Murataite-based ceramics produced by melt crystallization from a modified, zirconia-enriched batch (50 wt% TiO2, 10 CaO, 10 MnO, 5 Al2O3, 5 Fe2O3, 10 ZrO2, plus 10 wt% rare-earth or Th/U oxide) are candidate hosts for the actinide and rare-earth fractions of high-level radioactive waste. The source study left their scientific qualification incomplete: uranium in the ceramics is close to U(IV) by U L3-edge XANES but is not a simple U(IV)/U(VI) mixture, and its valence and local environment remain undetermined (surface XPS later suggested roughly equal U(IV) and U(V)); Raman bands cannot be assigned to specific cation valences and coordinations because perovskite, crichtonite, and zirconolite bands overlap; corrosion durability rests only on preliminary 90/150 C leach data; and perovskite, which concentrates Ce-group lanthanides and degrades durability, cannot be avoided under the studied chemistry. These issues remain unresolved in the literature, so the qualification problem remains open.

Background

High-level radioactive waste (HLW) from spent-nuclear-fuel reprocessing contains long-lived actinides (U, Np, Pu, Am, Cm), rare-earth (REE) fission products, and corrosion products that must be converted into durable solids for geological disposal. Because the borosilicate and aluminophosphate glasses now used industrially tolerate only limited waste loadings, crystalline titanate ceramics of the Synroc family (Synroc denotes the 'synthetic rock' ceramic hosts developed following Ringwood's proposal) have been studied since the late 1970s as higher-loading alternatives. Murataite is a rare cubic complex titanate; its synthetic counterpart is a fluorite-derivative structure (fluorite denotes the CaF2-type lattice from which many actinide host phases are derived) built as a superstructure of the fluorite subcell. It occurs as polysomes M3, M5, and M8, whose unit cells are 3-, 5-, and 8-fold multiples of the fluorite subcell, with an ideal composition near A4B2C7O22 (A = Ca, Mn, REE, actinides in eightfold coordination; B = Mn, Ti, Zr, tetravalent actinides; C = Ti, Al, Fe). This lets one structure accommodate cations of charge +2 to +5 over a wide radius range, which is the basis of its use as an actinide host. Russian groups (Sobolev, Laverov, Stefanovsky, Yudintsev, and co-workers) showed from 1997 onward that melt-crystallized murataite ceramics accept about 10 wt% actinide oxides, have radiation resistance comparable to titanate pyrochlore (pyrochlore is another fluorite-derivative actinide host phase), and exhibit low actinide leach rates; X-ray photoelectron spectroscopy (XPS, a surface-sensitive probe of core-level binding energies that reveals oxidation states) had been applied to Th, U, Np, and Pu in murataite as early as 2012 (Maslakov et al., Radiochemistry 54 (2012) 115).

The source study (Stefanovsky et al., Journal of Nuclear Materials 529 (2020) 151958) examined a modified, zirconia-enriched, slightly titania-depleted batch - 50 wt% TiO2, 10 CaO, 10 MnO, 5 Al2O3, 5 Fe2O3, 10 ZrO2, plus 10 wt% RE oxide (La, Ce, Nd, Ho) or actinide oxide (Th, U) - crystallized from a melt at 1500 C, following the group's companion study of the Ti-richer baseline composition (Yudintsev et al., Journal of Nuclear Materials 517 (2019) 371, on murataite-crichtonite phase formation; crichtonite is another complex titanate accessory phase). X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray analysis (SEM/EDX) showed polyphase ceramics: murataite polysomes major (75-90 vol% in the Th- and U-bearing samples, where perovskite was nearly suppressed), perovskite and crichtonite secondary, zirconolite/pyrochlore minor. Cerium-group light rare earths concentrate in perovskite, while Nd and Ho and the actinides U and Th enter mainly the murataite, which forms zoned grains with M5/M8 cores and M8/M3 rims; U and Zr contents decrease from the M5 to the M3 polysome. X-ray absorption near-edge spectroscopy (XANES, in which the position and shape of an element's absorption edge encode its valence) at the U L3L_3 edge showed uranium close to U(IV) with line shapes not matching a simple U(IV)/U(VI) mixture - a possible U(V) contribution - independent of whether the batch contained UO2 or UO3; cerium was retained as Ce(III) from either Ce2O3 or CeO2. Preliminary 28-day deionized-water leach tests gave uranium release rates of about 6×10−76\times10^{-7} g/(cm^2 day) at 90 C and 1.2×10−61.2\times10^{-6} g/(cm^2 day) at 150 C. No published study to date has resolved any of these four issues.

Problem Statement

The problem, as left open by the characterization study of the modified murataite ceramics and still standing in the subsequent literature, is to complete the scientific qualification of these ceramics as a host for the actinide and rare-earth fractions of HLW, under one common objective: a validated, phase-assemblage-controlled murataite waste-form specification. Four linked issues define the open core, each feeding the same qualification case for the same ceramics (melt-crystallized batches of 50 wt% TiO2, 10 CaO, 10 MnO, 5 Al2O3, 5 Fe2O3, 10 ZrO2, plus 10 wt% RE or Th/U oxide):

  1. Uranium speciation. Determine the valence state (the U(IV), U(V), and U(VI) fractions) and the local structural environment (coordination, bond lengths, occupied cation sites, and host polysome) of uranium incorporated in the murataite ceramics, resolving why bulk U L3L_3-edge XANES indicates uranium close to U(IV) yet not a simple U(IV)/U(VI) mixture, while surface XPS indicates approximately equal U(IV) and U(V).

  2. Vibrational fingerprint. Assign the Raman spectra of the murataite-based ceramics to specific cation valences and coordination states, which the source study could not do because the bands of coexisting perovskite, crichtonite, and zirconolite/pyrochlore overlap those of the murataite polysomes.

  3. Chemical durability. Establish the corrosion behavior of the Zr-enriched modified compositions beyond the preliminary 28-day data - leach rates of the actinide-bearing ceramics under standardized test protocols and their dependence on temperature and on the phase assemblage (in particular the perovskite fraction).

  4. Phase-assemblage control. Demonstrate a chemistry or processing route that avoids perovskite formation (perovskite being the phase that concentrates Ce-group lanthanides and degrades leach durability) and yields a quasi-monophasic murataite ceramic, that is, an assemblage in which perovskite no longer forms as a separate phase.

These issues are interlinked within one qualification objective: the speciation (item 1) constrains the charge-balance and redox model of the waste form; the Raman fingerprint (item 2) would provide non-destructive production control; the durability data (item 3) establish release behavior; and the phase-assemblage control (item 4) addresses the composition and processing constraints under which the other results apply. Any redesigned chemistry or processing route demonstrated under item 4 carries its own re-qualification: the redesigned ceramic must itself be characterized for uranium speciation in its own local environment, for assignment of its own Raman bands, and for its own leach durability.

Qualification is complete only when all four items are resolved; resolving a single item, however deeply, does not complete it, and each item is required. Item 1 is resolved by a determined valence distribution and local structural environment of uranium that accounts for both the bulk U L3L_3-edge XANES result and the surface XPS result; item 2 by a phase-resolved assignment of the Raman bands to specific cation valences and coordination states; item 3 by leach rates of the actinide-bearing compositions measured under a standardized test protocol, such as MCC-1 or PCT, over a stated temperature range and as a function of the perovskite fraction; and item 4 by a demonstrated perovskite-suppressed route whose product has a determined uranium valence and local structural environment, a phase-resolved assignment of its own Raman bands, and leach rates measured under a standardized test protocol over the stated temperature range at that product's own phase assemblage.

References

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