Based on our previous study on the experimental performance tests of the catalyst-packed type HI thermal decomposer and bayonet type H2SO4 thermal decomposer for a 50 NL-H-2/h SI test facility, which were directly heated using electrical heating chambers, semi-pilot scale H2SO4 and HI decomposers for the 1 N m(3)-H-2/h SI test facility coupled to an out-of-pile helium loop have been designed, and it was theoretically confirmed that the design specifications satisfy the hydrogen production capacity based on a Computational Fluid Dynamics (CFD) analysis. The effects of the overall heat transfer coefficient on the helium outlet temperatures and decomposition percentages of the decomposers were identified. The H2SO4 and HI decomposers proposed are capable of outlet helium temperatures of 734 degrees C and 383 degrees C for an overall heat transfer coefficient of 5 W/m(2) K, respectively, which satisfy the operating temperature conditions of the out-of-pile helium loop. The average thermal decomposition percentages of the proposed decomposers are 60.4% for sulfuric acid and 22.4% for hydrogen iodide. These decomposition percentages obtained from the numerical results are acceptable with a hydrogen production rate of 1 N m(3)-H-2/h. Modification points of the decomposers to increase the decomposition percentages are suggested, such as a minimization of heat loss into the atmosphere and optimization of component designs. (C) 2017 Elsevier Ltd. All rights reserved.