AU0221609

Synroc - Progress and Future Prospects

ADAMJOSTSONS Materials Australian Nuclear Science and Technology Organisation, ANSTO PMB 1 Menai NSW 2234 Australia

SUMMARY. Most of the early development of synroc focused on the synroc-C formulation for immobilising liquid HLW from the reprocessing of commercial LWR spent fuel. Subsequently, ANSTO has responded to developments in R&D on partitioning and transmutation, excess plutonium disposition and the needs of global remediation programs by developing a variety of titanate ceramic waste forms for specific applications. This paper reviews the progress in the development of titanate ceramics and ceramic/glass composites and addresses the relevance of this work in future radioactive waste management strategies.

1. INTRODUCTION The pyrochlore-rich Synroc-F was developed for disposal of non-reprocessed spent LWR fuel by The synroc strategy aims to immobilise Kesson and Ringwood (3) and for CANDU fuel radioactive wastes in durable multiphase by Solomah et al. (4). ceramics, with the phases chosen to be similar to titanate minerals that exist in nature and have 2. WASTE PARTITIONING/ immobilised U and Th for billions of years. CONDITIONING Most of the early development of synroc wasteforms and process technologies focused on Radioactive waste partitioning is usually the Synroc-C formulation for immobilising liquid considered in the context of partitioning and HLW from the reprocessing of commercial LWR transmutation (P/T) strategies aimed at reducing spent fuel. The actinides in Synroc-C partition the long-term potential hazard associated with into zirconolite (CaZrTi2O7) and perovskite HLW destined for geological disposal. P/T (CaTiO3). Perovskite is mainly designed to involves the separation of minor actinides and immobilise strontium and barium hollandite long-lived fission products via advanced (BaAl2Ti6016) is used to immobilise caesium reprocessing and their transmutation into although it also accepts formulation is flexible products of greatly reduced half-lives. Because and a single precursor composition can be used of practical difficulties in achieving high to immobilise HLW loadings in the range O to separation factors and efficient transmutation, 30wt.% without significant effects on the P/T concept have increased interest in chemical durability in aqueous media. This conditioning certain partitioned waste streams in flexibility is due largely to the extended special durable matrices such as synroc (5). solubility of the radionuclides in the titanate phases and the excess of titanium oxide as ANSTO has demonstrated (6, 7) that significant reduced rutile in the formulation. Even at 30wt.% reductions in waste volume for the HLW, the principal host phases, zirconolite, partitioning/conditioning strategy can be perovskite and hollandite, are undersaturated achieved through immobilisation in Synroc-C of with respect to the key radionuclides in HLW the heat-generating radionuclides, i.e. 137Cs, ""Sr, from spent fuel reprocessing. A summary of the 244Cm together with long-lived "Tc whilst the early work on Synroc-C, including process remaining waste is immobilised in glass. demonstration, has been published previously Extended near-surface storage of the Synroc (1). would be required for 100-200 years, while immediate disposal of borosilicate glass Early alternatives to Synroc-C were developed containing the remaining radionuclides could be for defence wastes at the Savannah River Site carried out. The combination of partitioned Cs 244 (Synroc-D) by Ringwood et al. (2). High and Sr with Cm ensures that the effects of a- concentrations of inert contaminants such as decay processes on the wasteform durability are minimised through self-annealing of a-decay NajO, SiO2 and iron oxides, rendered hollandite damage provided by radiogenic decay heat. The thermodynamically incompatible with zirconolite 244 and perovskite in Synroc-D where nepheline was present of Cm, which is not likely to be introduced as the host phase for Cs, Rb and Ba. recycled for transmutation, will become more

45 important in the future with reprocessing of 3.1 Zirconolite-Rich Titanate MOX fuels. Vance et.al., (8) have shown that under the reducing conditions employed in Zirconolite is the most durable of the original Synroc-C fabrication, the Tc losses by synroc phases and hence it is a natural host for volatilisation can be as low as 1 x 10'5 of the actinides in general and plutonium in particular. original inventory. Some of the earlier work at ANSTO on zirconolite-rich actinide wasteforms was A zirconolite-rich Synroc can also be considered summarised by Vance (9) in 1994. for the conditioning of the Am/Cm/rare earth stream from HLW partitioning. The very low As part of our collaborative program with LLNL, release rates from Synroc of Am, Cm and their ANSTO fabricated and characterised zirconolite- long-lived daughters suggest that there may be rich ceramics containing about 10wt.% each of little incentive on cost and radiological benefit hollandite and rutile 13.5wt.% PuO2 as well as grounds for the transmutation of Am and Cm. 10wt.% HfO2, 4 wt.% GdA and 4wt.% Sm2O3. Separation of 24IAm, which is a major source of The hollandite was included in the formulations M7Np, is essential in P/T strategies aimed at as a host phase for radioactive caesium to significantly decreasing the long term provide the gamma radiation source for a radiological risk from geological disposal. homogeneous immobilisation option in contrast Efficient separation from each other of these to the present reference can-in-canister approach. minor actinides and the rare earths on an The Gd and Sm oxides were each incorporated in industrial scale will require significant a 1:2 molar ratio with respect to Pu. The HfO2 technological improvements and could involve content could have been increased to provide additional costs to limit radiological exposures to additional neutron absorption since it only operators of subsequent fuel fabrication plants. replaced on third of the Zr in zirconolite in the above case. Full substitution of Hf for Zr is 3. PU IMMOBILISATION IN TTTANATE possible in zirconolites. CERAMICS These zirconolite-rich, hollandite plus rutile In the aftermath of the cold war, the US and ceramics were produced by two methods, either Russia agreed to large reductions in their by sintering of blended high-fired oxides in air stockpiles of nuclear weapons. The US initiated a for 4 hours at 1375°C or by hot isostatic pressing two-track strategy for the disposition of 50 (HIPing) at 1280°C/150 Mpa of calcined metric tonnes of surplus plutonium: alkoxide route powders in which the Pu was introduced as a nitrate solution prior to • using the plutonium as mixed-oxide fuel in calcination at 750°C in argon. The HEPed 3 existing commercial power reactors; and material was fully dense (~ 5.1 g/cm ) with all the Pu incorporated in the zirconolite which had • geologic disposal of plutonium immobilised the composition in a pyrochlore-rich titanate ceramic Ca0.54Pu0.3jGdo.,,Sm0.,,Zr0.2Hf0.4gTi,.73Al0.3407. The sintered material had a grain size of ~ 4 \lm in ANSTO began work on Pu immobilisation in the HIPed material and a density of ~ 93% of 1994 with Lawrence Livermore National theoretical. Less than 2% of the Pu inventory Laboratory, California, USA, the local laboratory remained atomically unincorporated in the for US DOE on Pu disposition. The initial effort sintered zirconolite - as a reacted oxide (Hf, was on zirconolite-rich formulations but with the REE, Pu) O2. change in US DOE programmatic requirements shifted to pyrochlore-rich titanate ceramics. 3.2 Pyrochlore-Rich Titanates

The avoidance of nuclear criticality is essential After the preliminary compositions of actual during wasteform processing, during its storage, waste streams for immobilisation became and over the very long term in the repository available, it was apparent (hat the feed streams environment. Avoidance of nuclear criticality is had on average about equal amounts of uranium ensured in Pu-rich titanate wasteforms by having and plutonium. This uranium content could be high loadings of neutron absorbers such as Gd, incorporated in a zirconolite-rich ceramic Sm and Hf in solid solution in the Pu host together with the Pu but with a significant loss of phases. Such conditions can be met by the rare earth neutron absorbers because of total zirconolite-rich and pyrochlore-rich titanates. solubility limits. Consequently, it was decided to Long-term criticality issues in a repository focus on a pyrochlore-rich wasteform in which: environment will be addressed later.

46 • uranium to plutonium ratio would be 2:1. waste remediation in USA. About 240,000 High 23SU loadings would ensure additional metric tonnes of process chemicals contaminated criticality control through limitation of the with about 250 MCi of radioactive elements are long term 235U23SU ratio with the decay of239 currently stored at Hanford in 177 steel tanks. Puto23^. These wastes are dominated by sodium nitrate and nitrites. Significant incentives exist to • gadolinium-to-plutonium mole ratio was 1:1. minimise the volume of conditioned HLW • hafnium to plutonium mole ratio would be because repository space is limited. Waste 1:1. loadings in the currently accepted borosilicate glass are about 20wt.%. The baseline waste form was designed to consist of 9 5 w t . % pyrochlore Our strategy is first to develop formulations that (Ca0.89Gd0.22Hf0.23U0.44Pu0.22Ti2O7) and 5wt.% of a will yield some of the well-known crystalline hafnium substitute rutile (Hf0 ,Ti09O2). The actual synroc phases such as zirconolite and perovskite product, formed after sintering for 4 hours at in a glass matrix. These titanate phases are 1350°C, contains some brannerite (UTi2O6) designed to act as hosts for the long-lived which also contains Pu, Gd and Hf. Similar to actinides in the waste. Secondly, the glass the zirconolite-rich titanates mentioned above, a matrix is designed so that the partitioning of the small (<2%) amount of the Pu inventory long-lived actinides into the glass is rninimal. remained unincorporated in the sintered We have demonstrated (10, 11) that waste pyrochlore made from high-fired oxides, as a loadings in the range 50-70 wt.% are feasible in reacted oxide (Hf,REE,Pu,U)O2, whereas in such composite wasteforms and that they possess materials made from alkoxide powders the good aqueous durability. Such high waste actinides were fully dissolved in the titanate loadings require melting temperatures of 1300- phases. The theoretical density of the baseline 1400°C that are not feasible in current joule 3 pyrochlore ceramic is about 6.0 gm/cm and the heated melters. Instead cold crucible melters observed density of ceramics sintered for 4 hours such as being developed in France (12) and 3 at 135O°C is about 5.6 gm/cm . Russian (13) would be required to process synroc/glass composites. The mineralogy and partitioning of Pu, U and the neutron absorbers among the phases in the Current development of synroc/glass composites pyrochlore-rich ceramic have been studied and various synroc for production by melting is extensively. It is influenced by impurities in the directed at waste streams at INEEL, Idaho, US Pu feed stocks destined for immobilisation. The and for wastes arising from partitioning. plutonium assay in the candidate materials varies from under 10wt.% to over 99wt.%. The 5. PROCESS DEVELOPMENT uranium content varies from trace depleted uranium in plutonium to trace plutonium in fully a5 The feasibility of synroc fabrication was enriched (93% U) uranium. In general, the demonstrated with non-radioactive HLW impurities in the existing feed stock include: surrogates at industrial scale in the Synroc aluminium, carbon, calcium, chlorine, iron, Demonstration Plant (SDP). In this plant a slurry fluorine, gallium, potassium, magnesium, of simulated HLW and synroc precursor powders molybdenum, sodium, silicon, tantalum, tungsten was calcined under reducing atmosphere to and zinc. The observed volume abundances of produce a free flowing powder. The powder was pyrochlore (60-90%), brannerite (0-22%), then uniaxially hot-pressed in bellows containers zirconolite (0-25%) and rutile (0-16%) depend on at around 1150°C. The synroc produced in the impurities in the various sources of Pu. plant was equal in quality to laboratory scale specimens. The UOj/PuO2 ratios do show some variations amongst the phases. Pyrochlore and brannerite ANSTO has subsequently examined alternative have UO2/PuO2 ratios close to 2:1 whereas it is unit operations for applications to specific waste about 1:3 in zirconolite. All primary Pu- streams. Dry feed blending and calcination, containing phases have been found to including HLW impregnated sol-gel spheres accommodate more neutron absorbers (Gd and allows the front-end footprint to be reduced from Hf) than plutonium on a molar basis as per the that in the SDP. Microwave and fluid bid original design. calcination has also been investigated. Synroc powder consolidation has also been demonstrated 4. SYNROC/GLASS COMPOSITES by hot isostatic pressing as well as cold press and sinter routes. The latter was essential for the Pu ANSTO's interest in synroc/glass waste form immobilisation program where a decision was development initially derived from Hanford tank

47 made by the US DOE to adapt existing MOX been defined by the DOE, slippages are still fuel technologies to Pu immobilisation. This also occurring. led to the development of appropriate dry milling/blending technologies using high-speed A subsidiary company, ANSTO Inc, was formed attritors. in the United States to develop strategic alliances with major nuclear engineering and technology Laboratory scale cold crucible melters have been organisations active in radioactive waste developed to test formulations of synroc/glass management projects. ANSTO Inc formed a composite waste forms. limited liability company with COGEMA Inc to submit a proposal in July 2000 to the USDOE for 6. DURABILITY the design of a Pu immobilisation facility at Savannah River Site, USA, with Burns and Roe Extensive laboratory tests of polyphase synrocs and Batelle. In April 2001 the USDOE and single-phase materials have been carried out announced that because of budgetary pressures it over the previous 20 years at ANSTO and in was suspending work on the immobilisation of laboratories overseas. Most of these experiments excess plutonium and procurement process for have been carried out at temperatures of 70- the design of the PIP facility. The work will be 150°C in deionised water, sodium chloride, resumed when the construction of the Mixed bicarbonate or silicate solutions. Extensive Oxide (MOX) Fuel Fabrication Facility and the investigations on zirconolite, the most durable of Conversion Facility in the United States are the synroc phases, have been carried out by further along. Malmstrom (14) under hydrothermal conditions 150-700°C, at pressures of 50 and 200 MPa. In ANSTO continues to build on the good linkages general, pyrochlore is the next most durable established with the US DOE national phase to zirconolite followed by perovskite and laboratories during the Pu immobilisation R&D brammerite. Titanates fabricated by ceramic program. These laboratories provide the key technologies (hot pressing or sintering) under inputs for wasteform regulatory acceptance. controlled redox conditions are more durable R&D linkages also continue with CEA, France than melted products at this point in their on comparison of cold-crucible melted titanates development. These laboratory studies, involving with those produced by ceramic processes in the leaching tests beyond 3000 days, continue to be context of partitioning/conditioning programs in supported by studies of natural minerals of the France. Other international collaborations are at relevant synroc phases from known geological various stages addressing the feasibility of environments. A comprehensive review of the immobilisation of specific waste streams using durability of actinide host phases in natural either solid-state ceramic processes or melting systems has been recently published by Lumpkin routes. Increasingly our niche is one of a (15). specialist waste form developer that can provide solutions for specific waste stream 7. CONCLUSIONS immobilisation in partnership with relevant nuclear engineering companies. The synroc strategy for the immobilisation of radioactive wastes in durable polyphase 8. ACKNOWLEDGMENT ceramics, similar to resistate titanate minerals, provides flexible solutions adaptable to treatment The author acknowledges the outstanding of a variety of radioactive waste streams. All contributions of his colleagues at ANSTO in the variants of synroc are underpinned by an development of durable synroc-based titanate extensive database from laboratory studies and waste forms for the safe disposition of an ever-increasing knowledge of the durability of radioactive wastes. natural mineral analogs. The technologies for synroc production can be varied depending on 9. REFERENCES the nature of the waste to be immobilised using traditional ceramic process steps or melting. 1. A. Jostsons, "Status of Synroc Development", proc. of 9th Pacific Basin The evolution of the directions of ANSTO's Nuclear Conference, Sydney, 1-6 May, synroc R&D programs reflects the developments 1994, N.R. McDonald, Ed., pp 865-871, in waste management policies globally. The near (1994). term opportunities for synroc-based wasteforms are in the US defence waste remediation 2. A.E. Ringwood, S.E. Kesson and N.G. including Pu disposition. Although the time Ware, "Immobilisation of US Defence frames for construction of new facilities have Nuclear Wastes Using the Synroc Process",

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