APPLICATION OF CHEMICALLY BONDED PHOSPHATE
CERAMICS TO LOW-TEMPERATURE STABILIZATION OF ASH
Bobby R. Seidel and David B. Barber
Argonne National Laboratory
Jenya Macheret
DOE-ID
Albert S. Aloy
Khlopin Radium Institute
Dieter A. Knecht
LMITCO
ABSTRACT
More than 19,700 kg of incinerator ash residues containing plutonium remain to be stabilized at the Rocky Flats Environmental Technology Site. The ash and ash heel materials have accumulated from the incineration of combustible waste derived from plutonium handling operations. The ash residues contain from less than 1% to more than 20 wt.% plutonium and about varying amounts of residual unoxidized organic matter. While vitrification is the baseline technology for stabilization of ash, it continues to face technical issues that may demand a high-temperature calcination pre-treatment.
Chemically bonded phosphate ceramic (CBPC) technology was recently identified as a potential backup technology to vitrification. CBPC is a simple, room-temperature process generating no secondary waste streams nor off-gases and possessing the potential to minimize worker exposure, cost and schedule impacts. In the CBPC process, the components are mixed at room temperature and allowed to cure for as little as a few hours.
A small test program is under way to evaluate the acceptability of CBPC as an alternative to vitrification. In the United States (U.S.), the testing has focused on magnesium oxide phosphates as applied to Rocky Flats ash; while in Russia, the efforts have focused on iron oxide phosphates with ash. Several laboratory and production scale monoliths have been fabricated with Rocky Flats ash and ash heel. The plutonium content in these specimens ranged from 0 to 5 wt.%. The organic content remaining from incomplete combustion during incineration ranged from 0 to about 10%. The monoliths were structurally stable and mechanically strong. Waste loadings of about 50% were achieved, and the feedstock materials, and processing costs were demonstrated to be small. Some of these monoliths, of about 400 g mass, were placed into individual gas monitoring containers to precisely measure hydrogen release. Initial results suggest that the hydrogen release rate from the monoliths is less than one-third that allowable for interim storage and shipping in the TRUPACT II. This paper describes the process, initial test program and preliminary results.
INTRODUCTION
More than 19,700 kg of incinerator ash residues containing plutonium remain to be stabilized at the Rocky Flats Environmental Technology Site. The ash and ash heel materials have accumulated from the incineration of combustible waste derived from plutonium handling operations. The ash residues contain from less than 1% to more than 20 wt.% plutonium and about varying amounts of residual unoxidized organic matter.
Vitrification of the residues is currently the baseline stabilization technology for the ash.1-3 An Ash Residues End State Trade Study4 analyzed 41 ash processing options and recommended vitrification. However, the Trade Study did not consider CBPC technology, which was not technically developed at the time of the study. CBPC is a simple, room-temperature process generating no secondary waste streams nor off-gases with the potential to minimize worker exposure, cost and schedule impacts. Since stabilization by vitrification continues to face technical issues that may demand a high-temperature calcination pre-treatment, stabilization using the CBPC technology process could provide a simpler alternative with no thermal treatment required.
The Department of Energy (DOE) has recently started a small test program to evaluate the acceptability of using CBPC to stabilize ash. In the U.S., the testing has focused on magnesium oxide phosphates as applied to Rocky Flats ash; while in Russia, the efforts have focused on iron oxide phosphates with ash. This paper provides an introduction to the process and reports preliminary results.
BACKGROUND ON PHOSPHATE CERAMICS
The bonding properties of phosphate cements and ceramics have been known for many years.5 Though largely overlooked in waste stabilization efforts to date, the phosphate cements and ceramics have the operational simplicity of a nonthermal batch mix-and-set process, like typical hydraulic cements, but produce final solids with mechanical strength and component binding properties similar to those of high temperature fused ceramics. Commercial versions of these materials are used for road surface repairs where short setting time and strength are important.5
The strength of these materials derives from their formation through acid-base reactions and the subsequent dominance of ionic and covalent bonding in the final structure. These bonds are much stronger and therefore preferable to the weaker hydration bonding which binds typical hydraulic Portland cement based concretes. These materials have come to be called acid-based concretes or chemically bonded ceramics.6
A particularly well studied version of chemically bonded ceramics is the magnesium-phosphate system7 which has been the subject of dozens of patents. Material scientists at Argonne National Laboratory (ANL) have developed a modified form of the magnesium-phosphate system which is being called Chemically Bonded Phosphate Ceramics (CBPC) or "Ceramicrete" (as discussed in references 7 and 8). This formulation brings reduced exothermicity and greater control over setting time which enable application of this superior low-temperature process to waste stabilization. The CBPC process has been applied successfully to the macroencapsulation and stabilization of low-level mixed9 and debris wastes.10
In a parallel development, researchers at the V. G. Khlopin Radium Institute (KRI) in St. Petersburg, Russia, investigated phosphate ceramics fabricated with oxides of iron,11 including magnetite and hematite. Tests with ash resulted in a dense and hard ceramic. In the case of magnetite, the reaction was rapid while for hematite, the reaction was slow. The latter had to be molded and heated to form monoliths.
A collaborative research program has been established between Argonne and KRI. Work is continuing at both laboratories to demonstrate first that full size monoliths can be fabricated and secondly to measure the release of hydrogen generated by the radiolytic decomposition of organics in the incinerator ash.
INITIAL APPLICATION TO RADIOACTIVE MATERIALS
Laboratory tests have been performed to scope the process and perform initial leaching and dissolution tests with surrogate and waste material similar to ash.10 Initial gas release measurements were sufficiently encouraging to proceed with tests at full scale using actual uncalcined Rocky Flats ash in the magnesium phosphate CBPC process. Laboratory and bench-scale process parameters and equipment have been easily scaled to production size which is limited by criticality safety and container sizes.
APPLICATION TO ROCKY FLATS INCINERATOR ASH
In tests at ANL, a test matrix of ten initial monoliths spanned plutonium concentrations in the matrix of 0.15 to 5 wt.%. The amount of organic material in the monolith was determined by relative ratios of uncalcined ash and ash heel; the ash heel contained no organic material. Powder binders, ash and water are simply mixed for thirty minutes and poured into the final container to set within a few hours and cure for a few days to harden to compressive strengths as high as 9,000 psi at a density of about 1.9 g/ml. Photos of the starting ash and ash heel material in the glovebox are presented in Figures 1 and 2. The mixer for the 2-liter scale monolith is shown in Figure 3. A 400 g monolith curing in a plastic bottle is shown in Figure 4.
Fig. 1. Photo of original RFETS ash in glovebox.
Fig. 2. Photo of original RFETS ash heel container in glovebox.
Fig. 3. Mixing of ash and components in glovebox.
Fig. 4. Cured 400 g test monolith.
For application of CBPC to the Rocky Flats ash residue waste stream, the primary performance criterion by which the final waste forms will be judged is the radiolytic gas generation rates being within acceptable limits. To meet interim storage and transportation requirements, the amount of hydrogen gas generated by radiolytic dissociation of water and hydrogenous material and released to any confinement layer in a 60-day transport period must be less than 5% of the contained volume.
To characterize radiolytic hydrogen gas generation from magnesium phosphate monoliths containing Rocky Flats ashen residues, a specific set of test specimens was fabricated at ANL with variance among parameters important to gas generation, namely ionizing energy deposition concentration and hydrogen source availability. This variation was with respect to alpha-decaying actinide concentration, from 0.15 to 5 wt.%, and in hydrogen availability both from organic matter, from 0 to 10 wt.%, and water used in the process, 18 to 24 wt.%.
Initial characterization at ANL of radiolytic gas release from these monoliths has shown a predictable first order response in hydrogen release with respect to organic content. However, the release of hydrogen from a subset of the gas characterization test array indicates that water of hydration which is bound in the crystaline matrix is considerably less available for hydrogen generation than additional excess water which may be found within pore space of the final waste form.
While these results are still preliminary, the hydrogen gas release data from the first round of test canister head space samples allowed the bracketing of expected gas generation rates which might be expected over the range of fabrication possibilities in application of this stabilization technology to the Rocky Flats ash residues. This analysis indicates that even under worst case conditions of highest anticipated organic loading and highest reasonably conceivable excess pore water content, the radiolytic hydrogen gas generation is still a factor of three below that which might be found unacceptable under storage or transport conditions such as in the TRUPACT II. Thus, the magnesium phosphate version of CBPC is a strong candidate for stabilization of Rocky Flats and other alpha-bearing finely divided wastes.
The KRI continues to focus on iron oxide-based monoliths and in particular, the radiolytic gas generation. The monoliths have been prepared with additions of Pu-238 to measure effects of internal alpha radiation. The effect on the ceramic will be characterized by measuring the dimensions, density, compressive strength, phase composition, and leach rate before and after radiation. Measurements of generated hydrogen as a function of absorbed dose will help predict the behavior of iron phosphate ceramics containing plutonium during long-term storage and disposal.
CONCLUSION
The chemically bonded phosphate ceramic process has been demonstrated to effectively stabilize Rocky Flats ashen residues in a waste form that is acceptable for interim storage and shipment to Waste Isolation Pilot Plant (WIPP). The process is being considered as a candidate to displace vitrification and become the baseline technology for processing of ash residues.
Iron oxide-based monoliths are also being evaluated to provide a lower cost alternative.
ACKNOWLEDGMENTS
The work is sponsored by the U.S. Department of Energy under Contracts DE-AC07-94ID13223 and W-39-109-Eng-38. This research has been supported by both the Plutonium Focus Area (PFA) and the Mixed Waste Focus Area (MWFA) with support of the Office of Science and Technology of DOE.
REFERENCES