Engineering topics
Lee, S
Publications and source records attributed to Lee, S.
High Efficiency Visible Photocathode Development
The alkali antimonides are of significant interest to the accelerator community, as they have a high quantum efficiency (QE) under illumination by green light. These cathodes are attractive for high-average-current photoinjector applications and understanding their fundamental properties are critical to the production of long lived, reliable cathodes. We report on ongoing work to characterize cathode formation during growth. In-situ X-ray Diffraction (XRD) has been used to compare grain size and texture in antimony layers, and X-ray Photoelectron Spectroscopy (XPS) has been used to investigate the solid-state chemistry of cathode formation.
RAMI Analyses of Heating Neutral Beam and Diagnostic Neutral Beam Systems for ITER
A RAMI (Reliability, Availability, Maintainability, Inspectability) analysis has been performed for the heating (and current drive) neutral beam (HNB) and diagnostic neutral beam (DNB) systems of the ITER device. The objective of these analyses is to implement RAMI engineering requirements for design and testing to prepare a reliability-centred plan for commissioning, operation, and maintenance of the system in the framework of technical risk control to support the overall ITER Project. These RAMI requirements will correspond to the RAMI targets for the ITER project and the compensating provisions to reach them as deduced from the necessary actions to decrease the risk level of the function failure modes. The RAMI analyses results have to match with the procurement plan of the systems.
THREE-DIMENSIONAL THERMAL MODELING ANALYSIS OF CST MEDIA FOR THE SMALL ION EXCHANGE PROJECT
The Small Column Ion Exchange (SCIX) project is designed to accelerate closure of High Level Waste (HLW) tanks at the Savannah River Site (SRS). The SRS tanks store HLW in three forms: sludge, saltcake, and supernate. An in-tank ion exchange process is being designed to treat supernate and dissolved saltcake waste. Through this process, radioactive cesium from the salt solution is adsorbed into Crystalline Silicotitanate (CST) ion exchange media packed within a flow-through column. A packed column loaded with radioactive cesium generates significant heat from radiolytic decay. The waste supernate solution within the ion exchange bed will boil around 120 C. Solution superheating above the boiling point within the column could lead to violent hazardous energy releases. System heating from loaded CST is also of concern in other process modules, such as the waste tank. Due to tank structural integrity concerns, the wall temperature limit for the SRS waste tanks is 100 C. The transfer of cesium-loaded CST to the tank could result in localized hot spots on the tank floor and walls which may exceed this limit. As a result, thermal modeling calculations have been conducted to predict the maximum temperatures achievable both in the column and in the waste tank. As specified in the associated Technical Task Plan, one objective of the present work was to compute temperature distributions within the ion exchange column module under accident scenarios including loss of salt solution flow through the bed and loss of coolant system flow. The column modeling domain and the scope of the calculations in this case were broadened relative to previous two-dimensional calculations to include vertical temperature distributions within the packed bed of ion exchange media as well as the upper column plenum region containing only fluid. The baseline design conditions and in-column modeling domain for the ion-exchange column module are shown in Figure 1. These evaluations assumed the maximum bounding cesium loading considered possible based on current knowledge regarding CST media and the anticipated feed compositions. Since this cesium loading was considerably higher than the nominal loading conditions in SRS waste, cases with lower loading were also evaluated. Modeling parameters were the same as those used previously unless otherwise indicated. The current model does not capture multi-phase cooling mechanisms operative when solution boiling occurs. This feature is conservative in the sense that it does not account for the large cooling effects associated with phase transfer. However, the potential transfer of heat to the plenum region associated with vertical bubble ascension through the column during boiling is also neglected. Thermal modeling calculations were also performed for the entire waste storage tank for the case where loaded and ground CST was transferred to the tank. The modeling domain used for the in-tank calculations is provided in Figure 2. The in-tank domain is based on SRS Tank 41, which is a Type-IIIA tank. Temperature distributions were evaluated for cylindrical, ground CST mounds located on the tank floor. Media grinding is required prior to vitrification processing of the CST in the SRS Defense Waste Processing Facility (DWPF). The location of the heat source region on the tank floor due to the accumulation of CST material was assumed to be just under the grinder. The shape of the CST mound was assumed to be cylindrical. This shape is believed to be most representative of the actual mound shape formed in the tank, given that submersible mixing pumps will be available for media dispersion. Alternative configurations involving other geometrical shapes for the CST mound were evaluated in the previous work. Sensitivity analysis for the in-tank region was performed for different amounts of CST media. As was the case for the in-column model, the in-tank model does not include multi-phase cooling mechanisms operative when solution boiling occurs. The in-column and the in-tank evaluations incorporated recently updated maximum cesium loading levels calculated using the current SCIX feed compositions, which resulted in significantly higher cesium loading than previously calculated. The calculations were conducted to ensure conservative predictions for the maximum temperatures achievable using the current baseline design. The degree of conservatism was reduced for in-column calculations relative to the previous work by using a three-dimensional modeling approach and selecting parameters which were nearer to expected conditions. The degree of conservatism for the in-tank calculations was also reduced by lowering the soil penetration depth below the tank from 150 to 20 feet. The primary goals of the extended thermal modeling effort were to determine whether fluid boiling or superheating are possible within the column module and to determine the maximum floor temperatures within the tank loaded with spent CST.
GROUT HOPPER MODELING STUDY
The Saltstone facility has a grout hopper tank to provide agitator stirring of the Saltstone feed materials. The tank has about 300 gallon capacity to provide a larger working volume for the grout slurry to be held in case of a process upset, and it is equipped with a mechanical agitator, which is intended to keep the grout in motion and agitated so that it won't start to set up. The dry feeds and the salt solution are already mixed in the mixer prior to being transferred to the hopper tank. The hopper modeling study through this work will focus on fluid stirring and agitation, instead of traditional mixing in the literature, in order to keep the tank contents in motion during their residence time so that they will not be upset or solidified prior to transferring the grout to the Saltstone disposal facility. The primary objective of the work is to evaluate the flow performance for mechanical agitators to prevent vortex pull-through for an adequate stirring of the feed materials and to estimate an agitator speed which provides acceptable flow performance with a 45{sup o} pitched four-blade agitator. In addition, the power consumption required for the agitator operation was estimated. The modeling calculations were performed by taking two steps of the Computational Fluid Dynamics (CFD) modeling approach. As a first step, a simple single-stage agitator model with 45{sup o} pitched propeller blades was developed for the initial scoping analysis of the flow pattern behaviors for a range of different operating conditions. Based on the initial phase-1 results, the phase-2 model with a two-stage agitator was developed for the final performance evaluations. A series of sensitivity calculations for different designs of agitators and operating conditions have been performed to investigate the impact of key parameters on the grout hydraulic performance in a 300-gallon hopper tank. For the analysis, viscous shear was modeled by using the Bingham plastic approximation. Steady state analyses with a two-equation turbulence model were performed with the FLUENT{trademark} codes. All analyses were based on three-dimensional results. Recommended operational guidance was developed by using the basic concept that local shear rate profiles and flow patterns can be used as a measure of hydraulic performance and spatial stirring. Flow patterns were estimated by a Lagrangian integration technique along the flow paths from the material feed inlet. The modeling results show that when the two-stage agitator consisting of a 45{sup o} pitched propeller and radial flat-plate blades is run at 140 rpm speed with 28 in diameter, the agitator provides an adequate stirring of the feed materials for a wide range of yield stresses (1 to 21 Pa) and the vortex system is shed into the remote region of the tank boundary by the blade passage in an efficient way. The results of this modeling study were used to develop the design guidelines for the agitator stirring and dispersion of the Saltstone feed materials in a hopper tank.