Simulated Low-activity Waste Research Articles

Effect of glass forming additives on low-activity waste feed conversion to glass

A significant effort was invested in the past to develop and refine mathematical models that relate the composition of nuclear waste glasses with their properties, such as viscosity, electrical conductivity, or chemical durability. However, less attention has been paid to the formulation of the melter feed itself, such as the chemical form and the particle size of the glass forming and modifying additives, which have a significant effect on the feed-to-glass conversion process during melting. To address this issue, we systematically changed the mineral composition of a simulated low-activity waste melter feed and inspected its melting behavior. When substituting minerals with corresponding oxides and hydroxides, we found that different alumina sources (kyanite, gibbsite, boehmite, or corundum) had the strongest effect on the feed melting process, whereas the sources of Ca, Mg, and Zr had little effect. The X-ray diffraction analysis showed that the alumina sources differ in their dissolution kinetics: early dissolving alumina sources, such as gibbsite (Al(OH)3) and boehmite (AlO(OH)), increase the transient glass-forming melt viscosity at early stages, when gases still evolve, causing extended foaming, whereas alumina sources that dissolve at high temperatures, such as kyanite (Al2SiO5) and corundum (Al2O3), keep the transient glass-forming melt viscosity low and lead to a faster foam collapse. Using a viscosity-composition relationship to estimate the viscosity of transient glass-forming melts in the primary foaming range, we found that the primary foam began to collapse at 360 to 800 Pa s, and fully collapsed between 65 and 260 Pa s. This result agrees with our previous studies, according to which, the glass-forming melt viscosity at the cold cap foam bottom ranged from 24 to 85 Pa s.

Halogen bonding and hydrophobic interactions: A new strategy for synergistically enhancing ReO4−/99TcO4− binding

Efficient removal of 99TcO4− from strongly acidic and alkaline nuclear wastewater is essential due to environmental protection and nuclear fuel cycle issues associated with the nuclear industry. However, the combined features of anion exchange materials with decent acid/base stability, high capacity, and high selectivity toward ReO4−/99TcO4− have not yet been achieved. Here, we explore a new strategy to overcome the aforementioned challenges by pre-organizing pyridinium-based cationic polymer network (TFTA-BPD) with sigma-holes and strongly hydrophobic fluorine atoms, which can effectively form strong halogen bonding and enhance hydrophobic interactions for selective ReO4−/99TcO4− binding. This emerging strategy not only significantly enhances the stability of the adsorbent, but also remarkably strengthens the specific affinity toward ReO4−/99TcO4− due to the synergistic effect of halogen bonding and hydrophobic interactions, compared to reported cationic polymer networks. More importantly, our TFTA-BPD exhibit fast removal kinetics, high capacity, excellent removal selectivity, and encouraging reusability, enabling efficient removal of ReO4− from simulated low activity waste (LAW) at Hanford and high-level waste (HLW) at the Savannah River Site in both dynamic column separation and batch experiments. Experimental and computational results indicate that the halogen-functionalized TFTA-BPD can regulate the electron distribution of adjacent N atoms and enhance charge separation through the halogen electron traction effect. This work demonstrates the enormous potential of halogen bonding and hydrophobic interactions in synergistically enhancing ReO4−/99TcO4− binding and paves the way for constructing high-performance adsorbents for HLW and LAW treatment.

Effect of feed composition on the production of off-gases during vitrification of simulated low-activity nuclear waste

During the vitrification of nuclear waste, hazardous and radioactive emissions are generated from the feed-to-glass conversion reactions, in addition to discharges from forced air bubbling and air inleakage. Although the major gaseous emissions are water vapor, nitrogen, and carbon dioxide, various monitored environmental pollutants are also released, such as nitrogen oxides or sulfur dioxide. In addition, reactions between organics and nitrates in the feed may also form products of incomplete combustion such as carbon monoxide and acetonitrile. Although off-gas emissions are commonly measured during both laboratory- and pilot-scale melter testing, no predictive tool is currently available to a priori estimate the composition of gaseous emissions during nuclear waste vitrification. This work forms a basis for the development of such predictive tool by measuring gas evolution from a broad range of simulated low-activity waste melter feeds using evolved gas analysis data and developing correlations between the feed and off-gas compositions. Using reaction stoichiometry and regression analysis, we demonstrate that next to the content of nitrogen and organic carbon in the feed, the gaseous emissions are affected by the feed reduction-oxidation conditions – the more the feed is reduced, the less nitrogen monoxide, and more carbon monoxide and acetonitrile evolves. The results presented in this work provide a first step towards reducing the amount of expensive physical melter testing and the regression analysis provides a simple tool for rapid optimization of feed composition with respect to off-gas composition.

The S solubility of Cr and Al containing simulated low-activity waste glass

The presence of sulfur-containing molten salt phases during vitrification is a particular problem which can be detrimental to the melter. Borosilicate compositions were designed to explore both the effects of a fixed amount of Cr, as Cr2O3 or Na2CrO4, and the impact of Al2O3, on sulfur solubility as determined by progressively adding elemental S. The NABS series (with Al2O3) has a higher SO3 solubility of ∼3.5 mol% while the NBS series (without Al2O3) has a SO3 solubility of ∼2 mol%. Crystalline Cr2O3 formed in the NABS series upon the addition of S but is not present in the NBS series. At the highest S addition, a separate salt phase always forms, and was identified as Na2(S,Cr)O4 via electron dispersive spectroscopy, ultraviolet-visible spectroscopy, x-ray diffraction, and thermal analysis. Magic angle spinning nuclear magnetic resonance identified no change in the B coordination in either series as S is added, suggesting that the Na is scavenged from other locations, such as the Si sites, to charge compensate sulfate, whether in the glass or in the salt. The Cr precursor identity did not affect the observed overall behavior regarding sulfur, but did impact Cr redox as identified with Raman spectroscopy.

Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide.

Here, Cr(VI) effects on Tc-immobilization by Fe(OH)2(s) are investigated while assessing Fe(OH)2(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe(OH)2(s) present, where complete Cr and Tc removal requires more Fe(OH)2(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe(OH)2(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe(OH)2(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr(III) and Tc(IV) substitute for octahedral Fe(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr(III) forms oxide and/or hydroxide phases, whereas Tc(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc(IV)-oxide species. As such, to fully incorporate Tc(IV) into the goethite crystal structure, additional Fe(OH)2(s) (>200 g/L of simulant) may be required.

The performance of Inconel 693 electrodes for processing an iron phosphate glass melt containing 26 wt.% of a simulated low activity waste

Iron phosphate glass is a candidate fixation medium for storing radioactive waste. The Department of Energy supported a program to assess the viability of using Fe-phosphate glass for vitrifying low activity waste in a Joule Heated Melter (JHM). In this study, Inconel 693 electrodes were tested in a research-scale joule-heated melter (RSM) at Pacific Northwest National Laboratory. After a 10-day test at 1030°C that yielded 124kg of glass, the electrodes exhibited a dimensional loss rate of ∼1.6mm/year, which is comparable to that of Inconel 690 electrodes used in a JHM for processing borosilicate melts. Microstructural changes occurred within the outermost 700μm of the electrodes and are consistent with an earlier study of Inconel coupons in Fe-phosphate melts. The results indicate that Inconel 693 should have an acceptable corrosion resistance as the electrode for JHM processing of iron phosphate melts.

Evaluating the Effects of Tributyl Phosphate (TBP) and Normal Paraffin Hydrocarbon (NPH) in Simulated Low-Activity Waste Solution on Ion Exchange

:Ultrafiltration and ion exchange are among the pretreatment processes selected for the Waste Treatment Plant at the U.S. Department of Energy’s Hanford Site. Envelope B AZ-101 waste may contain tributyl phosphate (TBP) and normal paraffin hydrocarbon (NPH)-dodecane, as dissolved species and also as a separate phase.This study is the second part of a two-part study on Evaluating the Effects of TBP and NPH (dodecane) in Simulated Low-Activity Waste Solution on Ultrafiltration and Ion Exchange. The first study dealt with ultrafiltration. The second part involved using filtrates from the ultrafiltration work to perform resin sorption experiments using SuperLig® 644 and SuperLig® 639 resins. SuperLig® 644 resin has a high selectivity for cesium, while SuperLig® 639 resin is highly selective for rhenium as perrhenate (the surrogate for pertechnetate). What is unknown is the effect the organics may have on these resins’ high-sorption capacities for cesium and rhenium.The goal of the study was to evaluate the effectiveness of the resins to remove cesium and rhenium from AZ-101 simulant containing organics by comparing to the baseline case (i.e., AZ-101 simulant containing no organics) using both batch contact and small-scale column tests.The batch contact work demonstrated there is no statistically significant effect of organics (TBP, dodecane, and their degradation products) on cesium and rhenium sorption by SuperLig® 644 and SuperLig® 639 resins, respectively, from AZ-101 simulant solution. Both the cesium and rhenium column breakthrough/elution profile curves for simulants with and without organics were virtually identical, which is an indication of a no-organics effect.

Corrosion behavior of Inconel 690 and 693 in an iron phosphate melt

The corrosion resistance of Inconel 690 and 693 coupons submerged in an iron phosphate melt has been investigated. After 155 days in an iron phosphate melt at 1050°C, which contained 30wt% of a simulated low activity waste (LAW at Hanford), the weight loss of Inconel 690 and 693 was 14% and 8%, respectively. The overall corrosion rate, calculated from the initial and final dimensions of each coupon, was 1.3 and 0.7μm/day for the Inconel 690 and 693, respectively. Scanning electron microscopy and X-ray diffraction of the submerged Inconel coupons after 155 days in the iron phosphate melt showed that an altered surface layer had formed which was depleted in nickel and consisted of a (Fe, Cr)2O3 solid solution. This altered layer appears to be chemically protective as indicated by the gradual reduction in weight loss which occurred with time in the iron phosphate melt. Inconel 693 appears to be a better candidate to use as an electrode in iron phosphate melts since its corrosion rate and weight loss was only about one half that of Inconel 690.

New Developments in Field Studies of Low Activity Waste Glass Corrosion and Contaminant Transport

ABSTRACTPerformance assessment calculations for low-activity waste glass to be disposed at the Hanford site depend on simulations of long-term glass corrosion behavior and contaminant transport that are being performed via reactive chemical transport modeling. Confidence in the underlying physical and chemical processes that are being approximated by the computer model could be significantly enhanced through carefully-controlled field testing, which includes studies of buried ancient glasses. Field tests with simulated low-activity waste glasses have been initiated on the Hanford site and at the Ballidon site in the United Kingdom. In addition, a joint PNNL – SIA RADON research project has been initiated to analyze a unique data set collected over 12+ years during a Russian in-situ testing program with actual low-activity waste glass. The glasses buried at Hanford are scaled down cylinders (45 kg mass) and include a representative glass composition for Hanford low-activity waste and a glass designed to be highly reactive. Each glass was doped with chemical analog tracers (Re, Se, and Mo). Design of the lysimeter test facility and sampling locations were done with aid of predictive calculations performed with the STORM code. These same glasses were also buried at Ballidon but in the form of small glass coupons. The coupons will be retrieved over a period of several years for detailed analysis along with core samples of the surrounding limestone, which will be analyzed for contaminant transport profiles. Analysis of porewater from underneath the Russian burial site ha s been compared with modeling calculations. Good agreement between the model and the field data has been obtained so far using estimated parameters for the glass corrosion. Independent laboratory tests are in progress to parameterize the STORM model for quantitative comparisons.

The effect of experimental conditions and evaluation techniques on the alteration of low activity glasses by vapor hydration

The vapor hydration test (VHT) is used to study the reaction between waste forms and water with the aim to gain insight into their alteration behavior. In nuclear-waste immobilization, the test is primarily used as a screening tool to identify durable/non-durable glasses and as a convenient method to generate and identify alteration products for use in performance modeling. The lack of a standard procedure for conducting this test has resulted in a number of reported test methods, thus decreasing the ability to directly compare test results from different sources. To optimize the VHT procedure, a series of tests was conducted on simulated low activity waste (LAW) glasses at temperatures ranging from 150 to 300 °C with different volumes of water, specimen holders, specimen-preparation techniques, and data-evaluation methods. Reaction progress was monitored by measuring the thickness of the remaining glass and the alteration layer. The alteration rate at which glass is converted into the alteration products was determined by linear regression from the remaining glass thickness plotted as a function of time. The resulting procedure for conducting VHT eliminates problems associated with the measurement of alteration layers and enables direct comparison of alteration rates for different materials.