[en] This report details our assessment of the chemistry of the planned Hanford Waste Vitrification Plant (HWVP) off-gas system and its impact on the applicability of known iodine removal and control methods. To predict the gaseous species in the off-gas system, we completed thermodynamic calculations to determine theoretical equilibrium concentrations of the various potential chemical species. In addition, we found that HWVP pilot-plant experiments were generally consistent with the known chemistry of the individual elements present in the off gas. Of the known trapping techniques for radioiodine, caustic scrubbing and silver-containing sorbents are, in our opinion, the most attractive methods to reduce the iodine concentration in the HWVP melter off gas (MOG) after it has passed through the high-efficiency particulate air (HEPA) filter. These two methods were selected because they (1) have demonstrated retention factors (RFs), ratio of amount in and amount out, of 10 to 1000, which would be sufficient to reduce the iodine concentration in the MOG to below regulatory limits; (2) are simple to apply; (3) are resistant to oxidizing gases such as NOx; (4) do not employ highly hazardous or highly corrosive agents; (5) require containment vessels constructed or common materials; (6) have received extensive laboratory development; (7) and the radioactive wastes produced should be easy to handle. On the basis of iodine trapping efficiency, simplicity of operation, and waste management, silver sorbents are superior to caustic scrubbing, and, or these sorbents, we prefer the silver zeolites. No method has been fully demonstrated, from laboratory-scale through pilot-plant testing, to be an effective iodine trap at the low iodine concentration (2 x 10-11 mol I/L) expected in the MOG of the HWVP in the presence of the other gaseous off gas components. In terms of compatibility of the trapping technology with the components in the MOG, there is some question about the resistance of the silver zeolite's aluminosilicate matrix to the fluoride component in the off gas. The caustic scrubber has no compatibility problems with the MOG off gas; however, the acidic components such as CO2 will increase the volume of waste produced and could affect the efficiency of the iodine trapping. To apply these gaseous iodine trapping technologies to the HWVP, further development work would be required. Neither method has been demonstrated at the very low iodine concentrations that exist in the off gas, which are 0.01% to 1% of the found in nuclear fuel dissolver off gases for which these technologies were developed. Furthermore, the large excess of other reactive and trappable gases in the HWVP off gas imposes a heavy load on the trapping medium, could impede iodine trapping, and could have deleterious effects on the trapping medium itself. For silver zeolites, other trappable gases such as chlorine, which are in gross excess of the iodine in the off gas, will compete for the active sites in the silver zeolite. In applying a silver zeolite to the HWVP, 99-9% of the silver would be used to trap chlorine with less than 0.1% of the silver employed in the zeolite bed used for iodine trapping. It is also difficult to predict what will happen when the aluminosilicate framework of the zeolite is exposed to the reactive gas, HF, which is also present in the off gas and is known to attack silicates. In the case of caustic scrubbing, because of the low iodine concentration in the off gas, essentially all of the caustic will be used for CO2 removal, a small fraction for chlorine and fluorine removal, and a trace amount for iodine removal. NO2, which should exist largely as NO, will not be removed

[en] This report provides an assessment of the use of nitrogen trifluoride for removing oxide and water-caused contaminants in the fluoride salts that will be used as coolants in a molten salt cooled reactor. The Pacific Northwest National Laboratory, in support of the Oak Ridge National Laboratory's program to investigate an advanced molten salt cooled reactor concept for the U.S. Department of Energy, evaluated potential nitrogen trifluoride (NF₃) use as an agent for removing oxide and hydroxide contaminants from candidate coolants. These contaminants must be eliminated because they increase the corrosivity of the molten salt to the detriment of the materials of containment that are currently being considered. The baseline purification agent for fluoride coolant salts is hydrogen fluoride (HF) combined with hydrogen (H₂). Using HF/H₂ as the reference treatment, we compare HF and NF₃ industrial use, chemical and physical properties, industrial production levels, chemical, toxicity, and reactivity hazards, environmental impacts, effluent management strategies, and reaction thermodynamic values. Because NF₃ is only mildly toxic, non-corrosive, and non-reactive at room temperature, it will be easy to manage the chemical and reactivity hazards during transportation, storage, and normal operations. Industrial experience with NF₃ is also extensive because NF₃ is commonly used as an etchant and chamber cleaner in the electronics industry. In contrast HF is a highly toxic and corrosive gas at room temperature but because of its significance as the most important fluorine-containing chemical there is significant industrial experience managing HF hazards. NF₃ has been identified as having the potential to be a significant contributor to global warming and thus its release must be evaluated and/or managed depending on the amounts that would be released. Because of its importance to the electronics industry, commercial technologies using incineration or plasmas have been developed and are used to destroy the NF₃ in a facility's gaseous effluent stream. A process has been developed and used to recover and recycle NF₃. The electronics industry is actively pursuing alternative methods to control NF₃ releases. In comparison, HF has not been identified to be a potential global warming gas nor has it been determined to have any other environmental affect. Also because of the high solubility of HF in water and aqueous caustic solutions, the HF industry has developed and used aqueous scrubbers to effectively prevent its release into the environment. Care appears to be necessary when using NF₃ in a plant. Precautions must be taken to prevent adiabatic compression and make sure that NF₃ thermal decomposition does not occur in unplanned locations. The system must be engineered to avoid the use of ball valves and sharp bends. The materials of construction that will be required to contain NF₃ and anhydrous HF will be similar. If water is present such as in the process effluent, HF is more corrosive than NF₃ and its containment would require nickel or nickel-based alloys. Both of these fluorinating agents become more reactive with increasing temperature and would require pure nickel or nickel-based alloys for containment until the gas stream has cooled. With respect to the cost of the fluoride, HF is about one third the cost of NF₃ on a fluorine basis. Of the fluorine-containing chemicals, more HF is produced than any other. NF₃ is produced on an industrial scale and its capacity has grown each year since being identified as a useful etchant. Both NF₃ and HF have been demonstrated to be effective at removing oxide, hydroxide, and water contamination from fluoride salts during melt processing of fluoride glasses while HF in combination with H₂ has been demonstrated to be effective for some of the candidate coolant salts and some of their individual constituents such as beryllium oxide (BeO). HF has a limited solubility in molten 66 mol% LiF-33 mol% BeF₂ indicating that treatment with HF will result in free F- in HF-treated fluoride salts. H₂'s flammability and potential explosivity introduces additional hazards to its use. With respect to chemical viability, as measured by reaction free energies, NF₃ is the stronger fluorinating agent when compared to HF. For all postulated contaminants the calculated free energies for treatment by NF₃ were negative, indicating that the reactions were favorable and should occur provided there are no kinetic barriers. In contrast, HF's fluorinating power declined with increasing temperature, and in a couple of instances the reaction free energy became slightly positive (e.g., BeO above 700 C), indicating that use of excess HF would be required for the fluorination to occur or that the product water would have to be removed to force the reaction to occur.

[en] The study of the fluorination of UO2 with various fluorinating agents using thermogravimetric analysis (TGA) is well documented. Additional research has examined the fluorination of other metals and metal oxides. Seeing the possible benefit of using analogous fluorination technologies to affect separations in used nuclear fuel, we have expanded the existing research to an investigation of the reaction of NF3 with UO2 and NpO2. The conversion fractions gathered from experimental results have been compared with those predicted by existing models for various rate-limiting phenomena.

[en] The baseline bulk-vitrification (BV) process (also known as in-container vitrification ICV(trademark)) includes a mixer/dryer to convert liquid low-activity waste (LAW) into a dried, blended feed for vitrification. Feed preparation includes blending LAW with glass-forming minerals (GFMs) and cellulose and drying the mixture to a suitable dryness, consistency, and particle size for transport to the ICV(trademark) container. The cellulose is to be added to the BV feed at a rate sufficient to destroy 75% of the nitrogen present as nitrate or nitrite. Concern exists that flammable gases may be produced during drying operations at levels that could pose a risk. The drying process is conducted under vacuum in the temperature range of 60 to 80 C. These flammable gases could be produced either through thermal decomposition of cellulose or waste organics or as a by-product of the reaction of cellulose and/or waste organics with nitrate or the postulated small amount of nitrite present in the waste. To help address the concern about flammable gas production during drying, the Pacific Northwest National Laboratory (PNNL) performed studies to identify the gases produced at dryer temperatures and at possible process upset conditions. Studies used a thermogravimetric analyzer (TGA) up to 525 C and isothermal testing up to 120 C to determine flammable gas production resulting from the cellulose and organic constituents in bulk vitrification feed. This report provides the results of those studies to determine the effects of cellulose and waste organics on flammable gas evolution

[en] Pacific Northwest National Laboratory has been tasked by Bechtel National Inc. on the River Protection Project-Hanford Tank Waste Treatment and Immobilization Plant (RPP-WTP) project to perform research and development activities to resolve technical issues identified for the Pretreatment Facility. The Pretreatment Engineering Platform (PEP) was designed, constructed, and operated as part of a plan to respond to issue M12, 'Undemonstrated Leaching Processes'. The PEP is a 1/4.5-scale test platform designed to simulate the WTP pretreatment caustic leaching, oxidative leaching, ultrafiltration solids concentration, and slurry washing processes. The PEP replicates the WTP leaching processes using prototypic equipment and control strategies. This report provides the lessons learned regarding the manufacture and delivery of simulated feeds for PEP testing.

[en] Recent work by Hanson [1] has demonstrated a clear dependence of the oxidation of Light Water Reactor spent fuel on burnup. Oxidation of spent fuel was shown to proceed via the two-step reaction UO2?UO2.4?UO2.67+x, where the U3O8-like phase does not form until conversion to UO2.4 is complete. The temperature-dependent activation energy (Ea) of the transition from UO2.4 to the hyperstoichiometric U3O8 was found to be ∼150 kJ mol-1. Each MWD/kg M burnup added ∼1.0 kJ mol-1. The work of McEachern et.al. [2], Choi et. al. [3], and You et. al. [4] have all verified this oxidation dependence on SIMFUEL or unirradiated doped-UO2. All present work agrees that the soluble actinides or fission products that substitute in the U matrix act to delay the onset of U3O8. However, no single model exists to explain the observed behavior, including the fact that most dopants actually allow an earlier onset for UO2.4 formation. The present work is part of a Nuclear Energy Research Initiative project attempting to develop a UO2-based matrix capable of achieving extended burnups by including soluble dopants. The resulting fuel should be highly oxidation and dissolution resistant, which will be beneficial during accident scenarios or for disposal in a geologic repository. In addition, the stabilized matrix may help delay the onset of fuel restructuring that occurs at higher burnups. Initial results of the oxidation tests to quantify effects as a function of ionic radii and charge of the dopant are presented

[en] This document describes the results of our investigations on the potential use of nitrogen trifluoride as the fluorinating and oxidizing agent in fluoride volatility-based used nuclear fuel reprocessing. The conceptual process uses differences in reaction temperatures between nitrogen trifluoride and fuel constituents that produce volatile fluorides to achieve separations and recover valuable constituents. We provide results from our thermodynamic evaluations, thermo-analytical experiments, kinetic models, and provide a preliminary process flowsheet. The evaluations found that nitrogen trifluoride can effectively produce volatile fluorides at different temperatures dependent on the fuel constituent.

[en] In this document, we describe the results of radiation damage testing and characterization for specimens that were resintered to re-establish crystallinity. The phases in these specimens have become amorphous from radiation induced damage over the 8 months since sintering

[en] A triple containment magic-angle spinning rotor insert system has been developed and a sample handling procedure formulated for safety analyzing highly radioactive solids by high resolution solid state NMR. The protocol and containment system have been demonstrated for magic angle spinning (MAS) experiments on ceramic samples containing 5-10 wt% 239Pu and 238Pu at rotation speeds of 3500 Hz. The technique has been used to demonstrate that MASNMR experiments can be used to measure amorphous atomic number fractions produced during accelerated internal radioactive decay. This will allow incorporated ?-emitters with short half-lives to be used to model the long-term radiation tolerance of potential ceramic radioactive waste forms. It is believed to be the first example of MASNMR spectroscopy on samples containing fissionable isotopes

[en] PNNL has demonstrated that cellulose effectively reduces the amount of molten ionic salt during Bulk Vitrification of simulated Hanford Low Level Waste (LLW). To address concerns about the potential reactivity of cellulose-LLW, PNNL used thermogravimetric analysis, differential thermal analysis, and accelerating rate calorimetry to determine in these preliminary studies that these mixtures will support a self-sustaining reaction if heated to 110 C at adiabatic conditions. Additional testing is recommended