R.A. Day

Plutonium and neptunium incorporation in zirconolite

The incorporation of Pu and Np in zirconolite (CaZrTi 2 O 7 ) has been investigated over a range of redox conditions. Zirconolite formulations designed to favour either trivalent or tetravalent Pu and Np were prepared by limiting the amount of charge compensating additives available to maintain electroneutrality. From near-edge X-ray absorption spectroscopy the Pu valence state was found to vary with the processing atmosphere, from completely tetravalent when fired in air, and located on either the Ca or Zr sites, to trivalent, when substituted on the Ca site after annealing in 3.5% H 2 /N 2 . Np was predominantly tetravalent over the range of redox conditions examined and was readily incorporated on either of zirconolites Ca or Zr sites. The charge compensation mechanisms at work in different zirconolites are also discussed.

Alteration of Cold Crucible Melter Titanate-Based Ceramics: Comparison with Hot-Pressed Titanate-Based Ceramic

Synroc ceramics were synthesized in an induction-heated cold crucible at laboratory scale (1 kg) from an oxide mixture, and at industrial prototype scale (45 kg) from Synroc previously produced by sintering under load at high temperature. After melting, both materials contained the major phases of Synroc-C. The chemical durability of both melted materials, as determined by static leaching of powder samples in initially pure water at 90°C with an SA/V ratio of 20000m −1 , was equivalent to that of conventional hot-pressed Synroc-C. Cerium, used in this investigation to simulate the presence of tri-and tetravalent actinides, was found in steady-state concentrations on the order of 1 ppb (i.e. NL(Ce) ≤ 10 −6 g·m −2 ). The concentration in the leachates was independent of the initial CeO 2 content of the Synroc (at least up to 10 wt%); moreover, it is similar to the results obtained with hot-pressed Synroc-C specifically formulated for conditioning long-lived actinides.

Leaching Behavior of Zirconolite-Rich Synroc Used to Immobilize High-Fired Plutonium Oxide

This study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu0 2 . The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low ( −4 g m −2 d −1 ) and comparable rates from the waste form.

Further Studies of Synroc Immobilization of HLW Sludges and Tc for Hanford Tank Waste Remediation

Synroc/glass composites were designed for simulated Hanford HLW sludges containing U (the current “All-Blend” formulation). The composite contained ∼ 50 wt% of simulated HLW (oxide equivalent), to which ∼ 6, 10, 10, and 24 wt% of CaO, Al 2 O 3 , TiO 2 and SiO 2 were added, and melted under an argon atmosphere at 1350°C. The phase assemblage consisted of zirconolite, perovskite, spinel, nepheline, whitlockite and glass as major phases. Seven-day PCT tests yielded values of 2 for all elements studied. The PCT results were tolerant to changes of ∼ 20% of the inventories of the additives, and to variations in redox conditions. Technetium separated out during decontamination of liquid Hanford wastes can be incorporated as metal in hot-pressed Synroc prepared under reducing conditions, and its leach resistance is good (∼ 10 ∼3 g/m 2 /day at 70°C), and can be improved by alloying with iron group metals. With a choice of “neutral” (P(O 2 ) ∼ 10 −4 atm., near the Ni/NiO buffer) hot-pressing conditions, Tc can also be incorporated as Tc 4+ , substituting for Ti 4+ in the ceramic phases, and in this form, it should be highly leach resistant also.

Structural Effect of Pu Substitutions on the Zr-Site in Zirconolite

As the level of Pu 4+ substituted on the Zr-site in CaZr 1-x Pu x Ti 2 O 7 zirconolite increased, from x=0.1 to 0.6, a series of structural transitions occurred from zirconolite-2M to zirconolite-4M and subsequently from zirconolite-4M to pyrochlore. The solid-solution limit for Pu 4+ substituted on the Zr-site in zirconolite-2M was ~ 0.15 formula units. Zirconolite-4M was only stable over a narrow compositional range, centered about CaZr 0.59 Pu 0.41 Ti 2 O 7 , whilst the pyrochlore structure was stabilized with CaZr 0.4 Pu 0.6 Ti 2 O 7 stoichiometry. The stability of the zirconolite polytypes is therefore sensitive to the average effective ionic size of the ions occupying the seven-coordinated Zr-site. The reduction in Pu from Pu 4+ to Pu 3+ destabilized the zirconolite-4M, producing a mixture of perovskite and possibly zirconolite-3T. The CaZr 0.4 Pu 0.6 Ti 2 O 7 pyrochlore was also predominantly transformed to perovskite as a result of this reduction of Pu.

Synroc derivatives for the Hanford waste remediation task

Three wt% each of Cs and Tc were mixed with the standard Synroc precursor and the ceramic was formed by hot-pressing. Attempts were made to incorporate the Tc as either metal or Tc{sup 4+}, using different redox conditions in processing. Volatile losses of Tc during calcination were <0.1% in all cases. Short-term Tc leach rates when the Tc was present as a metal alloy were in the order of 10{sup {minus}4} g/m{sup 2}/d at 90 C with frequently changed water, and decreased with increasing leaching time. The valence of the Tc was monitored by X-ray absorption spectroscopy at the drying and calcination stages of the production. The general viability of Synroc/glass composites for immobilizing the Hanford HLW sludges is further demonstrated by using further refinements of additive schemes for the inactive ``All-blend`` formulation and initial studies using the U-containing All-blend waste formulation.

Ionic Size Limits for A Ions in Brannerite (ATi 2 O 6 ) and Pyrochlore (CaATi 2 O 7 ) Titanate Structures ( A = tetravalent rare earths and actinides)

The lower limit of the size of the octahedral A 4+ ion in the ATi 2 O 6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U 5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi 2 O 6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi 2 O 6 showed that the average B site size could tolerate deviations of 4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400°C in the A site of brannerite. Lattice parameter data on different endmember ATi 2 O 6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi 2 O 7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO 6 , whose structure is still uncertain, plus brannerite and rutile.

Excess Pu disposition in zirconolite-rich Synroc containing nepheline

A titanate Synroc ceramic for the immobilization of Pu-bearing waste was designed to consist of 70 wt% zirconolite (CaZrTi{sub 2}O{sub 7}) + 15 wt% nepheline (NaAlSiO{sub 4}) + 15 wt% rutile (TiO{sub 2}). It contained 10 wt% of Pu plus 6 wt% of Gd as a neutron poison. The material was made by the standard sol-gel route, using a mixture of alkoxides and nitrates, followed by stir-drying and calcination. It was fabricated by hot-pressing at 1,150--1,250 C/20 MPa for 2 hours in a collapsible metal bellows. Though zirconolite was the majority phase, {approximately} 20 wt% of perovskite also formed. Some of the Na, intended for nepheline, partitioned into the titanate phases. 84-day differential total leach rates of Pu were in the order of 10{sup {minus}5} g/m{sup 2}/d at 90 and 200 C. Companion ceramics using molar substitution of Ce for Pu confirmed the idea that Ce is a good simulant of Pu from a solid state chemical view, but that there are limitations in terms of leach rate parallels.

Alpha-Spectroscopy and Leach Testing of Synroc Doped With Actinide Elements

Synroc specimens doped individually with {approximately}1 wt% of Np or Pu have been studied, after leaching for >2,500 days, using {alpha}-spectroscopy and SEM. In the last leaching period, the leachant was replaced on either a daily or monthly basis. When the leachant was replaced on a daily basis the release rate of Np was similar to that measured when the samples were first leached, whereas for the monthly replacement the release rates of Np or Pu were about a factor of 20 to 30 lower than that for daily replacement. These findings agree generally with the results obtained from surface examination of the samples which showed that the thickness of actinide-depleted surface layer of anatase was 0.3 {micro}m for monthly replacement but only 0.1 {micro}m when the leachant was replaced frequently. Overall, the results suggest that release of actinides from Synroc are controlled by solubility limiting effects at the surface area to volume ratios employed in MCC-1 tests.