A methodology for assessing the environmental impact of products and services is the life cycle analysis (LCA); which is a versatile tool to define the inclusion process and the scope of the production system, for different scenarios and selective comparison of environmental burdens. For the LCA developed in this work, the S I thermochemical cycle coupled to a high temperature gas nuclear reactor was selected. The defined system function is the production of hydrogen using nuclear energy and the functional unit is 1 kg of hydrogen at the plant gate. The product system was defined by the following steps: (i) extraction and manufacturing of raw materials (upstream flows), (ii) external energy supplied to the system, (iii) nuclear power plant, and (iv) hydrogen production plant. Particular attention was placed to those processes where there was limited information from literature about inventory data, like the TRISO fuel manufacture, and the production of iodine from caliches, which is supplied to the thermochemical process for hydrogen generation. The environmental impact assessment focuses on the emissions of greenhouse gases as comparative parameter related to global warming. The results showed low emissions when electric power was supplied from nuclear energy. When the electric power supply was changed to a mix of fossil fuels, the emissions were significantly higher. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.