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Crystallization Fouling Behaviour of Uranium Oxide and Alumnosilicate Scale in High Level Nuclear Waste Medium

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Crystallization Fouling Behaviour of Uranium Oxide and Alumnosilicate Scale in High Level Nuclear Waste Medium ECI Symposium Series, Volume RP5: Proceedings of 7th International Conference on Heat Exchanger Fouling and Cleaning - Challenges and Opportunities, Editors Hans Müller-Steinhagen, M. Reza Malayeri, and A. Paul Watkinson, Engineering Conferences International, Tomar, Portugal, July 1 - 6, 2007 CRYSTALLIZATION FOULING BEHAVIOUR OF URANIUM OXIDE AND ALUMNOSILICATE SCALE IN HIGH LEVEL NUCLEAR WASTE MEDIUM Jonas Addai-Mensah1, Jun Li1 and Bill Wilmarth2 1 Ian Wark Research Institute, ARC Special Research Centre. University of South Australia. Mawson Lakes, Adelaide. 5095. SA. Australia. 2Westinghouse Savannah River National Laboratory Aiken. South Carolina. USA Corresponding author E-mail: [email protected] ABSTRACT aluminosilicates and uranium (U) based species, the limiting During high level nuclear waste (HLNW) liquor reactants [3-6, 8-15]. processing in evaporators operating in the temperature The combination of high heat influx and liquor range 30 - 140 °C, dissolved silica, alumina, sodium supersaturation act to facilitate the poly-condensation of hydroxide, uranium-235 and transuranic species (e.g. solution Al(III) and SiO2 tetrahedral species and hence, the plutonium-238) invariably become concentrated. As the precipitation of SAS polytypes (e.g. zeolite, sodalite and liquor evaporation proceeds, the sodium aluminosilicate cancrinite) and occasionally sodium diuranate as scale. If (SAS) and radionuclides may exceed their solubility limits the fissile material precipitation fouling is not effectively and co-precipitate, fouling the tubes and walls of the mitigated, scale build up, approaching 3 times the critical evaporator. If the fouling process is not effectively mass, may result and pose a serious risk [1-3,7]. controlled or mitigated, radionuclide scale accumulation Mechanistically, fouling may generally occur by a exceeding the critical mass necessary for self-sustaining number of ways including: (i) high surface energy metal nuclear fission reaction may proceed at an alarming rate, substrate-mediated heterogeneous precipitation, (ii) posing a serious criticality concern. adsorption of existing particulate matter in suspension onto a substrate, (iii) chemical reaction solid product deposition To probe the mechanisms underpinning uranium onto an “inert” substrate, and (iv) substrate corrosion- oxide-sodium aluminosilicate co-crystallization fouling, mediated precipitation product deposition. The mechanisms fundamental studies simulating the process were and kinetics of SAS precipitation and polytypic phase undertaken. New knowledge and greater understanding transformations have been investigated under a variety of gleaned from the present work comprise crystallo-chemical conditions [8-28]. Precipitation from highly caustic structure characteristics, solubility and the fouling alumino-silicate media at low to moderately high mechanisms involved in the mixed oxides scale deposition. supersaturations is a metal substrate (e.g., steel) - mediated The implications of the findings with regards to uranium- heterogeneous process and may involve amorphous solid, based scale formation in HLNW plants are highlighted. zeolite A, sodalite and cancrinite crystals. These SAS polytypes tenaciously foul process vessel (e.g. evaporator) INTRODUCTION walls or heat exchanger tubes. The least thermodynamically A serious issue that sometimes confronts HLNW processing stable amorphous phase is kinetically pre-disposed to form plants in the USA is the co-deposition of sodium diuranate first, in contrast to the crystalline SAS phases, particularly and aluminosilicate scale as a consequence of at low temperatures (< 85 oC) [8,10,19-22, 26-28]. It rapidly heterogeneous precipitation within the evaporators used in transforms into zeolite A which, in turn, readily transforms liquor concentration at the temperature range 30 - 140 °C into sodalite with time. Upon further crystallization, sodalite [1-9]. The HLNW liquors are, characteristically, high ionic may also transform into cancrinite over several hours [10, strength (6 - 12 M) caustic solutions mostly comprising 26, 28]. sodium, hydroxide, silicate, aluminate, nitrite and nitrate Whilst many of the reported SAS precipitation studies ions and trace (< 300 mg·dm-3) radionuclides (e.g., uranium- [9-27] have considerably advanced our understanding of 235, plutonium-238 and caesium-137) species. As a aluminosilicate fouling behaviour, there is still a consequence of dramatic increase in solute concentration considerable paucity of knowledge of how the inherent and concomitant composition and speciation changes polytypism specifically influences or impacts on the co- occurring during continuous evaporation, the liquor precipitation of uranium containing solute species. In invariably becomes supersaturated with respect to sodium previous studies [9], we showed that at low U species concentrations (< 30 mg·dm3), reflecting low relative Produced by The Berkeley Electronic Press, 2016 91 Heat Exchanger Fouling and Cleaning VII [2007], Vol. RP5, Article 13 -3 supersaturations (σ < 2), U species incorporation into SAS 0.1 M SiO2, 6.6×10 -1.2 M Al(OH)3, 3.8 – 6.0 M NaOH solid by adsorption is dependent upon the nature of and plus either 0.38 M Na2CO3 for solutions (“Bayer spent -3 polytype adsorbent phase(s) present. Furthermore, U species liquor type”) or 21 – 3400 mg⋅dm U, 1.0 NaNO3 and 1.0 sorption loading was remarkably low, accounting for a very NaNO2 for those containing lower Al(OH)3 concentration minor component of the overall U species that may be (<6.6×10-3 M (HLNW liquor type). Before their use, they incorporated into SAS solids phases crystallized in HLNW were twice filtered through 0.45 µm membranes to give liquors [9]. Thus, co-precipitation of U species with SAS is optically clear liquors. believed to be the main mechanism for the former’s A 316 stainless steel, high pressure Parr autoclave appearance in HLNW evaporator solids. fitted with an external heater and an interval cooling system Given a set of solution conditions and the crystallization was used as the crystallizer for isothermal, batch of various SAS solid phases, it is unclear as to how the precipitation runs. The vessel was fitted with a central 4 nature and sequential evolution of the polytypes determine blade, 45°-pitch, 2-tier impeller which provided constant and/or facilitate the formation of uranium-based scale under agitation at 400 rpm to within ±2 rpm. Stainless steel strips industrially relevant conditions. In HLNW processing of dimensions 10 × 6 mm were attached to the shaft of the plants, uncontrolled fouling by radioactive solids facilitated impeller to routinely provide fouled substrates for high by SAS formation may occur at an alarming rate and pose a resolution scanning electron microscopy (SEM) and powder major criticality concern, warranting complete plant X-ray diffraction (XRD) analysis. The control of the shutdown for mitigation. Effective management and autoclave’s temperature, heating and agitation rates were mitigation of both SAS and radionuclide fouling of process achieved through an automatic proportional, integral and heat transfer equipment are, therefore, of significant derivative control system. To prevent boiling of solution importance to the HLNW industry. above 100 oC, the autoclave was pre-pressurized by using In the present work, unseeded and seeded crystallization H2O vapour-saturated N2 gas to a pressure of 3200 kPa, studies have been carried out to unravel the crystallo- prior to heating. This also ensured that there was no chemical characteristics, crystallization and the concomitant significant solution water loss by vaporization. Continuous, fouling behaviour of uranium and sodium aluminosilicate plug flow precipitation experiments mimicking plant based oxides, using simulant liquors and conditions tubular heat exchanger were also conducted under other reflecting HLNW evaporator processing. otherwise similar conditions. Unseeded (self-nucleating) and seeded isothermal crystallization / fouling tests were run over 4 h at 30 and 65 oC. Solution or slurry samples were periodically removed EXPERIMENTAL for crystalline product characterization and solution SiO2, Fresh, SiO2-free, synthetic sodium aluminate solutions Al(III) and U concentration analysis, the latter by ICP-MS were prepared from known masses of gibbsite (γ-Al(OH)3) (Spectro Analytical Instruments, Spectro SIM-SEQ ICP- (C-31 Hydrate, Alcoa Arkansas, USA), sodium hydroxide OES, Kleve, Germany). The experimental runs were (NaOH) (Ajax Chemicals, Australia, 97.5 % pure, 2.5 % repeated 3 times for each temperature. The relative errors in Na2CO3), sodium carbonate (Na2CO3)(Merck, Australia, SiO2, Al(III) and U species analysis were determined to be < 99.9 % pure) and Milli-Q water to give a liquor of 3 %. Seed crystals were prepared from unseeded concentration 2.20-2.33 M Al(OH)3, 5.4–6.0 M NaOH and crystallization experiments. Particle size and its distribution 3 0.49 M Na2CO3. 0.25 dm of the above liquor was placed in were measured by laser diffraction. The specific surface 3 a 0.6 dm stainless steel autoclave operating at 400 rpm area of the particles/crystals was determined by N BET 3 2 agitation rate at a constant temperature. A 0.05 dm solution analysis (Coulter Omnisorp 100, Hialeah Fl. USA). Carbon 3 containing 0.741 g of sodium metasilicate and 0.05 dm of coated samples were used for SEM imaging in secondary 6.0 M NaNO3 and 6.0 M NaNO2 for HLNW liquor type electron (SE) and backscattered electron (BSE)
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