Synroc
| Topics: |
This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
Contents
Introduction
Synroc is a particular kind of "Synthetic Rock", invented in 1978 by the late Professor Ted Ringwood of the Australian National University. It is an advanced ceramic comprising geochemically stable natural titanate minerals that have immobilized uranium (Synroc) and thorium for billions of years. They can incorporate into their crystal structures nearly all of the elements present in high-level radioactive waste (HLW) and thus immobilize them. Originally, some 57% of Synroc was titanium dioxide (rutile, TiO2).
Synroc can take various forms depending on its specific use and can be tailored to immobilize particular components in the high level nuclear waste (HLW). The original form, Synroc-C, was intended mainly for the immobilization of liquid HLW arising from the reprocessing of light water reactor fuel. However, by 1980 those reprocessing used fuel had chosen borosilicate glass as the medium for immobilization because it was the most technically mature technology.
The main minerals in Synroc-C are hollandite (BaAl2Ti6O16), zirconolite (CaZrTi2O7) and perovskite (CaTiO3). Zirconolite and perovskite are the major hosts for long-lived actinides such as plutonium (Pu), though perovskite is principally for strontium (Sr) and barium (Ba). Hollandite principally immobilises caesium (Cs), along with potassium (K), rubidium (Rb) and barium. Synroc-C can hold up to 30% HLW by weight.
Over the past few years, different forms of Synroc have been developed to deal with military radioactive wastes, including a substantial amount of plutonium. Other applications have been developed related to the partitioning and transmutation of wastes. This involves partitioning HLW into separate components, some of which can then be transmuted, or changed, into different forms that are less radioactive or shorter-lived (usually by neutron bombardment in a reactor or accelerator). Those not suitable for transmutation can then be immobilized in Synroc.
The waste form is the key component of the immobilization process, as it determines both waste loading (concentration), which directly impacts cost, as well as the chemical durability, which determines environmental risk. To achieve maximum cost savings and optimum performance, the Synroc waste forms are tailored to suit the particular characteristics of nuclear waste to be immobilized rather than adopting a single one-size fits all approach.
Background
Research and development on Synroc and its properties has been carried out at the Australian Nuclear Science and Technology Organisation (ANSTO) Research Laboratories at Lucas Heights, New South Wales, and at the Australian National University (ANU) in Canberra. From the early 1980s, funding was provided by the Australian Government. A pilot plant to manufacture Synroc using only non-radioactive material was designed, constructed at Lucas Heights. Synroc became the flagship of an ANSTO program that has now broadened into other wasteforms and maintains an international profile.
At the Australian Government's request, a Synroc Study Group (SSG) was set up in 1989 by four Australian companies, BHP, CRA (now Rio Tinto), Energy Resources of Australia (ERA) and Western Mining Corporation (WMC), in association with ANSTO and ANU to consider commercialization. The situation was complicated by the fact that existing spent fuel reprocessing plants in France and the UK, and the plant now being built in Japan, are committed to the use of borosilicate glass for immobilization.
Although the advantages of the synroc approach have been well recognized internationally, billions of dollars have been invested in glass technology for high-level waste (HLW) arising from the [[reprocessing of nuclear fuel. While glass is appropriate for a large fraction of HLW, considerable quantities of waste exist that are very difficult to incorporate in glass and it is these waste streams in particular that ANSTO is targeting, with its tailored range of waste forms. Initially, ANSTO oriented its Synroc research to military wastes, and in particular to the clean-up problems faced by the US Department of Energy (DOE) at their Savannah River Site, South Carolina, and at [Hanford Site, Washington]]].
Development for US military wastes
Early alternatives to Synroc-C were developed for military wastes at the U.S. DOE's Savannah River Site. Because of problems involving high levels of non-radioactive contaminants in the wastes, the new formulation, Synroc-D, contained nepheline - (Na, K)AlSiO4 instead of hollandite as host for Cs, Rb and Ba. Another variant, Synroc-F, was rich in pyrochlores - (Ca, Gd, U, Pu, Hf)2Ti2O7, and was developed for the disposal of unreprocessed spent fuel from light water and CANDU reactors.
In 1994, ANSTO began working with the U.S. Lawrence Livermore National Laboratory (LLNL) to develop a Synroc variant for plutonium disposition. Early efforts focused on zirconolite-rich titanates, since zirconolite is the most durable of the original Synroc phases and hence a natural host for actinides in general and plutonium in particular. These were also loaded with neutron absorbers such as gadolinium, hafnium and samarium to reduce the risk of criticality during and after processing. Zirconolite variants containing hollandite and rutile were produced either by sintering the ingredients in air at 1375°C or by hot isostatic pressing of calcines at 1280°C and 150 MPa in argon.
But this work was eventually dropped in favor of the pyrochlore-rich ceramic. This was found to be more efficient for immobilizing the uranium contained in the waste stream without the loss of rare-earth neutron absorbers from the crystalline structure (which tended to be displaced by uranium). The plutonium feedstocks have significant levels of diverse impurities.
The specialized form of Synroc that has emerged for this plutonium program is a pyrochlore-rich titanate ceramic with an increased loading of natural uranium and neutron absorbers (Gd, Sm, Hf) needed for nuclear criticality control. Pyrochlore is similar to zirconolite and can incorporate up to 50% by mass of PuO2 and/or UO2. There is twice as much U as Pu (23%:11.5% by mass) so that high uranium-238 levels will ensure additional criticality control as plutonium-239 decays to 235U. There is as much Hf and Gd (each) as Pu. The baseline wasteform is 95% pyrochlore and 5% rutile (TiO2) with Hf replacing one-tenth of the Ti. However, there are large variations according to feedstock composition and impurities.
In 1997, Synroc was tested with real high-level waste using technology developed jointly by ANSTO and the US DOE's Argonne National Laboratory. The project to compress the waste in a remotely controlled hot cell provided the first remote demonstration of hot isostatic pressing on a commercial scale.
In 1998, DOE chose the pyrochlore-rich form of Synroc from among 70 contending treatments for HLW management. This process adapts some of the ceramic technology from mixed-oxide (MOX) fuel production. The DOE hoped to have the plutonium immobilization facility operational at the Savannah River Site by 2007, with the product going to geological disposal. Cans containing pressed and sintered ceramic discs containing plutonium would be surrounded in canisters by vitrified HLW to provide an external gamma radiation barrier that would deter attempts at Pu recovery.
In anticipation, ANSTO set up a joint venture with Cogema of France through their US subsidiaries to bid for the contract to build the plutonium immobilization plant. The venture also included U.S. companies Burns and Roe, and Battelle. The bid was submitted in mid-2000, but in April 2001, the DOE announced that it was deferring immobilization plans in favor of the MOX plant, representing the other policy track for plutonium disposition.
ANSTO is again discussing with DOE the use of Synroc for immobilizing a range of problematic high-level wastes primarily arising from Cold War nuclear activities. It is relevant to note the advantages of the pyrochlore-rich ceramic over the alternative of lanthanide borosilicate glass: it is more chemically insoluble, giving better proliferation resistance; it is easy and safer to process; it is criticality-safe since it incorporates neutron absorbers in the same phase as the actinides and depleted uranium; higher actinide waste loadings are possible, resulting in about half the volume of solid waste; and neutron dose rate to workers is significantly lower.
There has also been collaboration with Minatom for treatment of Russia's high-level wastes, and a 20t/yr pilot plant is envisaged.
Composite wasteforms
Where there is a lot of either sodium or silicon in the wastes, these can form a glass as part of a durable glass-ceramic composite wasteform. ANSTO is now developing a Synroc-glass combination where the radioactive materials are typically incorporated into extremely durable crystalline titanate phases such as zirconolite and pyrochlore (that will hold actinides, particularly plutonium) within a glass matrix. Waste loadings of 50-70% by weight have been demonstrated in such composite wasteforms formed from dry sintered wastes. These composite wasteforms are typically melted at 1200-1400°C, although the use of hot isostatic pressing has some advantages.
The process was originally developed to deal with wastes at the DOE Hanford Site, Washington, which are highly contaminated with sodium nitrate and nitrites. The wastes were being vitrified in borosilicate glass (with about 20% waste loading), but space is at a premium and a synroc-glass composite would reduce volumes. Calcined waste (with no Pu) from the 1960s at DOE's Idaho laboratory (INEEL) were also part of the program.
Composite wasteforms are also the subject of a collaborative research program with the French Atomic Energy Commission. This includes developing Synroc-glass waste forms using French cold-crucible melting technology. These achieve a 50% waste loading and may be used for French P/T programs.
In 2005, ANSTO entered an agreement with Nexia Solutions, part of British Nuclear Group, to use a composite Synroc glass-ceramic waste form for 5 tonnes of impure plutonium waste at Sellafield in the UK. The glass-ceramic mix will be subject to hot isostatic pressing.
Waste partitioning and conditioning
Radioactive waste partitioning is usually considered in the context of partitioning and transmutation (P/T) strategies aimed at reducing the long-term potential hazard associated with high-level waste (HLW) destined for geological disposal. P/T involves the separation of minor actinides and long-lived fission products in advanced reprocessing and their transmutation into products of greatly reduced half-lives. Because of practical difficulties in achieving the high separation factors required for efficient transmutation, P/T concepts have increased interest in conditioning certain partitioned waste streams in durable matrices such as Synroc.
ANSTO has demonstrated that significant reductions in waste volume for the partitioning/conditioning strategy can be achieved through immobilization in Synroc-C of the heat-generating radionuclides, i.e., Cs-137, Sr-90, and Cm-244 together with long-lived Tc-99, while the remaining waste is immobilized in glass. Extended near-surface storage of the Synroc would be required for 100 to 200 years, while disposal of borosilicate glass containing the remaining radionuclides could be carried out immediately. The combination of partitioned cesium and strontium with curium-244 ensures that the effects of alpha-decay processes on the waste form durability are minimized through self-annealing of alpha-decay damage due to the radiogenic decay heat. The presence of curium, not likely to be recycled for transmutation, will become more important in the future with reprocessing of mixed oxide (MOX) fuels.
A zirconolite-rich Synroc can also be considered for the conditioning of the Am/Cm/rare-earth stream from HLW partitioning. The very low release rates from Synroc of Am, Cm and their long-lived daughters suggest that there may be little incentive on cost and radiological benefit grounds for the transmutation of Am and Cm. Separation of Am-241, which is a major source of Np-237, is essential in P/T strategies aimed at significantly decreasing the long-term radiological risk from geological disposal. Efficient separation from each other of these minor actinides and the rare earths on an industrial scale will require significant technological improvements and could involve additional costs to limit radiological exposures to operators of subsequent fuel fabrication plants.
Further Reading
- WNA paper on Synroc
