Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li–Cs, Tl, Ag, Be–Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98–99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.


Introduction
Crystalline ceramics, aiming to immobilize high-level radioactive waste (HLW), are important for the current stage of development of modern nuclear technology in the world.
The crystal-chemical principle is used to design multicomponent ceramics with needed structures. The approach to designing mineral-like crystalline materials is based upon the structural features of materials and isomorphism concept. The choice of the structural forms of compounds for discussion here was based upon the following criteria: (1) The ability of the structure to include various cations in different combinations and ratios.
(2) Known high stability of structure to the action of the destructive factors of the environment during their prolonged exposure ("mineral-like" compounds preferred while "the nature suggests") such as high temperatures, thermal "stresses", radiation levels, the corrosive action of water and other chemical solutions. Criteria for the resistance of materials to such effects are justified by

Theoretical Aspects of Substitution
The crystal-chemical substitutions in crystalline waste-forms must be electrically balanced [45,46] which is important when relying on the long range order (LRO) of crystals accounting for the size and coordination of the crystallographic site, which will act as host to a given radionuclide, or its decay product upon transmutation (see [15] for natural analogs). Moreover, if a monovalent cation transmutes to a divalent one, the substitutions must be coupled to retain the electrical balance of the host phase without destroying the integrity of the phase. It means that the lattice site must be of suitable size and have a bond coordination able to accept the cation resulting from transmutation. The bond system of a crystalline ceramic can only maintain its charge balance if: (1) Sufficient lattice vacancies exist in the structure or, (2) A variable valence cation such as Fe or Ti is present in a neighboring lattice site balancing the charge.
Both above ways assume that the variable valence cations do not change lattice sites, and that the charge balancing cations are in the nearby lattice sites of the same host phase. The lattice site must be of close size flexible enough to accommodate the transmuting cation. Better flexibility is characteristic to host phases with lattice sites having irregular coordination or are distorted, as shown in some examples below. The flexibility (solubility) of waste-form mineral phase(s) as hosts for a different valence substituted cation can be analyzed by performing coupled substitutions. When the number of cations changes during the substitution, a vacancy is either created or consumed, however the substitution must maintain electrical neutrality. These types of substitution are characteristic for polymorphic changes such as [47], where denotes a vacancy: + Ba 2+ → 2K + , or + Ca 2+ → 2Na + , or + Na + + 2Ca 2+ → 3Na + + Ca 2+ In these coupled substitutions it is implicit that the exchanging cations occupy the same lattice sites, have the same coordination, and thus the crystallographic symmetry is maintained. These substitutions are typically written using Roman numerals that designate the number of oxygen atoms that coordinate around a given cation, e.g., VIII [45][46][47]. Ca 2+ is normally in VIII-fold coordination in the apatite and has a 1.12 Å atomic radius [47][48][49][50]. The Nd 3+ cation in VIII-fold coordination also has an atomic radius of 1.11 Å [50], which is very close to the Ca 2+ atomic radius in VIII-fold coordination. It has been shown that the rare earth elements from La 3+ through Lu 3+ can substitute for Ca 2+ and form oxyapatites,  [51]. It was also shown [3] that even more complex but coupled substitutions were possible in the oxyapatite structure, such as: where the atomic radius, r, of Cs + in VIII-fold coordination is 1.74 Å, Ce 4+ in VIII-fold coordination is 0.97 Å, and Sr 2+ in VIII-fold coordination is 1.26 Å. In this case small radii cations e.g., Ce 4+ are mixed with larger radii cations such as Cs + , so that individual lattice sites can distort without perturbing the entire crystal structure of the host mineral. It should be noted that the exchanging cations are always in the same lattice site of the same host phase [3,45,46,51].
The substitutions such as those given above for the oxyapatites were also demonstrated to be possible in many other Ca-bearing mineral phases such as larnite (Ca 2 SiO 4 or b-C 2 S), alite (calcium trisilicate or Ca 3 SiO 5 or C 3 S), C 3 A (Ca 3 Al 2 O 6 ) and C 4 AF (Ca 4 Al 2 Fe 2 O 10 ), characteristic for cements [45,46]. This allowed Jantzen, et. al. [52,53] to make substitutions for Ca 2+ in each phase (up to~15 mole%) and prove possible the following additional substitutions: It should be noted that the number of lattice sites have to be equivalent on the left-hand side and right hand site of the above equations.
These types of crystal-chemical substitutions have been studied in several waste-forms including SYNROC (SYNthetic ROCk) titanate phases containing zirconolite (CaZrTi 2 O 7 ), perovskite (CaTiO 3 ), and hollandites (nominally Ba(Al,Ti) 2 Ti 6 O 16 ) [54], and in high alumina-tailored ceramic phases such as magnetoplumbites. Notable that magnetoplumbites were also found as a minor component of SYNROC, which immobilizes waste with high contents of Al [55].
Monoclinic CaZrTi2O7, has a fluorite-derived structure closely related to pyrochlore, where Gd, Hf, Ce, Th, U, Pu and Nb may be accommodated on the Ca/Zr-sites, as in the case of Ca(Zr,Pu)Ti2O7. Structure: Trigon., Pr. gr. C2/c. Ceramics were prepared by cold pressing and sintering. CaTiO3 has a wide range of compositions as stable solid-solutions; orthorhombic; consists of a 3-dimensional network of corner-sharing TiO6 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)TiO3. The octahedra can also tilt to accommodate larger cations in the Ca site. Structure: Cubic, sp. gr. Pm3m; rombohedral, Sp. gr. Pnma; may include: Ca, Y, REE, Ti, Zr, U and Pu. Ceramics were prepared by cold pressing and sintering, and by hot pressing enabling densities up to 90-98% of theoretical. 6. Perovskite [110,134,140,[151][152][153][154][155][156][157][158][159], Figure 6. CaTiO 3 has a wide range of compositions as stable solid-solutions; orthorhombic; consists of a 3-dimensional network of corner-sharing TiO 6 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)TiO 3 . The octahedra can also tilt to accommodate larger cations in the Ca site. Structure: Cubic, sp. gr. Pm3m; rombohedral, Sp. gr. Pnma; may include: Ca, Y, REE, Ti, Zr, U and Pu. Ceramics were prepared by cold pressing and sintering, and by hot pressing enabling densities up to 90-98% of theoretical. 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)TiO3. The octahedra can also tilt to accommodate larger cations in the Ca site. Structure: Cubic, sp. gr. Pm3m; rombohedral, Sp. gr. Pnma; may include: Ca, Y, REE, Ti, Zr, U and Pu. Ceramics were prepared by cold pressing and sintering, and by hot pressing enabling densities up to 90-98% of theoretical.    8. Garnet [87,89,104,105,, Figure 8.
where X is the charge balancing counter-ion, n is the charge of the counterion, x is the number of charge-deficient alumina sites, and y is the number of charge-neutral silica sites. Zeolites are characterized by internal voids, channels, pores, and/or cavities of well-defined size in the nanometer range, ≈4-13 Å. The channels and/or cavities may be occupied by charge compensating ions and water molecules. Zeolites like Ag-Mordenite selectively sorbs I2 ( 129 I); certain zeolites can be converted to condensed oxide ceramics by heating. This process is particularly attractive for waste-form synthesis because contaminants capture and immobilization is performed with minimal steps. Structure of Zeolite-A showing alternate Al and Si atom ordering but omitting the tetrahedral oxygens around each Al and Si may include Na, K, NH4 + , Cs, Mg, Ca, Sr, Co, Fe, Ga, REE and Ti. 45 natural zeolites and 100 artificial ones are known. Ceramics were prepared by hot pressing.  [75,[250][251][252][253][254][255][256][257][258][259][260][261][262][263][264][265][266], Figure 16.

Zeolites
(X x/n [(AlO 2 ) x (SiO 2 ) 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 y is the number of charge-neutral silica sites. Zeolites are characterized by internal voids, channels, pores, and/or cavities of well-defined size in the nanometer range, ≈4-13 Å. The channels and/or cavities may be occupied by charge compensating ions and water molecules. Zeolites like Ag-Mordenite selectively sorbs I 2 ( 129 I); certain zeolites can be converted to condensed oxide ceramics by heating. This process is particularly attractive for waste-form synthesis because contaminants capture and immobilization is performed with minimal steps. Structure of Zeolite-A showing alternate Al and Si atom ordering but omitting the tetrahedral oxygens around each Al and Si may include Na, K, NH 4 + , Cs, Mg, Ca, Sr, Co, Fe, Ga, REE and Ti.
zeolites can be converted to condensed oxide ceramics by heating. This process is particularly attractive for waste-form synthesis because contaminants capture and immobilization is performed with minimal steps. Structure of Zeolite-A showing alternate Al and Si atom ordering but omitting the tetrahedral oxygens around each Al and Si may include Na, K, NH4 + , Cs, Mg, Ca, Sr, Co, Fe, Ga, REE and Ti. 45 natural zeolites and 100 artificial ones are known. Ceramics were prepared by hot pressing.  17. Pollucite [37,87,212,214,215,259,, Figure 17.   Figure 19.
(1) Sodalite Na8Cl2Al6Si6O24, also written as (Na,K)6[Al6Si6O24]·(2NaCl) to demonstrate that 2Cl and associated Na atoms are in a cage structure defined by the aluminosilicate tetrahedra of six adjoining NaAlSiO4, is a naturally occurring feldspathoid mineral. It incorporates   Figure 19.
Ca4-xRE6+x(SiO4)6-y(PO4)y(O,F)2 can be actinide-host phases in HLW glass, glass-ceramic wasteforms, ceramic waste-forms and cements. The actinides can readily substitute in apatite for rare-earth elements as in Ca2(Nd,Cm,Pu)8(SiO4)6O2, and fission products are also readily incorporated. However, the solubility for tetravalent Pu may be limited without other charge compensating substitutions.
Ca 4-x RE 6+x (SiO 4 ) 6-y (PO 4 ) y (O,F) 2 can be actinide-host phases in HLW glass, glass-ceramic waste-forms, ceramic waste-forms and cements. The actinides can readily substitute in apatite for rare-earth elements as in Ca 2 (Nd,Cm,Pu) 8 (SiO 4 ) 6 O 2 , and fission products are also readily incorporated. However, the solubility for tetravalent Pu may be limited without other charge compensating substitutions.
The first studies of materials with such a structure were carried out by the authors [379][380][381][382][383] in 1976-1987. They substantiated the crystal-chemical approach when choosing the composition of substances and their structural modifications with ion-transforming properties (Li+, Na+, etc.): NASICON, Langbeinite. Such materials have a frame structure: Na 1 + x Zr 2 Si x P 3-x O 12 , Na 3 M 2 (PO 4 ) 3 (M = Sc, Cr, Fe), Na 5 Zr(PO 4 ) 3 , Li x Fe 2 (WO 4 ) 3 , Li x Fe 2 (MoO 4 ) 3 . Elements in oxidation states 3-6 were introduced into the frame positions: Sc, Cr, Fe, Si, Zr, P, W and Mo. It was also the first time in 1987 that the rationale for the use of such structural analogs for the consolidation of HLW and transmutation of minoractinides [384] was presented. The development of such materials-Structural analogues of NASICON, NZP, Langbeinite-and their research, was continued in subsequent years. NaZr 2 (PO 4 ) 3 . The NZP structure can incorporate a complex variety of cations, including plutonium; a three dimensional network of corner-sharing ZrO 6 octahedra and PO 4 tetrahedra in which plutonium can substitute for Zr, as in Na(Zr,Pu) 2 (PO 4 ) 3 . Complete substitution of Pu 4+ 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. Phosphates with the mineral kosnarite structure (NaZr 2 (PO 4 ) 3 type, NZP) form a wide family. They can contain various cations in the oxidation state from 1+ to 5+. The structure consists of several positions and so many various cations can occupy it. These are MI = Li, Na, K, Rb, Cs; H, Cu(I) and Ag; MII = Mg, Ca, Sr, Ba, Mn, Co, Ni, Cu, Zn, Cd and Hg; MIII = Al, Ga, In, Sc, Y, La, Ce-Lu, Am, Cm, V, Cr, Fe, Sb and Bi; MIV = Ge, Sn, Ti, Zr, Hf, Mo, Ce, Th, U, Np and Pu; MV = Sb, Nb and Ta. Structure: Rhombohedral, Sp. gr. R 3 c, R3c, R3. This fact is extremely important, and can be useful for the synthesis of single-phase crystalline products of the solidification of radioactive waste whose cationic composition, as a rule, is extremely complicated. Ceramics were prepared by cold pressing and sintering (ρ = 80-98%), hot pressing (ρ = 96%) and Spark Plasma Sintering with high relative density (up to 98-99.9%).
that the rationale for the use of such structural analogs for the consolidation of HLW and transmutation of minoractinides [384] was presented. The development of such materials-Structural analogues of NASICON, NZP, Langbeinite-and their research, was continued in subsequent years. NaZr2(PO4)3. The NZP structure can incorporate a complex variety of cations, including plutonium; a three dimensional network of corner-sharing ZrO6 octahedra and PO4 tetrahedra in which plutonium can substitute for Zr, as in Na(Zr,Pu)2(PO4)3. Complete substitution of Pu 4+ 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. Phosphates with the mineral kosnarite structure (NaZr2(PO4)3 type, NZP) form a wide family. They can contain various cations in the oxidation state from 1+ to 5+. The structure consists of several positions and so many various cations can occupy it. These are MI = Li, Na, K, Rb, Cs; H, Cu(I) and Ag; MII = Mg, Ca, Sr, Ba, Mn, Co, Ni, Cu, Zn, Cd and Hg; MIII = Al, Ga, In, Sc, Y, La, Ce-Lu, Am, Cm, V, Cr, Fe, Sb and Bi; MIV = Ge, Sn, Ti, Zr, Hf, Mo, Ce, Th, U, Np and Pu; MV = Sb, Nb and Ta. Structure: Rhombohedral, Sp. gr. , R3c, R3. This fact is extremely important, and can be useful for the synthesis of single-phase crystalline products of the solidification of radioactive waste whose cationic composition, as a rule, is extremely complicated. Ceramics were prepared by cold pressing and sintering (ρ = 80-98%), hot pressing (ρ = 96%) and Spark Plasma Sintering with high relative density (up to 98-99.9%). 27. Langbeinite [18,87,89,211,293,[416][417][418][419][420], Figure 27.
Phosphates with the whitlockite structure (analog β-Ca 3 (PO 4 ) 2 ) were proposed as matrices for radioactive waste immobilization. Their origin is both biogenic and cosmogenic. Whitlockite samples from meteorites, rocks of the Moon, Mars and other cosmogenic bodies, preserve the crystalline form under the action of natural thermal "stress" and cosmic radiation. They contain small amounts of uranium and thorium, and it is presumed to contain plutonium. It is known to form isostructural compounds with H, Li, Na, K, Cu, Mg, Ca, Sr, Ba, Al, Sc, Cr, Fe, Ga, In, La, Ce, Sm, Eu, Gd, Lu, Th and Pu. Thermal stability is up to 1200 • C, thermal expansion up to 1 × 10 −5 deg −1 (25-1000 • C) are close to Synroc and zirconolite; hydrothermal stable -leach rates at 90 • C up to 10 −5 g·sm −2 ·day −1 , radiation stable. Structure: Trigonal, Sp. gr. R3c. Ceramics were prepared by cold pressing and sintering (ρ = 92-97%) and Spark Plasma Sintering with high relative density (up to 95-98%).
Materials with the structure of the scheelite mineral (calcium tungstate CaWO4) based on individual molybdates and tungstates and solid solutions may contain elements in oxidation degrees from 1+ to 7+: Li, Na, K, Rb, Cs and Tl; Ca, Sr, Ba, Mn and Cu; Fe, Ce, La-Lu and Y; Th, U, Np and Pu; Nb, Ta-in Ca-positions and Mo, W, Re, I, V and Ge in W-positions. The structural analog CaWO4 crystallizes in the tetragonal structure, Sp. gr. I4/c. The structure is constructed of CaO8 polyhedral and WO4 tetrahedrals connected through common oxygen vertices. For some compounds ceramics were prepared by the Spark Plasma Sintering (SPS) method, with a relative density of 92%.

Summary of Crystalline Ceramic Waste-forms
Crystalline materials including oxides-simple and complex, salts-silicates, phosphates, tungstates with various compositions and different structural modifications (30 structure forms) intended for nuclear waste immobilization were developed using various approaches and accounting for criteria of enough high durability (see e.g., [15,238,[458][459][460]) requested for nuclear wasteforms. These are presented in Table 1.
From the analysis of the presented data of various compounds with various compositions and structural forms it is clear that researchers in the field of materials for nuclear waste immobilization have many variants available for work. While materials are mineral-like the principle "from nature to nature" can be realized. Although many structures were included herewith, some could be missed, for example brannerite [15,99], which is currently considered for actinide immobilization [461]. Among most investigated structures one can note oxide ceramics. Some of crystalline ceramics such as monazite were synthesized using real (radioactive) actinides [15,235], whereas most of researchers use surrogate (non-radioactive) cations for investigations.

1.
Ceramic waste-forms for nuclear waste immobilization are investigated in different countries with a focus on improving environmental safety during storage, transport and disposal.

2.
Inorganic compounds of oxide and salt character, having structural analogs with natural minerals, are being studied as most perspective materials for the immobilization of radioactive waste.

3.
Approaches based on crystallochemistry principles are used when choosing the most favorable structural forms. They are based on the materials science concept "composition-structure-method of synthesis-property" accounting for the real task to be achieved. The basic principle is the isomorphism of cations and anions in compounds when choosing a real structure. Possible isomorphic substitutions in both cationic and anionic structural sites were considered in the works analyzed.

4.
Crystalline ceramic waste-forms are intended to increase the environmental safety barrier when isolating radioactive materials (containing both actinides and fission products) from the biosphere. Among the methods of obtaining ceramic waste-forms, special attention in recent years is paid to sintering methods which ensure the formation of ceramics that, first, are almost non-porous e.g., have a relative density of up to 99.0-99.9% of theoretical, and, second, can be obtained within a small processing time e.g., within a few minutes (i.e., 2-3 min). These requirements are met by high-speed electric pulse sintering processes e.g., so-called Spark Plasma Sintering (SPS), although hot pressing enables the synthesis of very dense ceramics as well.
Professor Albina Orlova is working in the field of new inorganic materials used in nuclear chemistry for the rad-waste immobilization of dangerous isotopes, for actinide transmutation, as well for construction materials. She uses the structure properties and physico-chemical principles for the elaboration of new ceramics with mineral-like crystal forms.
Professor Michael Ojovan is known for the connectivity-percolation theory of glass transition, the Sheffield model (two-exponential equation) of viscosity of glasses and melts, condensed Rydberg matter, metallic and glass-composite materials for nuclear waste immobilization, and self-sinking capsules to investigate Earth's deep interior.