JHM, ССІМ.

Applications to commercial and defense wastes, including decontamination of Three Mile Island containment water together with core debris; melt at 1400-1500°C and controlled cooling after casting the glass into containers; JHM, ССІМ; arc melting.

Cast glasses crystallized by holding at 1200°C for 16h; Ті phases retain the actinides; JHM, ССІМ; arc melting.

 

Iron enriched basalt (IEB) [4, 8, 61, 261]

 

Iron spinel; feldspars NaAISi308 to CaAI2Si208; augite (Ca, Mg, Fe)2Si206; fluroapatite, Ca5(P04)3F; zircon, ZrSi04; fluorite CaF2; cristobalite Si02; hematite, Fe203, mullite AI6Si2013

Same as above plus: zirconolite, pseudobrookite Fe2Ti05, chevkinite

Ce4Fe2Ti3Si4022

 

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Iron enriched basalt (IEB) with Ti02 and Zr02 [4, 8, 61, 261]

Magnesium aluminosilicate (MAS) [4, 8]

Phosphate based

Apatite/monazite glass ceramics [274-276]

 

Alumino-silicate

glass

 

Enstatite MgSi03lndialite/Corderite Mg2AI4Si5018

 

Used as an encapsulant for Zr alloy cladding wastes; accommodates 20% Zr02; press and sinter.

 

Magnesium

alumino-silicate

 

Apatite Ca5(P04)3(F, CI)Monazite (Ce, U) P04

 

Apatite hosts Ca, P,F, Cl, S, Sr, Cs, As, Pb, Ba, Hg, Cd, Cr, U, and Ce, melted at 1400°C, crystallized at 1150°C and allowed to furnace cool; investigated primarily for phosphate-rich or fluoride-rich waste streams including Idaho National Laboratory CaF2 wastes; JHM, AJHM, ССІМ.

 

Calcium

phosphate

 

Adapted from [11].

 

Table 6.7 Attributes of homogeneous and multiphase ceramic (mineral) waste forms

Waste form(s) Single phase oxides/ Multiphase oxides/minerals/

minerals/metals metals (granular or monolithic)

(granular or

monolithic)

Description

 

Individual phases contains one radionuclide or hazardous species or a solid solution, i. e. UO2-ThO2 (shown).

 

Individual phases contain different or multiple radioactive or hazardous species (see solid solution indicated between UO2- ThO2). Some phases do not incorporate radionuclides or hazardous species at all.

Chemical incorporation

 

Radionuclide

immobilization

mechanism

 

Chemical

incorporation

 

Key

image70

 

* $

Cs U

 

image71"image72"

image113

Подпись: Waste loading(s)/ durability

(a)

high waste

(a)

loading for single

(b)

radionuclide or hazardous species

(c)

(b)

durability

(c)

easy to model

species released from a single phase

(d)

(d)

may require precalcining for certain

(e)

technologies to work efficiently

(f)

(e)

high waste loadings superior overall durability difficult to model durability of species released from multiple phases and grain boundaries

need to tailor for species partitioning amongst phases

need to determine species partitioning and source terms from each phase may form an intergranular glassy phase that sequesters species of concern

may require precalcining for certain processes to work efficiently

Immobilization Hot Isostatic Pressing (HIPing >40 wt% crystals), Hot

technologies Uniaxial Pressing (HUPing >90 wt% crystals), Press

and sinter (> 90 wt% crystals), Fluidized Bed Steam Reforming (>90 wt% crystals)

Simple oxides

X02 Oxides [277-280]

 

Cubic Zirconia, 1C

 

Murataite, 3C

 

Zr02, U02, Th02,Hf02, Pu02 have the simple fluorite CaF2 cubic structure; make by HIP, HUP, press and sinter, melt and crystallize.

 

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Pyrochlore, 2C

 

Complex oxides

Pyrochlore [281-284]

 

A derivative of the fluorite structure type, A2B207, where A-site contains large cations (Na, Ca, U, Th, Y and lanthanides) and the В-site contains smaller, higher valence cations (Nb, Та, Ті, Zr, Fe3+).

Also a derivative of the isometric fluorite structure A6BuC5TXt0-x. with multiple units of the fluorite unit cell; hosts U, Pu, Cm, and BEs including Gd a neutron absorber. Forms in solid solution with pyrochlore.

Monoclinic CaZrTi207 has a fluorite-derived structure closely related to pyrochlore, where Pu, U, Gd and Hf may be accom­modated on the Ca/Zr-sites, as in the case of Ca(Zr, Pu)Ti207.

 

V Zr4* • Y3+ • Tl4* • Zn2+ # Gd3* b Fe3* # 02~__________________

 

Murataite [3, 7, 285-293]

 

Projection along [111] direction for zirconia, pyrochlore, and murataite structures [from 284]

 

Zirconolite [294-299]

 

image73"image74"image75"

image078

Crystalline ceramic phase Perovskite [296, 300]

 

Comments

CaTi03 has a wide range of compositions as stable solid-solu­tions; orthorhombic; consists of a three-dimensional network of corner-sharing TiOe octahedra, with Ca occupying the large void spaces between the octahedra (the corner-sharing octahedra are located on the eight corners of a slightly distorted cube). Plutonium, other actinides, and rare-earth elements can occupy the Ca site in the structure, as in (Ca, Pu) Ti03. The octahedra can also tilt to accommodate larger cations in the Ca site [from 301].

 

Structure

 

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[from 301]

 

Ba12(AI, Ti)8016 tunnels between Ti06 octahedra accommodate 133Ba, 137Cs and soSr.

 

Ba-Hollandite [302, 303]

 

Ferrite garnet [283] Garnet [304-307]

 

[slA3[6lB2[Ti04]3, e. g.[sl(Ca, Gd, actinides)[6lFe2[4lFe3012

A3B2(X04)3; distorted cubic structure; B06 octahedra and X04 tetrahedra establish a framework structure alternately sharing corners; A and В sites can host actinides, BEs, and X = Si4+, Fe3+, Al3+, Ga3+, Ge4+ and V5+ making silicate, ferrite, aluminate, gallate, germinate, and vanadate garnets.

 

image76"image77"

Na2AI2(Ti, Fe)6016 a spinel-based phase suitable for incorporating Al-rich wastes from Al fuel cladding/decladding. The A site can accommodate Na, К while the different octahedral sites can accommodate Mg, Co, Ni, Zn, Al, Ті3*’, Cr, Fe, Ga, Si and Nb.

image120
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Sr, La, Ce, Y positions are indicated by the solid circles. Other cations are in the octahedral positions, [from 309]

 

Freudenbergite [310]

 

[from 311]

 

Simple silicates

Zircon/Thorite [312, 313]

 

ZrSiO/ThSiO,,; zircon is an extremely durable mineral that is commonly used for U/Pb age-dating, as high uranium concen­trations (up to 20,000 ppm) may be present; the PuSi04 end member is known and Ce, Hf and Gd have been found to substitute for Zr.

 

image78"image79"image80"

image073

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Crystalline ceramic phase Comments

 

Structure

 

Titanite (sphene) [261, 314] CaTiSi05 can sequester cations such as Ba, Sr, and fission

product oxides (~15wt%), U, Cr, and Ni in the Ca sites (dark circles). Tetrahedra are Si and octahedral are Ті.

 

Britholite (silicate apatite) Also known as oxy — apatites in the literature.

[54, 82, 87, 315-318]

 

(BEE, Ca)5(Si04,P04)3(0H, F); i. e. Ca2Nd8(Si04)602, Ca2La8(Si04)602; based on ionic radii of Nd3+, La3+, and Pu3+, an extensive range of solubility for Pu3+ substitution for the Nd or La, particularly on the 6h site, is expected. Since there is an extensive range in the Ca/BE ratio in these silicate apatites, a fair amount of Pu4+ substitution may be possible; La34 through Lu3+ can substitute for Ca2+ and form oxyapatites, ВЕ4і67іИо. зз[Зі04]зО; can also accommodate Sr and Cs, Th, U, Np.

 

image81"image82"

(Ca, Na)2AI2Si4012»2H20; host for fission products such as 137Cs CsTiSi2065

Framework silicates

Zeolites [320-325]

 

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(Xxlnl(A2)x (Si02)y], where X is the charge balancing counter-ion, n is the charge of the counter-ion, x is the number of charge — deficient alumina sites, and у is the number of charge-neutral silica sites; characterized by internal voids, channels, pores, and/or cavities of well-defined size in the nanometer range, = 4-13A; channels and/or cavities may be occupied by charge — compensating ions and water molecules. Zeolites like Ag-Mor — denite selectively sorbs l2 (12SI); certain zeolites can be converted to condensed oxide ceramics by heating. This process is particularly attractive for waste form fabrication because capture and storage is performed with minimal steps.

 

Structure of Zeolite-A [326] showing alternate Al and Si atom ordering but omitting the tetrahedral oxygens around each Al and Si.

 

Pollucite [327-332]

Pollucite Cs/Ti Version [333-342]

 

image83"image84"

Подпись: Published by Woodhead Publishing Limited, 2013image85Crystalline ceramic phase Comments Structure

Nepheline [49, 343-348] NaAISi04 silica ‘stuffed derivative’ ring-type structure; some

polymorphs have large nine-fold cation cage sites while others have 12-fold cage-like voids that can hold large cations (Cs, K,

Ca). Natural nepheline structure accommodates Fe, Ті and Mg.

Подпись: Two-dimensional representation of the structure of nepheline showing the smaller eight oxygen sites that are occupied by Na and the larger nine oxygen sites that are occupied by К and larger ions such as Cs and Ca. [357]

Leucite* KAISi206; К analogue of nepheline

Подпись: Published by Woodhead Publishing Limited, 2013Подпись: Sodalite group (name of mineral changes with anions sequestered in cage structure) [49, 343, 347-356]Sodalite Na8CI2AI6Si6024 also written as(Na, K)6[AI6Si6024H2NaCI) to demonstrate that 2CI and associated Na atoms are in a cage structure defined by the aluminosilicate tetrahedra of six adjoing NaAISi04; a naturally occurring feldspathoid mineral; incorporate the alkali, alkaline earths, rare earth elements, halide fission products, and trace quantities of U and Pu (sodalite was and is being investigated as a durable host for the waste generated from electro-refining operations deployed for the reprocessing of metal fuel); minor phases in high level waste (HLW) supercalcine waste forms* where they retained Cs, Sr, and Mo, e. g. Na6[AI6Si6024](NaMo04)2; sodalite struc­tures are known to retain B, Ge, I, Br, and Be in the cage-like structures

Nosean, (Na, K)6[AI6Si6024](Na2S04)), silica ‘stuffed derivative’ sodalite cage-type structure host mineral for sulfate or sulfide species.

Hauyne, (Na)6[AI6Si6024]((Ca, Na)S04)1_2 sodalite family; can accommodate either Na2S04 or CaS04

Подпись: (b) (c) Подпись: Structure of Sodalite showing (a) two-dimensional projection of the (b) three-dimensional structure and (c) the fourfold ionic coordination of the Na site to the CF ion and three framework oxygen bonds [357].Helvite (Mn4[Be3Si3012]S : Be can be substituted in place of Al and S2 in the cage structure along with Fe, Mn, and Zn

Continued

 

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Danalite (Fe4[Be3Si3012]S)

Genthelvite (Zn4[Be3Si3012]S)

Lazurite, (Ca, Na)6[AI6Si6024]((Ca, Na)S, S04,CI)x; can accommodate either S04 or S2,Ca or Na and Cl (Na, Ca, K)6[AI6Si6024]((Na, Ca, K)2C03)l 6»2.1H20 only found in hydroceramic waste forms

 

Cancrinite [358]

 

Crystalline SilicoTitanate
(CST) [336, 359-363]

 

[(Ca, Na, K,Ba)AISi04 incorporates Ca, Na, K, Ba, Cs, and Sr

 

Crystal structure of Cs exchanged Nb-titanium silicate. The dark and light grey spheres represent Cs+ cations and water molecules, respectively [from 364].

 

image87"

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The following dehydroxylated micas have been synthesized phase pure: ИА^ізО,,, ІМаАІзЗізОц, КАІзЗізОц, ИЬАІзЗізОц, СвАІзЗізОп, ТІАІзЗІзОц, Сао. БПо. БАЇзЗізОп, Зго. бПо. бАізЗІзОц, Вао. БПо. БАЇзЗізОп, Ьао. ззПо. ббАІзЗізОп. In the Cs-mica up to 30wt% Cs20 can be accommodated, in the Rb-mica up to 22wt% Rb20 can be accommodated, and in the Ra-mica up to 19wt% RaO can be accommodated. Mg, Fe2+, Fe3+, Mn, Li, Cr, Ті and V can substitute for Vl-fold coordinated Al3+.

 

Micas (dehydroxylated)

[365-367]

 

[from 368]

 

Continued

 

image88"

Подпись: Published by Woodhead Publishing Limited, 2013Phosphates

Monazite [218, 274, 369-373] CeP04 or LaP04; very corrosion resistant and can incorporate a

large range of radionuclides including actinides and toxic metals into its structure; it has been proposed as a potential host phase for excess weapons plutonium and as a host phase for radionuclides and toxic metals in glass-ceramic waste forms for low-level and hazardous wastes.

Xenotime [218] YP04

Apatite [10, 54, 45, 218, 274, Ca4xRE6+x(Si04)6y(P04)y(0,F)2; actinide-host phases in HLW glass,

317, 318, 372, 375-384] glass-ceramic waste forms, ceramic waste forms and cement;

image89Подпись: Alternating chains of P04 tetrahedra and REOg polyhedra [from 374]. actinides can readily substitute for the rare earth elements in the crystal structure, as in Ca2(Nd, Cm, Pu)8(Si04)602, and fission products are also readily incorporated. However, the solubility for tetravalent Pu may be limited without other charge com­pensating substitutions; has been proposed as a potential host phase for Pu and high-level actinide wastes.

Sodium zirconium phos­phate (NZP) [218, 386-391]

 

NaZr2(P04)3; structure can incorporate a complex variety of cations, including plutonium; three-dimensional network of corner-sharing Zr06 octahedra and P04 tetrahedra in which plutonium can substitute for Zr, as in Na(Zr, Pu)2(P04)3. Com­plete substitution of Pu4+ for Zr has been demonstrated in NZP. Cs and Sr can substitute for Na while fission products and actinides substitute for Zr in octahedral positions. P is tetrahe­dral.

 

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[from 388]

 

Thorium phosphate diphos­phate (TPD) [218, 392-394]

 

Th4(P04)4P207; a unique compound for the immobilization of plutonium and uranium; partial substitution of Pu for Th has been demonstrated (up to 0.4 mole fraction), complete substi­tution is not possible.

 

Continued

 

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Подпись: Published by Woodhead Publishing Limited, 2013image93Aluminates

Magnetoplumbites [22, 62, Nominally X(AI, Fe)12019, where X = Sr, Ba,(Cs05 + La05) and (Na05 396-399] + La0.5>. The X site is XIl-fold coordinated and both Cs+/

Ba2+-Fe3+/Fe2+ or Cs /Ba2 — Ti47Ti3 — type substitutions can occur. Accommodating structures because they are composed of spinel blocks with both IV-fold and Vl-fold coordinated sites for multivalent cations and interspinel layers which have unusual V-fold sites for small cations. The interspinel layers also accommodate large cations of 1.15-1.84A, replacing oxygen in XIl-fold sites in the anion close packed structure. The large ions may be monovalent, divalent, or trivalent with balancing charge substitutions either in the interspinel layer (Na0i5 + La0i5) or between the interspinel layer and the spinel blocks (Cs+/ Ba2+-Fe37Fe2+ or Cs /Ba2 — Ti47Tr). [16]

Table 6.9 Attributes and examples of encapsulant and embedded waste forms (cements, geopolymers, ceramicrete, hydroceramics, and bitumen)

 

Waste form(s)

 

Encapsulated waste forms

 

Embedded waste forms

 

Liquid waste is mixed with concrete or other binder — hydrated phases occur that can incorporate the radionuclides or hazardous species weakly or retain them by sorption. Example Cs and U sequestered by C-S-H hydrates and U and Tc sequestered by secondary fly ash granules. The remaining species are trapped on the grain boundaries of the interlocking C-S-H phases.

Encapsulation and some chemical incorporation

 

Liquid waste is mixed with concrete of other binder — primary phases and any secondary phases created by hydration (if an active mechanism) do not retain or sorb the radionuclide or hazardous species. Example shows Tc, Cs, U, and Pu all on the grain boundaries.

Encapsulation and no chemical incorporation

 

Description

 

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Radionuclide immobilization mechanism

 

Key

О Phase or binder without radionuclides

* $ I

Cs U Tc Pu xl.

 

(a) low waste loadings, (b) lower overall durability, (c) difficult to model radionuclide release from hydrated secondary phases and grain boundaries, (d) easy to process — usually mix and cast

 

(a) low waste loadings, (b) lower overall durability, (c) difficult to model radionuclide release from grain boundaries, (d) easy to process — usually mix and cast, (e) in case of bitumen, must be heated to flow so embedding can occur

 

Waste loading(s)/durability

 

Mix and pour (cement, geopolymer, ceramicrete), Heat, mix, and pour (bitumen), Mix, pour, cure at slightly elevated temperatures (hydroceramics)

 

Immobilization technologies

 

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image073

Waste form(s) Composite

 

Composite

 

Multiphase granular oxides/minerals/metals (must be monolithed due to disposal requirements if not containerized)

 

Previously made waste forms in need of remediation

(monolithing agents can be numerous and include glass — see GCMs above)

Chemical incorporation and encapsulation/embedding

 

Description

 

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Radionuclide

immobilization

mechanism

Phase or

О binder without radionuclides

* # о — f %

Cs U Tc Pu xl.

 

Chemical incorporation and encapsulation/embedding

 

(a) high waste loadings only if binder (monolithing agent) is minimized, (b) superior overall durability — double containment,

(c) difficult to model radionuclide release from multiple phases,

(d) need to tailor for and determine radionuclide partitioning amongst phases, (e) may require precalcining for certain processes to work efficiently

 

(a) high waste loadings only if binder (monolithing agent) is minimized, (b) superior overall durability — double containment, (c) difficult to model radionuclide release from multiple phases, (d) need to determine radionuclide partitioning amongst phases

 

Waste loading(s)/ durability

 

Immobilization technologies for matrix phase

 

Mix and pour (cement, geopolymer, ceramicrete), Heat and pour (glass or metal), Heat, mix, and pour (bitumen), Mix, pour, cure at slightly elevated temperatures (hydroceramic)

 

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image144

in simulated waste glasses (Table 6.4). These more highly ordered or polym­erized regions of MRO, often have atomic arrangements that approach those of crystals and are often referred to as quasi-crystalline species or quasi-crystals. Crystalline ceramics incorporate radionuclides and hazard­ous species by a combination of SRO, MRO, and LRO. The LRO defines the periodic structural units characteristic of crystalline ceramic structures. In glass, glass-ceramics, glass composite materials (GCMs), and crystalline ceramics, the radioactive and hazardous constituents are atomically bonded by a combination of SRO, MRO, and LRO. In GCMs there is additional encapsulation of the ceramic components in the glass matrix.

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