A number of reports have identified four volatile radionuclides that arise from the reprocessing of nuclear fuel ( 3 H, 14 C, 85 Kr, and 129 I) that require controls limiting their environmental release from reprocessing facilities in order to meet US regulatory requirements. Of these, 129 I has the longest half-life and highest potential biological impact.The study of inorganic iodide in aqueous reprocessing facility off-gas systems has been almost exclusively limited to I 2 , and the focus of organic iodide studies has been CH 3 I as a surrogate for various organic iodides that could be present. A study conducted in 2015 focused on those inorganic and organic iodine-bearing species in reprocessing plants that have the potential to be poorly sequestered with traditional capture methods. This study found that even if high decontamination factors (DFs) are achieved for the dissolver off-gas (DOG), losses of iodine to the head-end cell, if greater than about 0.1% of the total iodine inventory, can limit the overall plant DF unless sufficiently high DFs are also achieved for the cell off-gas. The same is true for the losses of iodine to the vessel off-gas (VOG), if those losses are greater than about 0.1%. The study further indicated that high DFs on the VOG may be difficult to achieve as it is the stream where penetrating, or difficult to remove, organic iodide species are most likely. These two factors prompted studies examining organic iodide removal from prototypic VOG streams at both Oak Ridge National Laboratory (ORNL) and Idaho National Laboratory (INL). These studies included an initial evaluation of reduced silver-exchanged mordenite (Ag 0 Z) and silver-functionalized silica-aerogel (AgAerogel) for the capture of CH 3 I and I 2 under VOG conditions by Jubin et al. (2017a) and for the capture of iodobutane under DOG conditions at INL (to be reported in 2018.)The objective of this report is to develop a robust experimental program that will evaluate the adsorption rate of long-chain organic iodides and identify any significant variations in mass transfer zone and byproducts produced during the adsorption on Ag 0 Z and AgAerogel under both DOG and VOG conditions expected for a used nuclear fuel aqueous reprocessing facility. Organic species of interest include both short-chain alkyl iodides such as methyl iodide (CH 3 I) and longer alkyl iodides up to iodododecane (C 10 H 21 I). The primary product of this report is a test plan for the experimental work that could be conducted as part of the ongoing off-gas abatement R&D efforts at both INL and at ORNL.
In fiscal year 2016 an Engineering Evaluation of an integrated off-gas system was conducted. This study resulted in a report entitled Engineering Evaluation of an Integrated Off-Gas Treatment System for Used Nuclear Fuel Reprocessing Facilities (Jubin et al. 2016a), from here referred to as the "Engineering Evaluation." This study focused on the capture and retention of the volatile radionuclides ( 3 H, 14 C, 85 Kr, and 129 I), selected semi-volatile radionuclides, specifically 106 Ru, and chemical species like NO and NO 2 generated during the dissolution of the used nuclear fuel. The study examined the design of the combined head-end off-gas streams and the vessel off-gas stream. The study drew upon the available literature to conduct the equipment sizing and sorbent usage. A number of assumptions were required to complete this analysis and major gaps in the available data were identified that, if resolved, could increase the fidelity of the engineering design.This assessment looks at those identified data gaps and the more subtle assumptions that were required in the Engineering Evaluation and provides a detailed look at the specific data needs for each major system. This assessment of the data gaps builds on a study conducted early in fiscal year 2016 that established a set of performance criteria for capture and immobilization technologies. The gaps identified in the engineering assessment also provided a check on the breadth of the criteria and metrics that were previously developed. During the analysis of the data gaps it was necessary to expand the metrics slightly to capture co-adsorption effects and to address desorption, neither of which were included in the original set of metrics. Across the six major off-gas control systems evaluated a number of common data gaps became apparent. These include: Capacity and adsorption rate data needs appear common for virtually all the unit operations considered. These two parameters dictate the dimensions and operating conditions of the sorbent beds and wet scrubber systems. The capacity dictates the mass and volume of Ru, tritium, I, and Kr/Xe sorbent beds for a given adsorption duration; the adsorption rate dictates the needed depth of the solid sorbent beds and the height of the wet scrubbers. Co-adsorption of other species is important to the extent that this interferes with the adsorption of the target species, impacts process operation, or impacts handling, recycle, or disposal of the spent sorbent/scrub solutions, and the waste form and disposal of the captured species. This is of secondary importance compared to demonstrating and optimizing capacity and adsorption rates for the target species; but co-adsorption and impacts of coadsorption are areas where generally even less is currently known for both sorbent beds and wet scrubber systems. Sorbent particle and bulk densities impact the volume of the adsorption system for a given mass of sorbent, but these physical properties data are readily measured. For systems including regenerable sorbents, data o...
Nuclear fuel reprocessing produces off-gas streams containing several radioactive components that must be captured for further treatment or storage. As part of the Off-Gas Sigma Team, parallel research at INL and PNNL has produced several promising sorbents for the selective capture of xenon and krypton from surrogate off-gas stream compositions. In order to design full-scale treatment systems, sorbents that are promising on a laboratory scale must be tested under process conditions prior to consideration for pilot testing and eventual full-scale application. To that end, a bench-scale multi-column system with capability to test multiple sorbents was designed and constructed at INL. This report details multi-column testing of CaSDB MOF produced at PNNL.
Nearly all previous testing of HZ-PAN and AgZ-PAN was conducted at the same flow rate in order to maintain consistency among tests. This testing was sufficient for sorbent capacity determinations, but did not ensure that sorbents were capable of functioning under a range of flow regimes. For this reason, a series of tests were conducted on both HZ-PAN and AgZ-PAN at superficial velocities between 20 and 700 cm/min. For HZ-PAN, Kr capacity increased from 60 mmol/kg to 110 mmol/kg as superficial velocity increased from 21 to 679 cm/min. Results for AgZ-PAN were similar, with Xe capacity ranging from 72 to 124 mmol/kg over the same range of superficial velocities. These results are promising for scaling up to process flows, demonstrating flexibility to operate in a broad range of superficial velocities while maintaining sorbent capacity. While preparing for superficial velocity testing it was also discovered that AgZ-PAN Xe capacity, previously observed to diminish over time, could be recovered with increased desorption temperature. Further, a substantial Xe capacity increase was observed. Previous room temperature capacities in the range of 22-25 mmol Xe/kg AgZ-PAN were increased to over 60 mmol Xe/kg AgZ-PAN. While this finding has not yet been fully explored to optimize activation and desorption temperatures, it is encouraging.
Previous research studies have shown that INL-developed engineered form sorbents are capable of capturing both xenon and krypton from various composite gas streams. The existing experimental test bed provided the capability of single column testing for capacity evaluations over a broad temperature range. To advance research capabilities, the employment of an additional column to study selective capture of target species to provide a defined final gas composition for waste storage was warranted. The second column addition also allows for compositional analyses of the final gas products to provide for final storage determinations.The INL krypton capture system was modified by adding an additional adsorption column to create a multi-column test bed. The purpose of this modification was to allow for the investigation of the separation of xenon from krypton supplied as a mixed gas feed. An extra (upstream) column was placed in a Stirling Ultra-low Temperature Cooler, capable of controlling temperatures between 190 and 253K and the column was filled with AgZ-PAN sorbent to capture the xenon from the feed gas. The effluent from this column would then be routed to the column in the cryostat filled with HZ-PAN to capture the krypton. Additional piping and valves were incorporated into the system to allow for a variety of flow path configurations.A limited scope of testing was performed to evaluate the performance of this updated test bed and to determine xenon and krypton separation, adsorption selectivity, desorption, and final concentrations in the desorbed gas streams. Sampling and analysis methods used in these tests included on-line GC-TCD analysis of the AgZ-PAN column outlet gas for xenon and krypton, as has been used in prior tests, and the addition of GC-MS evaluation of evacuated sample bombs. The sample bombs were analyzed after each test completion to measure xenon and krypton desorption from the columns as they were heated to above room temperature, with no purge gas flow.Two separation tests were performed utilizing a feed gas consisting of 1000 ppmv xenon and 150 ppmv krypton with the balance made up of air. The AgZ-PAN column temperature was held at 295 or 253K while the HZ-PAN column was held at 191K for both tests. The effluent from the AgZ-PAN column was monitored for xenon and krypton via GC-TCD during the tests. No xenon was detected exiting the AgZ-PAN column during the adsorption phase of either test.During the desorption phase of each test, gas samples from each column were taken via evacuated sample bombs and were analyzed by GC-MS analysis. No purge gas flowed during the sample bomb collection; the sample collection relied only on the evacuated bombs that drew gas from the sorbent beds when the connecting valve was opened.Results of these evaluations verified that the system operated as designed and demonstrated that AgZ-PAN exhibits excellent selectivity for xenon over krypton in air at or near room temperature, and that krypton with only small amounts of xenon was captured in the HZ-PAN column ...
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