Scholarly article on topic 'The Intermediate Level Liquid Molybdenum-99 Waste Treatment Process at the Australian Nuclear Science and Technology Organisation'

The Intermediate Level Liquid Molybdenum-99 Waste Treatment Process at the Australian Nuclear Science and Technology Organisation Academic research paper on "Materials engineering"

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{Glass-ceramic / "hot-isostatic pressing" / HIP / ILLW / molybdenum-99 / "waste treatment" / "waste form"}

Abstract of research paper on Materials engineering, author of scientific article — Catherine K.W. Cheung, Eric (Lou) R. Vance, Martin W.A. Stewart, Daniel R.M. Brew, Walter Bermudez, et al.

Abstract ANSTO manufactures 99Mo for radiopharmaceutical use. Alkaline Intermediate Level Liquid Waste (ILLW) from this process, plus legacy acidic waste, are planned to be treated by converting both wastes into stable, solid, waste forms with oxide-basis loadings of 25-35 wt% and 30-50 wt%, respectively. The hot-cell plant design utilises the same unit process steps to treat both wastes. Hot-Isostatic Pressing (HIP) is employed to consolidate the processed waste and achieve substantial waste volume reductions compared to a cementation option. In this paper an overview of the treatment process and selected waste forms for ANSTO's 99Mo production ILLW is given.

Academic research paper on topic "The Intermediate Level Liquid Molybdenum-99 Waste Treatment Process at the Australian Nuclear Science and Technology Organisation"

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Procedia Chemistry 7 (2012) 548 - 553

ATALANTE 2012

International Conference on Nuclear Chemistry for Sustainable Fuel Cycles

The Intermediate Level Liquid Molybdenum-99 Waste Treatment Process at the Australian Nuclear Science and Technology

Organisation

Catherine K. W. Cheung , Eric (Lou) R. Vance, Martin W. A. Stewart, Daniel R.M. Brew, Walter Bermudez, Tina Eddowes and Sam Moricca

Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Road, Lucas Heights, NSW 2234, Australia

Abstract

ANSTO manufactures 99Mo for radiopharmaceutical use. Alkaline Intermediate Level Liquid Waste (ILLW) from this process, plus legacy acidic waste, are planned to be treated by converting both wastes into stable, solid, waste forms with oxide-basis loadings of 25-35 wt% and 30-50 wt%, respectively. The hot-cell plant design utilises the same unit process steps to treat both wastes. Hot-Isostatic Pressing (HIP) is employed to consolidate the processed waste and achieve substantial waste volume reductions compared to a cementation option. In this paper an overview of the treatment process and selected waste forms for ANSTO's 99Mo production ILLW is given.

© 2012Elsevier B.V...Selectionand/orpeer-review under responsibility of the Chairman of the ATALANTE 2012 Program Committee

Keywords: glass-ceramic; hot-isostatic pressing; HIP; ILLW; molybdenum-99; waste treatment; waste form

1. Background

ANSTO produces 99Mo at its Lucas Heights facility as a precursor to the widely used radiopharmaceutical 99mTc. In recent years worldwide shortages of supply have arisen because of increased demand and shutdowns in the aging nuclear reactor fleet used to irradiate the targets for 99Mo production [1]. ANSTO has a new reactor,

Corresponding author. Tel.: +61-2-9717-7142 ; fax: +61-2-9717- 9225 . E-mail address: catherine.cheung@ansto.gov.au .

1876-6196 © 2012 Elsevier B.V...Selection and/or peer-review under responsibility of the Chairman of the ATALANTE 2012 Program Committee doi: 10.1016/j.proche.2012.10.083

OPAL and there is a potential to increase the capacity of its production to satisfy the immediate and longer-term market needs. A treatment plant to process the waste generated from any increased production, current waste and legacy waste has been proposed and is awaiting funding approval.

There are two main types of ILLW at Lucas Heights from the production of 99Mo. In the past UO2 pellets (1.82.2% enriched) were used as targets in ANSTO's HIFAR reactor (now shut down). The 99Mo was extracted using an acidic process. The ILLW formed was a uranyl nitrate solution containing fission products and process impurities. Some of the waste was further treated by drying to form a solid uranyl nitrate cake. The legacy ILLW is a mixture of primary (150-200g U/L) and secondary (~20g U/L) ILLW. There are approximately 6000 L of legacy ILLW and 560 kg of legacy solid intermediate level waste (ILW) waste stored on site.

Since 2007 the current reactor, OPAL has been in operation and the irradiated Low Enriched Uranium (LEU) Al-U targets (19.7% enriched) are now plates of U-Al alloy encased in Al metal. The 99Mo is extracted via an alkaline route. Consequently the ILLW stream is now a mixture of NaOH and NaAlO2 plus fission products (with a Na concentration of 6-7 M). Allowing 2-3 years of decay, the amount of alkaline ILLW produced is estimated to be 1500-2000 L/y.

The alkaline ILLW is presently stored in tanks onsite at ANSTO. In preparation for treatment of this liquid waste for final storage the waste will be converted to a solid. It is therefore desirable to convert the waste into a form that meets regulatory guidelines for ILW, is suitable for transport to, and storage at, a national radioactive waste store, and that also renders it suitable for disposition in a future national waste repository.

ANSTO examined several options for the treatment of waste from an economic life cycle and environmental perspective. If the alkaline ILLW was to be disposed of using cementation, it would create around 50 x 200 L drums per year, which would then be over-packed and grouted into a 1000 L concrete container creating 50,000 L/y of concrete waste. In comparison an analysis of the synroc HIP waste treatment option showed that it would only produce around 500 L of waste form from the same volume of 99Mo ILLW feed. Furthermore, the HIPed product does not require a further encapsulation barrier in a concrete over-pack, as the waste has been immobilized in a glass ceramic matrix and is hermetically sealed in a stainless steel container. It still requires appropriate shielding for transportation and storage. The waste volume reductions achievable when using the synroc process would result in significant life cycle cost savings from reductions in the storage building size, the number of transport shipments and the eventual repository disposal space required. Furthermore, ANSTO already has temporary storage available for the synroc route and hence the route offers further advantages to ANSTO in that it will not have to construct new storage facilities, which it would if it pursued a cementation route.

In addition, CO2 emission analysis for the two routes shows that the synroc route has ~ 20 % fewer life-cycle emissions for a solidify-and-storage route than a cementation route. However, the main CO2 reductions arise from the reduction in waste volumes achievable via a synroc-route, when a 30 year life-cycle with a repository is analysed. The synroc route produces 20 times less CO2 emissions of the cementation route [2].

2. Waste forms for the Acidic and Alkaline Waste Streams

The chemistries of the legacy and alkaline ILLW are very different and give rise to different designs required of the final waste forms. However, the plant has been designed so that both wastes can be treated using the same unit process operations.

A synroc derivative has been selected to treat the legacy waste based upon in-house research into treatment of U-bearing wastes [3]. The waste form design builds on ANSTO's extensive experience on synroc and actinide-doped waste forms, such as that from the plutonium immobilisation project [4] and makes use of data from a research program on the behaviour of uranium in pyrochlore [5]. Synroc is short for "synthetic rock" and the synroc concept utilizes a suite of naturally occurring minerals which incorporate the radioisotopes into their crystal structure. There is a range of synroc designs available with varying amounts and types of phases that are tailored to suit the waste stream. For this waste form we have selected a pyrochlore-rich formulation [3] somewhat similar to Synroc-F [6], which was developed for the immobilisation of spent nuclear fuel. The basis of the waste form is a precursor design to contain 30-50 wt % waste and form brannerite to contain the uranium,

hollandite to incorporate fission products such as Cs [7,8] and titanium dioxide. Small amounts of perovskite (CaTiO3) may also be present.

The waste form is designed to easily accommodate the small amounts of fission products present in the waste, e.g., transition metals substitute for Ti and Al in the titanate phases and rare earths can substitute into the pyrochlore and perovskite. The primary role of Ba-hollandite in the design is to incorporate the Cs via substitution of 2Cs+for Ba2+. Hollandite has a variable composition, but is nominally stated as BaU4Al2.28Ti6O16 for Synroc applications. Strontium can also be incorporated into Ba-hollandite [7] and will also be incorporated into perovskite if it is present. Titanium oxides such as rutile (TiO2) or Magneli phases (TinO2n-1) are present as a buffer and from the oxidation of added Ti metal.

The waste form for alkaline ILLW was built on work at ANSTO to develop waste forms to immobilise Na-rich wastes, such as those arising from the conversion of Na metal coolant from fast-reactors. The waste form is a borosilicate glass that also contains small amounts of crystals such as nepheline. The waste loading achieved ranges from 25-32 wt% (oxide basis) with the loading dependent upon the Na molarity in the waste feed and the waste form chemistry.

A typical microstructure for the legacy waste form is shown in Fig. 1. This sample was prepared at a laboratory scale following the processing steps in Fig. 1. A 200 g oxide basis batch (40 wt% waste loading) of powder was prepared by adding UO2(NO3)2 solution (200 g U/L) to stirred precursor slurry (~25 wt% solids, oxide basis). Part of the mixed slurry was then added to an IKA Kneader Mixer (MKD- 0.6 H60), to a level just above the shaft. The kneader was heated by passing hot oil (250oC) through the kneader jacket. An infra-red heating lamp was also placed over the kneader to heat the surface of the slurry. The temperature of the kneader blades was measured at ~ 60-70oC. As the slurry thickened it was topped up by the remaining slurry. The kneader produces a more homogeneous feed than tray drying in an oven, which is prone to settling and segregation. The dried powder was placed in an alumina crucible and calcined in air at 750 oC for 1 hour with a 2-hour heating and cooling period in air in a muffle furnace. 4 wt% Ti metal was added for redox control. The material was loaded into a 38 mm diameter stainless steel HIP can. The HIP can lid was welded in place; the can was evacuated through a stem tube, heated to 650 oC and then sealed. The can was HIPed under Ar at 1250oC /35MPa/2h.

Fig. 1. (a) and (b): Backscattered electron SEM micrographs of a HIPed legacy waste form sample. The Matrix consists of a mainly of a mixture of pyrochlore (grey, P) and brannerite (light grey, B,); rutile (R, dark-grey-black) and Ba-hollandite (H). A few fine UO2 crystals (< 1 |xm, white spots) are also present, as is a small number of closed pores (O).

Analysis of the HIPed sample was carried out employing a Scanning Electron Microscope (SEM) {fitted with an Energy Dispersive X-Ray analysis unit (EDS)} and X-Ray Diffraction (XRD). The matrix is composed mainly of a mixture of fine ~ 2-10 ^m brannerite (Ca0.02U0.97Tij.9Zr0.04Al0.01Oe) (and pyrochlore ~ (Ca0.95Zr0 09-0.i2U0.83-0.84Ti!.99-2.04Al0.05-0.07O7) grains (Fig. 1). There is also a fine scattering ~ (1 vol %) of < 2 ^m UO2 crystals. Titanium oxide, detected as rutile by XRD, was observed in two forms; some < 5 ^m particles scattered in the matrix and some as ~ 25-100 ^m regions derived from the Ti metal addition. Ba-hollandite (Ba1. !3Cao.25Alo.76Ti6.72Zro.o2O!6) was also associated with these Ti oxide relics and was observed at the interface between them and the matrix, and some smaller crystals of hollandite were also detected in the matrix. The matrix is dense with < 2 vol. % closed porosity. At the macro-scale the structure appears to be composed of 100300 ^m rounded agglomerates formed on drying. The boundaries of these are marked by the Ti metal relics and slightly darker, rings of crystals.

Both of the waste forms are designed to be durable, the pyrochlore-rich waste forms have been tested by both MCC-1[9] and Product Consistency Test (PCT-B) [10] methods commonly used to test waste form durability and have been shown to be durable [3,4]. In this respect, the pyrochlore titanate will have a geological durability similar to those reported for synroc, titanate minerals and their natural analogues [11]. Similarly, the glass-ceramic waste form for the alkaline waste has been tested via PCT-B type tests and passes the standards set for high level waste glasses [12].

3. Waste treatment process and plant

Although the compositions of the final waste forms for the legacy and alkaline wastes are different, the waste treatment steps are the same (Fig. 2). There are four major treatment stages of operations in the proposed waste treatment plant. Firstly, precursor additives are reacted with the ILLW to form a slurry-like mixture which is put through a dryer. Secondly, the resulting dried powder is heat treated to remove nitrates, chemically bound water or other volatiles and renders the powder suitable for HIPing. Thirdly, this powder is mixed with back-end additives and the combined mixture is placed in proprietary designed HIP cans. Finally, the filled cans are evacuated, sealed and then processed through a HIP to consolidate the waste form.

Fig. 2. Process flow diagram for the treatment.

The unit processes for the production of the waste forms are broadly the same as those originally chosen for ANSTO's Synroc-C process, designed for the treatment of high-level fuel reprocessing waste, and for which a 10 kg/hr inactive demonstration plant was constructed and operated [2,13]. One difference is the use of HIP technology in the current design, which replaces the hot-uniaxial pressing technology used in the original synroc demonstration plant. For example, the nitrate legacy waste is wet mixed with an alkoxide/hydroxide-route slurry precursor [8], dried and calcined to remove the waste and nitrate ions. The preparation route for this precursor is

the same as that used to produce Synroc precursor powder at large-scale for the Synroc demonstration Plant [13]. The resulting calcined powder is mixed with metal powder for redox control, loaded into a HIP can sealed and then HIPed to produce a dense synroc waste form. The alkaline ILLW is treated similarly but with changes afforded to the chemistry of the additives and the process parameters.

The design approach for the plant and waste form has been to maximise the waste loading by tailoring the waste form chemistry whilst maintaining chemical durability in order to meet regulatory requirements. One focus has also been on reducing off-gas emissions to simplify plant design. Therefore, HIPing was employed as the consolidation step. By containing the waste form within a sealed HIP can, the radioisotopes such as Tc and Cs that are volatile at glass melting or pressureless sintering temperatures can be retained within the waste form. Furthermore, the use of a HIP enables the treatment of a variety of waste forms; glasses and glass ceramics that require melting, and full ceramics that are (pressured) sintered. Hence one plant can be used to treat both forms, as is the case here where the legacy waste form is a full-ceramic and a glass-ceramic is the alkaline waste form. HIPing also enables higher waste loadings and provides a means to process wastes that are problematic or uneconomic to process via melting, as unlike melting, there are no viscosity or conductivity constraints [14].

ANSTO has developed proprietary, HIP can designs, can filling systems, HIP vessel and ancillary equipment designs and features that ensure the safe operation and easy maintenance of a HIP within a hot-cell and minimise contamination from powder handling [2]. The cans are designed so as to collapse to a near cylindrical shape and volume reductions of up to 70% are achieved, depending upon the packing density of the feed powder. The proposed Synroc waste treatment plant will be operated to process around 5000 L of liquid waste per year with a higher capacity possible. This will enable it to cope with any possible future expansion of 99Mo production. The process is scalable to larger volumes and ANSTO has demonstrated an inactive can of ~ 100L containing ~ 200kg of waste form. Furthermore, the secondary waste from the HIP plant will be less than that from a glass melting route as there will be no used glass melters and the HIP pressure vessel is designed to exceed the life of the plant and is readily able to be decontaminated. This will simplify future decommissioning and cleanup.

The proposed plant to be built at Lucas Heights uses industrially mature plant and equipment that has been modified to be able to be operated remotely. The focus was on having a high technical readiness level (TRL) to minimise technical risk involved in a "first of a kind" plant. The plant will be able to treat the waste coming from ANSTO's radiopharmaceutical production including any increase in production and the legacy inventory. A view of conceptual design for a plant to treat the legacy and current ILLW is shown in Fig. 3. Preliminary engineering is near completion on a slightly larger plant design and pending regulatory and government funding the plant is expected to be operational by late 2015.

Fig.3. An initial conceptual plant design of a series of hot cells in which powder and additives are mixed and transferred.

Acknowledgements

Thanks to I. Watson, C. Grant, N. Webb, S. Deen and K. Olufson for the assistance with the experimental work and to the design team of G. Beamish, J. Chapman, S. Chung, A. Murray and A. Gonzalez. Thanks also to our administrative help A. Tzigeras and K. Brown.

References

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[10]ASTM C 1285-02: Determining Chemical Durability of Nuclear, Hazardous and Mixed Waste Glasses and Multiphase Class Ceramics: The Product Consistency Test (PCT). West Conshohocken , PA: ASTM; 2002.

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[12] US DOE-EM. Waste Acceptance Product Specification for Vitrified High-Level Waste Forms; EM-WAPS Revision 2. Dec. 1996.

[13]Bartlett JR, Woolfrey JL, Buykx WJ. Production of Synroc Powders by Alkoxide Hydrolysis. in Mellinger GB editor Nuclear Waste Management III, Ceramic Trans. 9. Westerville, OH: Am. Ceram. Soc;1990, p. 45-55.

[14] Stewart MWA, Begg BD, Moricca S and Day RA. Low-Risk Tailored Waste Forms for Problematic High-level and Long-lived Nuclear Wastes. 15th Pacific Basin Nuclear Conference 2006. 2006, 15-20 Oct., Sydney, Australia: ANA; CD-ROM; 2006.