Troy G. Garn

Radioactive Iodine and Krypton Control for Nuclear Fuel Reprocessing Facilities

The removal of volatile radionuclides generated during used nuclear fuel reprocessing in the US is almost certain to be necessary for the licensing of a reprocessing facility in the US. Various control technologies have been developed, tested, or used over the past 50 years for control of volatile radionuclide emissions from used fuel reprocessing plants. The US DOE has sponsored, since 2009, an Off-gas Sigma Team to perform research and development focused on the most pressing volatile radionuclide control and immobilization problems. In this paper, we focus on the control requirements and methodologies for 85Kr and 129I. Numerous candidate technologies have been studied and developed at laboratory and pilot-plant scales in an effort to meet the need for high iodine control efficiency and to advance alternatives to cryogenic separations for krypton control. Several of these show promising results. Iodine decontamination factors as high as 105, iodine loading capacities, and other adsorption parameters including adsorption rates have been demonstrated under some conditions for both silver zeolite (AgZ) and Ag-functionalized aerogel. Sorbents, including an engineered form of AgZ and selected metal organic framework materials (MOFs), have been successfully demonstrated to capture Kr and Xe without the need for separations at cryogenic temperatures.

Polyacrylonitrile-Chalcogel Hybrid Sorbents for Radioiodine Capture

Powders of a Sn2S3 chalcogen-based aerogel (chalcogel) were combined with powdered polyacrylonitrile (PAN) in different mass ratios (SnS33, SnS50, and SnS70; # = mass% of chalcogel), dissolved in dimethyl sulfoxide, and added dropwise to deionized water to form pellets of a porous PAN-chalcogel hybrid material. These pellets, along with pure powdered (SnSp) and granular (SnSg) forms of the chalcogel, were then used to capture iodine gas under both dynamic (dilute) and static (concentrated) conditions. Both SnSp and SnSg chalcogels showed very high iodine loadings at 67.2 and 68.3 mass%, respectively. The SnS50 hybrid sorbent demonstrated a high, although slightly reduced, maximum iodine loading (53.5 mass%) with greatly improved mechanical rigidity. In all cases, X-ray diffraction results showed the formation of crystalline SnI4 and SnI4(S8)2, revealing that the iodine binding in these materials is mainly due to a chemisorption process, although a small amount of physisorption was observed.

Pilot-Scale TRUEX Flowsheet Testing for Separation of Actinides and Lanthanides from Used Nuclear Fuel

Testing of a TRUEX flowsheet has been performed using a thirty-stage, 5-cm centrifugal contactor pilot plant. This testing was performed using a non-radioactive feed surrogate and data were collected and analyzed to evaluate removal efficiencies of the lanthanides, mass transfer efficiency of the lanthanides in the extraction and strip sections of the flowsheet, and the temperature profile of the process solutions throughout the centrifugal contactor pilot plant. Results indicate >99.9% separation for all lanthanides and mass transfer efficiencies typically ranging from 85% to 100%. Energy input into the process under flowsheet conditions resulted in only a moderate temperature rise and did not appear to have a large effect on separations performance. The general process performance is also described.

Development of a hydrogen mordenite sorbent for the capture of krypton from used nuclear fuel reprocessing off-gas streams

A novel new sorbent for the separation of krypton from off-gas streams resulting from the reprocessing of used nuclear fuel has been developed and evaluated. A hydrogen mordenite powder was successfully incorporated into a macroporous polymer binder and formed into spherical beads. The engineered form sorbent retained the characteristic surface area and microporosity indicative of mordenite powder. The sorbent was evaluated for krypton adsorption capacities utilizing thermal swing operations achieving capacities of 100 mmol of krypton per kilogram of sorbent at a temperature of 191 K. A krypton adsorption isotherm was also obtained at 191 K with varying krypton feed gas concentrations. Adsorption/desorption cycling effects were also evaluated with results indicating that the sorbent experienced no decrease in krypton capacity throughout testing.

Thirty Stage Annular Centrifugal Contactor Thermal Profile Measurements

A thirty stage 5 cm annular centrifugal contactor cascade was assembled and tested to obtain thermal profiles during both ambient and heated input conditions of operation. Thermocouples were installed on every stage as well as feed inputs, and real-time data was taken during experiments lasting from two to eight hours at total flow rates of 0.5 to 1.4 liters per minute. Ambient temperature profile results show that only a small amount of heat is generated by the mechanical energy of the contactors. Steady state temperature profiles mimic the ambient temperature of the lab but are higher toward the middle of the cascade. Heated inlet solutions gave temperature profiles with smaller temperature gradients, more driven by the temperature of the inlet solutions than ambient lab temperature. Temperature effects of solution mixing, even at rotor speeds of 4000 rpm, were not measurable.

Advanced Remote Maintenance Design for Pilot-Scale Centrifugal Contactors

Abstract Advanced designs of spent nuclear fuel recycling processes and radioactive waste treatment processes are expected to include more ambitious goals for aqueous-based separations, including higher separation efficiency, high-level waste minimization, and a greater focus on continuous processes to minimize cost and footprint. Therefore, annular centrifugal contactors are destined to play a more important role for such future processing schemes. Pilot-scale testing will be an integral part of development of many of these processes. An advanced design for remote maintenance of pilot-scale centrifugal contactors has been developed and a prototype module fabricated and tested for a commercially available pilot-scale centrifugal contactor (CINC V-02, 5-cm rotor diameter). Advanced design features include air-actuated clamps for holding the motor/rotor assembly in place, an integral electrical connection, upper flange O-rings, a welded bottom plate, a lifting bale, and guide pins. These design features will allow for rapid replacement of the motor/rotor assembly, which can be accomplished while maintaining process equilibrium in the operating contactors during replacement of a unit. This means that fluids in the operating contactors remain at equilibrium with respect to composition and that process solutions are ready to resume discharge when the contactor is replaced and feed solutions are restarted. Hydraulic testing of a three-stage prototype unit was also performed to verify that design changes did not impact performance of the centrifugal contactors. Details of the pilot-scale remote maintenance design, results of testing in a remote mock-up test facility, and results of hydraulic testing of the advanced design are provided.

Development of Steam Reforming for the Solidification of the Cesium and Strontium Separations Product from Advanced Aqueous Reprocessing of Spent Nuclear Fuel

Abstract Steam reforming is one option currently being investigated for stabilization of the cesium/strontium strip products from spent fuel reprocessing solvent extraction processes because it can potentially destroy the nitrates and organics present in these aqueous, nitrate‐bearing solutions, while converting the cesium and strontium into leach resistant aluminosilicate minerals, such as pollucite. To produce pollucite and other mineral analogs of the alkaline metals, the feeds must be mixed with aluminosilicate compounds and thermally sintered or calcined to activate solid‐state crystal formation. Scoping tests completed indicated that the cesium/strontium in these organic and acid solutions can be converted into aluminosilicate materials using steam reforming.

A CENTRIFUGAL CONTACTOR DESIGN TO FACILITATE REMOTE REPLACEMENT

Abstract Advanced designs of nuclear fuel recycling and radioactive waste treatment plants are expected to include more ambitious goals for solvent extraction-based separations, including higher separation efficiency, high-level-waste minimization, and a greater focus on continuous processes to minimize cost and footprint. Therefore, annular centrifugal contactors (ACCs) are destined to play a more important role for such future processing schemes. Previous efforts defined and characterized the performance of commercial 5-cm, model V-02; and 12.5-cm, model V-05, single-stage ACCs in a nonradioactive environment. The next logical step, the design and initial evaluation of remote-capable, pilot-scale ACCs for use in a “hot” or radioactive environment has been completed. This work continues the development of remote designs for ACCs that can process the large throughputs needed for future nuclear fuel recycling and radioactive waste treatment plants. Novel designs were developed for the remote interconnection of contactor units, clean-in-place (CIP) and drain connections, and a new solids removal collection chamber. A three-stage, 12.5-cm-diam rotor module has been constructed and is being evaluated for use in highly radioactive environments. This prototype assembly employs three standard CINC V-05 CIP units modified for remote service and replacement via new methods of connection for solution inlets, outlets, drain, and CIP. Hydraulic testing and functional checks were successfully conducted, and then the prototype was evaluated for remote handling and maintenance. Removal and replacement of the center position V-05R contactor in the three-stage assembly was demonstrated using an overhead rail mounted PaR manipulator. Initial evaluation indicates a viable new design for interconnecting and cleaning individual stages while retaining the benefits of commercially reliable ACC equipment. Replacement of a single stage via remote manipulators and tools is estimated to take [approximately]30 min, perhaps fast enough to support a contactor change without loss of process steady-state equilibrium. The design presented in this work is scalable to commercial ACC models from V-05 to V-20 with total throughput rates ranging from 20 to 650 l/min.

Fundamental Chemistry of the Universal Extractant (UNEX) for the Simultaneous Separation of Major Radionuclides (Cesium, Strontium, Actinides, and Lanthanides) from Radioactive Wastes

Scientists at the INEEL and KRI collaboratively developed and validated the concept of a Universal Extractant (UNEX) for simultaneously removing the major radionuclides (Cs, Sr, actinides, and lanthanides) from acidic radioactive waste in a single solvent extraction process. The UNEX solvent incorporates three active extractants: chlorinated cobalt dicarbollide, polyethylene glycol, and a carbamoylmethylphosphine oxide derivative, dissolved in a suitable organic diluent to simultaneously extract target radionuclides. The process chemistry is unique, but complicated, since the extractants operate synergistically to extract the radionuclides. Furthermore, interactions with the diluent are quite important as the diluent strongly influences the extraction properties of the solvent system. We are currently studying the fundamental chemical phenomena responsible for the selective extraction of the different species to understand the underlying mechanisms and facilitate enhancements i n process chemistry. Our efforts to date have relied on a combination of classical chemistry techniques, infrared (IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy to identify and explain the structures formed in the organic phase, elucidate the operative chemical mechanisms, and evaluate the diluent effects on extraction properties.

Superficial Velocity Effects on HZ-PAN and AgZ-PAN for Kr/Xe Capture

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. 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 capacity ranging from 72 to 124 mmol/kg over the same range of superficial. 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.