[en] The High Temperature Gas Cooled Reactor (HTGR) with outlet coolant temperature up to 950 .deg. C is considered as an efficient reactor for the thermochemical SI(Sulfur Iodine) cycle to be coupled for the hydrogen production. The small scale high temperature test loop which is able to simulate the part of sulfuric acid decomposition process of SI cycle was installed in KAERI facility. The test loop consists of primary and secondary system. One of the key components in the test loop is the SO₃ decomposer, which receives heat from the nitrogen gas circulating in the primary system and transfers it to the sulfuric acid gas mixture circulating through the SO₃ decomposer in the secondary system. The purpose of the test loop is to confirm the integrity of the SO₃ decomposer under these high pressure and high temperature operating conditions. In this paper, the secondary system of the test loop is simulated using aspen plus chemical process simulator to predict the behavior of the small scale test loop