A very high temperature reactor (VHTR) has been considered for generation of hydrogen by water-splitting through thermochemical processes using process heat. The development of this technology requires understanding of the coupled behavior of VHTR and the hydrogen generation plants and key safety issues during transients. A system level modeling of the coupled VHTR and sulfur-iodine hydrogen generation process is carried out where the thermal and neutronic models of the VHTR and the chemical models of the sulfur-iodine cycle were developed and benchmarked with available experimental data, models and simulation codes. Transient analysis of the possible accidents emanating either from VHTR or hydrogen plants was carried out. The results indicate that on VHTR side the response of the reactor is affected by the temperature feedback and xenon buildup feedback. On the hydrogen plant side the slowest reaction controlled the speed of the overall response of the chemical plant. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.