Hanford Site Tank Research Articles

A newly proposed isotherm model to predict Cs exchange with crystalline silicotitanate in tank waste simulants

ABSTRACT The Zheng Anthony Miller (ZAM) computer model, a multicomponent ion exchange model used to predict the exchange of Group I metals onto crystalline silicotitanate (CST), has historically been used to predict Cs distribution coefficients from Hanford and Savannah River Site (SRS) tank waste simulants. Comparison of experimentally determined Cs distribution coefficients from tank waste simulants with ZAM isotherm model predictions indicate overprediction of Cs and K distribution coefficients for simple and complex simulants with the engineered form of CST. Additionally, recent changes in chemical composition/manufacturing of IONSIV TM R9140-B have resulted in increased Cs capacity from high-salt, highly alkaline solutions. This work served to assess different isotherm models and refine equilibrium parameters to develop a model that can be applied to Hanford and SRS tank waste Cs removal efforts. Toward this goal, the Campbell Westesen Peterson (CWP) model was developed. This model utilized the experimentally determined Cs capacity, and simplified ZAM equilibria expressions to include only the binary substitution of Cs+ or K+ on the Na+ sites. Equilibrium constants for these equations were refined using experimentally determined distribution coefficients. Overall, the CWP model significantly improved our ability to predict both Cs and K loading capacity from complex matrices.

Uranium-bearing phases in Hanford nuclear waste

Uranium is the most prevalent radioactive element in the 210 million liters of nuclear waste at the Hanford Site but the uranium speciation has not been determined in the waste tanks. The present study determined the common uranium-bearing solid phases in Hanford Site tank waste with X-ray diffraction and scanning electron microscopy. Sodium uranates were the most common species but uranium was also routinely co-precipitated with iron oxides. Na–U–O–P-bearing species were occasionally present. Two morphologies of sodium uranates were observed, long acicular and small spherical agglomerates up to 50 μm in size.

Effects of hydrated lime on radionuclides stabilization of Hanford tank residual waste

Chemical stabilization of tank residual waste is part of a Hanford Site tank closure strategy to reduce overall risk levels to human health and the environment. In this study, a set of column leaching experiments using tank C-104 residual waste were conducted to evaluate the leachability of uranium (U) and technetium (Tc) where grout and hydrated lime were applied as chemical stabilizing agents. The experiments were designed to simulate future scenarios where meteoric water infiltrates through the vadose zones into the interior of the tank filled with layers of grout or hydrated lime, and then contacts the residual waste. Effluent concentrations of U and Tc were monitored and compared among three different packing columns (waste only, waste + grout, and waste + grout + hydrated lime). Geochemical modeling of the effluent compositions was conducted to determine saturation indices of uranium solid phases that could control the solubility of uranium. The results indicate that addition of hydrated lime strongly stabilized the uranium through transforming uranium to a highly insoluble calcium uranate (CaUO4) or similar phase, whereas no significant stabilization effect of grout or hydrated lime was observed on Tc leachability. The result implies that hydrated lime could be a great candidate for stabilizing Hanford tank residual wastes where uranium is one of the main concerns.

Single-pass flow-through test elucidation of weathering behavior and evaluation of contaminant release models for Hanford tank residual radioactive waste

Contaminant release models are required to evaluate and predict long-term environmental impacts of even residual amounts of high-level radioactive waste after cleanup and closure of radioactively contaminated sites such as the DOE’s Hanford Site. More realistic and representative models have been developed for release of uranium, technetium, and chromium from Hanford Site tanks C-202, C-203, and C-103 residual wastes using data collected with a single-pass flow-through test (SPFT) method. These revised models indicate that contaminant release concentrations from these residual wastes will be considerably lower than previous estimates based on batch experiments. For uranium, a thermodynamic solubility model provides an effective description of uranium release, which can account for differences in pore fluid chemistry contacting the waste that could occur through time and as a result of different closure scenarios. Under certain circumstances in the SPFT experiments various calcium rich precipitates (calcium phosphates and calcite) form on the surfaces of the waste particles, inhibiting dissolution of the underlying uranium phases in the waste. This behavior was not observed in previous batch experiments. For both technetium and chromium, empirical release models were developed. In the case of technetium, release from all three wastes was modeled using an equilibrium Kd model. Formore » chromium release, a constant concentration model was applied for all three wastes.« less

Thermodynamic Model for Uranium Release from Hanford Site Tank Residual Waste

A thermodynamic model of U solid-phase solubility and paragenesis was developed for Hanford Site tank residual waste that will remain in place after tank closure. The model was developed using a combination of waste composition data, waste leach test data, and thermodynamic modeling of the leach test data. The testing and analyses were conducted using actual Hanford Site tank residual waste. Positive identification of U phases by X-ray diffraction was generally not possible either because solids in the waste were amorphous or their concentrations were not detectable by XRD for both as-received and leached residual waste. Three leachant solutions were used in the studies: deionized water, CaCO3 saturated solution, and Ca(OH)2 saturated solution. Analysis of calculated saturation indices indicate that NaUO2PO4·xH2O and Na2U2O7(am) are present in the residual wastes initially. Leaching of the residual wastes with deionized water or CaCO3 saturated solution results in preferential dissolution Na2U2O7(am) and formation of schoepite. Leaching of the residual wastes with Ca(OH)2 saturated solution appears to result in transformation of both NaUO2PO4·xH2O and Na2U2O7(am) to CaUO4. Upon the basis of these results, the paragenetic sequence of secondary phases expected to occur as leaching of residual waste progresses for two tank closure scenarios was identified.

Development of occupational exposure limits for the Hanford tank farms

Production of plutonium for the United States’ nuclear weapons program from the 1940s to the 1980s generated 53 million gallons of radioactive chemical waste, which is stored in 177 underground tanks at the Hanford site in southeastern Washington State. Recent attempts to begin the retrieval and treatment of these wastes require moving the waste to more modern tanks and result in potential exposure of the workers to unfamiliar odors emanating from headspace in the tanks. Given the unknown risks involved, workers were placed on supplied air respiratory protection. CH2MHILL, the managers of the Hanford site tank farms, asked an Independent Toxicology Panel (ITP) to assist them in issues relating to an industrial hygiene and risk assessment problem. The ITP was called upon to help determine the risk of exposure to vapors from the tanks, and in general develop a strategy for solution of the problem. This paper presents the methods used to determine the chemicals of potential concern (COPCs) and the resultant development of screening values and Acceptable Occupational Exposure Limits (AOELs) for these COPCs. A total of 1826 chemicals were inventoried and evaluated. Over 1500 chemicals were identified in the waste tanks headspaces and more than 600 of these were assigned screening values; 72 of these compounds were recommended for AOEL development. Included in this list of 72 were 57 COPCs identified by the ITP and of these 47 were subsequently assigned AOELs. An exhaustive exposure assessment strategy was developed by the CH2MHILL industrial hygiene department to evaluate these COPCs.

Leaching Characteristics of Hanford Ferrocyanide Wastes

A series of leach tests were performed on actual Hanford Site tank wastes in support of the Hanford Tank Waste Treatment and Immobilization Plant (WTP). The samples were targeted composite slurries...

Multiple Ion Exchange Column Tests for Cesium (CS+) Removal from Hanford Site Tank 241‐AW‐101 with SuperLig® 644 Resin

Six cycles of loading, elution, and regeneration were performed to remove cesium (Cs+) from a Hanford Site tank waste sample using SuperLig® 644 resin. The sample, which was retrieved from Tank 241‐AW‐101, was diluted to 5.09 M Na+ and processed through dual ion exchange columns to remove 137Cs. Each column had an inside diameter of 1.45 cm and a height of 30 cm; and contained 15 mL of wet resin in the sodium form. The columns, designated as primary (lead) and polishing (lag), were connected in series during loading, but they were separated during elution and regeneration. The cesium loading on the primary column during the six cycles ranged from 160 to 225 bed volumes (BV) at <50% breakthrough. A gradual decline of the resin loading performances was observed as a function of number of loading cycles. For all cycles, the percent removal of cesium (137Cs) was greater than 99.99% and the decontamination factors (DFs) achieved were higher than 1.0 × 104. Elution of the resin with 0.5 M nitric acid at 25 ± 2°C was effective. Approximately 99% of the 137Cs bound on the resin was eluted with less than 15 BV of the eluent (0.5 M nitric acid). The cumulative dose absorbed by the resin in the primary column was 1.99 × 107 rad with a 20% loss of ion exchange capacity was at 50% breakthrough after completing six loading cycles.

Technetium Removal from Hanford and Savannah River Site Actual Tank Waste Supernates with Superlig® 639 Resin

SuperLig 639 elutable, organic resin was selected for technetium (as pertechnetate ion) removal from Hanford Site radioactive waste samples as part of the River Protection Project—Waste Treatment Plant (RPP-WTP) design. In support of the RPP-WTP flow sheet development, column tests have been performed at the Savannah River Technology Center with SuperLig 639 resin using actual Hanford Site tank waste samples. The resin was shown to be highly effective at pertechnetate removal from these caustic, high-sodium, aqueous waste samples. Pertechnetate ion was subsequently eluted from the columns with water. An additional column test conducted on a Savannah River Site waste sample revealed exceptional performance, presumably due to the fact that lower concentrations of competing anions (primarily nitrate) were present in the sample.

Immobilized Low-Activity Waste Performance Assessment:2001 Version

ABSTRACTThe Hanford Immobilized Low-Activity Waste Performance Assessment[1] examines the longterm environmental and human health effects associated with the planned disposal of the vitrified low-activity fraction of waste presently contained in Hanford Site tanks. The tank waste is the byproduct of separating special nuclear materials from irradiated nuclear fuels over the past 50 years. This waste is stored in underground single- and double-shell tanks. The tank waste is to be retrieved, separated into low-activity and high-level fractions, and then immobilized by vitrification. The U.S. Department of Energy (DOE) plans to dispose of the low-activity fraction in the Hanford Site 200 East Area. The high-level fraction will be stored at the Hanford Site until a national repository is approved.This report provides the site-specific long-term environmental information needed by the DOE to modify the current Disposal Authorization Statement for the Hanford Site[2]. The original Disposition Authorization Statement was based on the 1998 version[3] of this performance assessment, which was conditionally accepted by DOE [4].The calculations in this performance assessment show that a “reasonable expectation” exists that the disposal of the immobilized low-level fraction of tank waste from the Hanford Site can meet environmental and health performance objectives. As shown by the sensitivity studies, this conclusion remains valid despite the conceptual designs of the disposal facility and the ILAW packaging having undergone changes.

Purification of Alkaline Waste Solutions from Np and Pu

AbstractCoprecipitation of Np and Pu with Co(OH)3 and Fe(OH)3, obtained by the Method of Appearing Reagents from Co(NH3)6 Cl3, K2FeO4, and Na2[Fe(CN)5NO]2H2O from alkaline solutions and from simulants of Hanford Site tank wastes has been studied. Removal of Np and Pu from pH 10.5 to 0.4 M NaOH is semiquantitative with decontamination factors of 80 to 400. The presence of EDTA and HEDTA interferes with the precipitation of Co(OH)3 and Fe(OH)3. The deleterious effect of EDTA was eliminated by Ca2 addition at a 1:1 molar ratio of Ca2:EDTA. The deleterious effect of HEDTA was eliminated by the addition of 0.11 M excess precursor Co(NH3)6Cl3 to increase the content of carrier Co(OH)3.

Results of modeling and experimental measurements for the design of a neutron surface moisture measurement sensor

Computer modeling and experimental testing have been performed to produce a design for a neutron-moderation-based surface moisture sensor. Results indicate that this sensor should be capable of providing information about the moisture concentration profile in the top 15 cm of material. Tests show that the sensor should be capable of operating in the expected high temperature and gamma-ray exposure rate fields within the Hanford Site tanks. A field-ready sensor and an in-tank deployment system were fabricated for application of this technology to the Hanford Site tank farms.

A moisture probe based upon neutron moderation for in SITU use in a cone penetrometer system

A moisture probe based upon neutron moderation has been designed, tested and built for measuring the moisture concentration of hazardous waste stored in the underground waste tanks at the DOE Hanford site. The probe is 1 inch in diameter, 24 inches long, and is deployed using a computer controlled winch into a cone penetrometer for in situ moisture measurements. Two thermal neutron detectors in a near-far geometry and a low-intensity /sup 252/Cf source are used in the probe to count thermalized neutrons for determining the moisture content. Several testbeds were constructed and a variety of experiments were performed to determine the optimal detector-source geometry and to rest that the sensor is capable of operating in the expected high temperature and very high gamma-ray fields within the tanks. Several calibration curves of the near-far count rate ratio were determined for tanks contaminated with various levels of neutron absorbing materials. A field-ready probe and a winch deployment system have been delivered for application of this technology to the Hanford Site tank farms.

Liquidus Temperature of Spinel Precipitating High-Level Waste Glasses

ABSTRACTThe liquidus temperature (TL) often limits the loading of high-level waste in glass through the constraint that TL must be at least 100°C below the temperature at which the glass viscosity is 5 Pa-s. In this study, values of TL for spinel primary crystalline phase were measured as a function of glass composition. The test glasses were based on high-iron Hanford Site tank wastes. All studied glasses precipitated spinel (Ni,Fe,Mn)(Cr,Fe)2O4 as the primary crystalline phase. TL was increased by additions of Cr2O3, NiO, Al2O3, Fe2O3, MgO, and MnO; while Li2O, Na2O, B2O3, and SiO2 had a negative effect. Empirical mixture models were fitted to data.

High-priority Hanford Site radioactive waste storage tank safety issues: An overview

High-activity radioactive waste has been stored in large underground storage tanks at the U.S. Department of Energy's Hanford Site in eastern Washington State since 1944. Since then, more than 227,000 m 3 (60 Mgal) of waste have accumulated in 177 tanks. These caustic wastes consist of many different chemicals. The waste forms include liquids, slurries, salt cakes, and sludges. A number of safety issues have been raised about these wastes, and resolution of these issues is a top priority of the U.S. Department of Energy. A waste tank safety program has been established to resolve these high-priority safety issues. This paper will deal with three of these issues. The issues described are the release of flammable vapors from single- and double-shell tanks, and the existence of organic chemicals and/or ferrocyanide ion-containing fuel-rich mixtures of nitrate and nitrite salts in single-shell tanks. Extensive management controls are employed to ensure that the tanks in question continue to be maintained a safe manner through issue resolution. In addition, comprehensive monitoring, characterization, and applied and basic research efforts have been initiated to support resolution of issues and to prevent creation of future problems associated with potentially incompatible wastes. The safety efforts will also support actions related to the planned retrieval and disposal of the wastes in these storage tanks. Such efforts will also provide the basis for safe near-future remediation of these tanks on an as-needed basis and will define the envelope of safety to support the disposal of all high-level waste in the Hanford Site tanks.