Materials Development by a Surface Modification for the Sulfuric Acid Decomposer in Iodine-Sulfur(IS)cycle for Nuclear H
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1215-V18-01
Materials Development by a Surface Modification for the Sulfuric Acid Decomposer in IodineSulfur(IS)cycle for Nuclear Hydrogen Production System. Jae-Won Park, Hyung-Jin Kim and Yongwan Kim Korea Atomic Energy Research Institute, Daejon-City, South.Korea
ABSTRACT Efforts have been made to develop long term sustainable materials for use above 900°C in the SO3/SO2 environments utilized in Nuclear Hydrogen Production Systems. In this study, the surface of Hastelloy X was serially modified by evaporative SiC coating, ion beam mixing (IBM), additional coatings, and final ion beam hammering (IBH). Subsequent heating above 900°C results in no peelingoff of the SiC coating layer in spite of the huge difference in their coefficients of thermal expansion (CTE). It was also found that ion beam hammering (irradiation) suppresses the vacuum sublimation of the low density (~40% of bulk) ceramic film (bulk materials begin to sublime at ~ 750°C in a vacuum of 1.5x10-5 torr). The sublimation rate is ≥ 30% of the bulk rate after an annealing at 950 for 2 hrs but is decreased to ≤10% of the bulk rate after irradiation with 70 keV ions to a total dose of 1x1017 N ions/cm2. Further irradiation (up to 4x1017 N ions/cm2) does not further decrease the rate. Both an immersion test in 98% sulfuric acid and the potentiodynamic polarization test suggest that the surface modified Hastelloy X has a greatly prolonged life time in the corrosive sulfuric acid atmosphere, suggesting the serial surface modification process is applicable to the thermo-chemical system for the nuclear hydrogen production. INTRODUCTION Since it was reported that massive production of hydrogen could be possible using the high temperature reactor with the aid of thermochemical methods [1], High Temperature Gas Cooled Reactor (HTGR) combined with the Iodine-Sulfur (IS) cycle has been regarded as the most efficient system for a mass production of hydrogen [2-3]. In the IS cycle, a process heat exchanger (PHE) comprised of channels for He and decomposed sulfuric acid gas (SO2/SO3/H2O) is needed. The material used for the sulfuric acid gas channels is of importance because it is subjected to severe corrosion environment. Currently there is no suitable commercial metallic material available. To surmount this obstacle, consideration is being given to surface modification of metallic materials or a development of ceramic PHEs. In this work, the surface modification option was studied. We selected Hastelloy X as the metallic substrate due to its good mechanical properties at a high temperatures and SiC as a corrosion inhibiting coating material (its corrosion resistance being due to the very strong covalent bonding between silicon and carbon [4-5]). This ceramic-coating-on-metal system certainly has considerable merit because that it does not hamper the manufacturabilty of the system as compared to a ceramic PHE system. In developing a surface coated metallic system, consideration needs to be given to how best to ensure the adhesion of the coating layer and how to re
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