Hanford Waste Treatment Plant Research Articles

Impact of feed variability on cesium removal with multiple actual waste samples from the Hanford site

ABSTRACT The Tank Side Cesium Removal (TSCR) system, under development by Washington River Protection Solutions (WRPS), will send initial low-activity Hanford waste tank supernate feeds to the Hanford Waste Treatment and Immobilization Plant (WTP) Low-Activity Waste (LAW) Facility. In addition to entrained solids removal from the supernate, the primary goal of TSCR is to remove cesium-137 (137Cs) by ion exchange, allowing contact handling of the liquid effluent product at WTP. Crystalline silicotitanate (CST) ion exchange media, manufactured by Honeywell UOP (product IONSIV™ R9140-B), was selected as the ion exchange media at TSCR. This lot of material was found to have superior performance in comparison to historic batches of CST. Ion exchange column and batch contact testing with supernate from Hanford tanks AP-105, AP-107 and AW-102 was performed to assess the impact of feed variability on system performance. These tests demonstrated that batch contact measurements provide a reasonable prediction of column capacities with some deviation in performance attributed to column dynamics. In addition, the variability in CST capacity for cesium in the various actual waste samples does not track with historical understanding of the competing cations, suggesting that tank waste samples contain other components that may significantly impact cesium loading.

The effects of mixing multi-component HLW glasses on spinel crystal size

The Hanford Waste Treatment and Immobilization Plant will vitrify radioactive waste into borosilicate glass. The high-level waste (HLW) glass formulations are constrained by processing and property requirements, including restrictions aimed at avoiding detrimental impacts of spinel crystallization in the melter. To understand the impact of glass chemistry on crystallization, two HLW glasses precipitating small (∼5 μm) spinel crystals were individually mixed and melted with a glass that precipitated large (∼45 μm) spinel crystals in ratios of 25, 50, and 75 wt%. The size of spinel crystals in the mixed glasses varied from 5 to 20 μm. Small crystal size was attributed to: (1) high concentrations of nuclei due to the presence of ruthenium oxide and (2) chromium oxide aiding high rates of nucleation. Results from this study indicate that the spinel crystal size can be controlled using chromium oxide and/or noble metal concentrations in the melt, even in complex mixtures like HLW glasses. Smaller crystals tend to settle more slowly than larger crystals, therefore smaller crystals would be more acceptable in the melter without a risk of failure. Allowing higher concentrations of spinel-forming waste components in the waste glass enables glass compositions with higher waste loading, thus increasing plant operational flexibility. An additional benefit to the presence of chromium oxide in the glass composition is the potential for the oxide to protect melter walls against corrosion.

99Tc immobilization from off-gas waste streams using nickel-doped iron spinel.

Technetium-99 (99Tc) incorporation within stable spinel phases is a novel method for 99Tc removal and immobilization from waste streams. In this study, transformation of Ni-doped Fe(OH)2(s) to spinel minerals, e.g. trevorite (NiFe2O4), is explored as a method for removing 99Tc from Hanford Waste Treatment and Immobilization Plant (WTP) primary off-gas waste stream simulant. The Fe(OH)2(s) transformation process was found to reduce 99Tc(VII) to 99Tc(IV) and incorporate reduced Tc(VI) into the produced spinel simultaneously. Nickel doping was applied in the mineral transformation to inhibit potential reoxidation of 99Tc(IV). Solid phase characterization by XRD and XANES confirmed the formation of nickel substituted ferric-spinel, and suggest incorporation of 99Tc(IV) in the final spinel. Furthermore, in the primary off-gas stream, which contains both redox-sensitive contaminants Cr(VI) and 99Tc(VII), results from solution analysis and solid digestion indicate that nearly 100% Cr and over 80% 99Tc can be simultaneously removed by adding Fe(OH)2(s) to solution with a solid to solution ratio of 5 g/L under near neutral and alkaline conditions. The 99Tc removal approach developed herein provides an alternative treatment method to eliminate the proposed recycle process of the off-gas waste stream, which ultimately can reduce WTP mission cost and operation time.

Remediation of potentially acidified Hanford wastes using tri-n-octyl phosphine oxide extraction chromatographic materials

Abstract As the Hanford site undergoes remediation, significant economies could be realized if aluminum and chromium are kept from High Level Waste glass produced at the Hanford Waste Treatment Plant (WTP). An acidic scrub of the Hanford sludge could enhance Al removal, although such treatment could lead to the mobilization of transuranic elements. If mobilization were minor, a chromatographic secondary cleanup of the acidic waste stream may be preferred to allow preconcentration of radionuclides prior to processing through the Hanford WTP. This study examines tri-n-octyl phosphine oxide coated resins as a chromatographic means for the removal of transuranics from a secondary waste stream. Metal uptake kinetics and mechanisms for transuranics and a simulant transuranic (europium) with the developed resin are characterized in both batch and column operation modes. Results indicate up to 99% of the radioactive material present from an acidic sludge leach may be recovered using extraction chromatography providing an effective avenue for high aluminum content tank pre-treatment.

Application of QMU to the design of a nuclear waste storage tank

Defining the reliability of complex physical systems is crucial for system certification and guaranteeing safety, but is often complicated by the presence of many uncertainties and system sub-components. One such example is the certification of the Hanford Waste Treatment Plant, where radioactive and toxic chemical waste that spontaneously generates flammable gases is stored in large tanks. We introduce the QMU methodology to quantify the system safety of waste storage tanks in the event of a loss of tank ventilation, which poses a safety risk to workers. We also show how QMU can be used as a design tool to find operating regimes that maintain a desired confidence level. A simplified physical model is used to analyze the build-up of hydrogen gas in storage tanks and conduct an uncertainty propagation study to quantify the operating margin with respect to the flammability conditions. A demonstration problem is shown in which the radiation loading in a waste tank is iteratively modified until a desired confidence ratio is achieved. The results show that the resulting designs are sensitive to the use of interval or full QMU to achieve a confidence ratio, and it is not always possible to realize a design at a particular confidence ratio.

Tc removal from the waste treatment and immobilization plant low-activity waste vitrification off-gas recycle

ABSTRACTVitrification of Low Activity Waste in the Hanford Waste Treatment and Immobilization Plant generates a condensate stream from the off-gas processes. Components in this stream are partially volatile and accumulate to high concentrations through recycling, which impacts the waste glass loading and facility throughput. The primary radionuclide that vaporizes and accumulates in the stream is 99Tc. This program is investigating Tc removal via reductive precipitation with stannous chloride to examine the potential for diverting this stream to an alternate disposition path. Research has shown stannous chloride to be effective, and this paper describes results of recent experiments performed to further mature the technology.

Review of a non-probabilistic sampler versus a Vezin sampler on low weight percent solids slurries

Author Summary: The Hanford Tank Operations Contractor (TOC) and the Hanford Waste Treatment and Immobilization Plant (WTP) contractor are both engaged in demonstrating mixing, sampling, and transfer system capability using simulated Hanford High-Level Radioactive Waste (HLW) formulations. This work represents one of the remaining technical issues with the high-level waste treatment mission at Hanford – the TOC’s ability to adequately sample high-level waste feed to meet the Waste Treatment and Immobilization Plant (WTP) Waste Acceptance Criteria Data Quality Objectives. A full-scale sampling loop was used at a cold test facility to evaluate sampler capability. The sampler under investigation for deployment is non-probabilistic but radioactive environment friendly. A Vezin sampler (probabilistic) was used to obtain reference samples and accurately characterize the simulant as it flowed through the test loop. The two samplers are located in series, allowing for multiple samples to be taken from both samplers over the same time period (sample pairs) and direct sample comparison. The Vezin sampler was modified to minimize material build up allowing for steady state operation. This report discusses modifications made to the Vezin sampler and the results of sampler comparison.

Solidification and stabilization of silver mordenite used to control radioiodine emissions from Hanford’s Waste Treatment Plant

The United States Department of Energy is building the Hanford Waste Treatment and Immobilization Plant (WTP) in south-central Washington State to immobilize a portion of the stored high-level radioactive wastes generated by decades of producing nuclear defense materials. To comply with United States Environmental Protection Agency (EPA) and Washington State air emission regulations, the WTP is planning to use reduced silver mordenite to control releases of any volatilized waste constituent radioiodine (129I t1/2=1.7×107a). Because the EPA and Washington State have designated silver as a hazardous or dangerous waste constituent, the disposal form for the waste reduced silver mordenite must comply with their land disposal regulations. We used the mandated EPA Toxic Characteristic Leach Procedure to evaluate and qualify a disposal form for the waste reduced-silver mordenite. Our development effort resulted in a regulation-compliant disposal form which used cast American Society for Testing and Materials (ASTM) Portland Type III cement modified to include calcium iodide to solidify and stabilize the reduced-silver mordenite and control silver release to regulatory levels.

Setting and stiffening of cementitious components in Cast Stone waste form for disposal of secondary wastes from the Hanford waste treatment and immobilization plant

Cast Stone is a cementitious waste form, a viable option to immobilize secondary nuclear liquid wastes generated from the Hanford Waste Treatment and Immobilization Plant. However, no study has been performed to understand the flow and stiffening behavior, which is essential to ensure proper workability and is important to safety in a nuclear waste field-scale application. X-ray diffraction, rheology, and ultrasonic wave reflection methods were used to understand the specific phase formation and stiffening of Cast Stone. Our results showed a good correlation between rheological properties of the fresh mixture and phase formation in Cast Stone. Secondary gypsum formation was observed with low concentration simulants, and the formation of gypsum was suppressed in high concentration simulants. A threshold concentration for the drastic change in stiffening was found at 1.56M Na concentration. It was found that the stiffening of Cast Stone was strongly dependent on the concentration of simulant.

Impact of Antifoam Agent Addition on Hydrogen Formation in the Hanford Waste Treatment and Immobilization Plant

Ventilation and mixing systems in the Hanford Waste Treatment and Immobilization Plant (WTP) are being designed to account for the flammable gas hydrogen that will form in process streams, just as it also does in the radioactive liquid wastes awaiting immobilization at the Hanford Tank Farms. Tank wastes forming hydrogen at the highest rates do so by reactions involving dissolved organic complexant compounds, even though hydrogen is also formed by the better known radiolysis pathway. Hydrogen generation rates (HGRs) are predicted with a correlation relating waste properties to reaction pathways involving radiolysis of water and the degradation of organic compounds. This correlation accounts only for aqueous phase reactions. An antifoam agent (AFA) will be added to waste processed in the WTP. This organic liquid mixture is immiscible in aqueous systems and will therefore form a nonaqueous phase liquid layer on the processed waste, unless some of its compounds are unstable in the hostile physical/chemical environment and break down into soluble degradation products. Dissolved organic species increase the organic source term in the WTP HGR correlation, but the correlation requires adaptation to address hydrogen formed from immiscible organic liquids. Here, we report our initial evaluation of the hydrogen formed by 60Co gamma irradiation of a waste simulant containing Dow Corning Q2-3183A AFA with an adapted WTP HGR correlation.

Physical Property Modeling of Concentrated Cesium Eluate Solutions—Part I: Derivation of Models

Both experimental and process simulation studies were performed to develop physical property models of the concentrated cesium ion-exchange eluate solutions in one of the Hanford Waste Treatment Plant evaporators. The physical properties of interest included the bulk solubility, density, viscosity, and heat capacity of the evaporator bottoms, and the proposed model of each response was a linear mixture model containing 12 coefficients. A unique feature of this work is that the values of these coefficients were determined by the regression of the “virtual” experimental data, which were not measured but were calculated using a computer process model that simulated the semibatch evaporation of cesium eluate solutions. To improve the accuracy of calculated virtual experimental data and the resulting physical property models, a series of benchscale evaporation tests was also conducted to provide the necessary experimental data for the development of a multielectrolyte thermodynamic database on which the computer process model was built. Specifically, the solubility and other physical properties of selected binary, ternary, and higher-order systems were measured to support the optimization of a sexenary database for the Na-K-Cs-Al-HNO3-H2O system. As the input to the virtual experimental runs, a matrix of cesium eluate simulants was designed within the bounding concentrations of the major analytes identified in radioactive samples. The computer process model was then run in conjunction with the sexenary thermodynamic database to calculate the physical properties of each matrix solution concentrated to the target end points of 80 and 100% saturation. The calculated physical properties were analyzed statistically and fitted into the 12-coefficient mixture function of temperature and the concentrations of major analytes in the unevaporated eluate. Over the concentration and temperature ranges considered, the resulting empirical physical property models were found to correlate the computer-generated data well without significant bias.

Estimation of Ultrafilter Performance Based on Characterization Data

AbstractBecause of limited availability of test data with actual Hanford tank waste samples, a method was developed to estimate expected filtration performance based on physical characterization data for the Hanford Waste Treatment and Immobilization Plant in Richland, Washington. A test with simulated waste was analyzed to demonstrate that filtration of this class of waste is consistent with a concentration polarization model. Subsequently, filtration data from actual waste samples were analyzed to demonstrate that centrifuged solids concentrations provide a reasonable estimate of the limiting concentration for filtration.

Ultrafilter Conditions for High‐Level Waste Sludge Processing

Optimal filtration conditions were evaluated for the ultrafiltration process planned for pretreating high‐level waste (HLW) sludge in the Hanford Waste Treatment Plant. This sludge must be filtered in the pretreatment process to remove sodium and, consequently, reduce the number of canisters for storage. The evaluation, which was based on Hanford HLW slurry test data, was performed to identify the optimal pressure drop and crossflow velocity for filtration at both high and low solids loading. Results from this analysis indicate that the actual filtration rate achieved is relatively insensitive to these conditions under anticipated operating conditions. The maximum filter flux was obtained by adjusting the system control valve pressure to between 400 kPa and 650 kPa while the filter feed concentration increased from 5 wt% to 20 wt%. However, operating the system with a constant control‐valve pressure drop of 500 kPa resulted in a reduction of less than 1% in the average filter flux. Also, allowing the control valve pressure to swing as much as ±20% resulted in less than a 5% decrease in filter flux. This analysis indicates that a back pressure setting of 500 kPa±100 kPa will give effectively optimal results for the system of interest.

Vitrification and testing of a Hanford high-level waste sample. Part 2: Phase identification and waste form leachability

A sample of Hanford high-level radioactive waste from Tank AZ-101 was vitrified into borosilicate glass and tested to demonstrate its compliance with regulatory requirements. Compositional aspects of this study were reported in Part 1 of this paper. This second and last part presents results of crystallinity and leachability testing. Crystallinity was quantified in a glass sample heat treated according to the calculated cooling curve of glass at the centerline of a Hanford Waste Treatment Plant canister. By quantitative X-ray diffraction analysis and image analysis applied to scanning electron microscopy micrographs, the sample contained 7 mass% of spinel, a solid solution of franklinite, trevorite, and other minor spinels. Glass leachability was measured with the product consistency test and the toxicity characteristic leaching procedure. Measured data and model estimates were in reasonable agreement. Leachability results were close to those obtained for the non-radioactive simulant. Models were used to elucidate the effects of glass composition of spinel formation and to estimate effects of spinel formation on glass leachability.