This publication presents the outcomes of The International Project on Demonstration of Safety for Geological Disposal (GEOSAF Project), implemented by the IAEA from 2013 to 2021. It outlines an approach for addressing the relationship between the operational phase and the post-closure safety of a geological disposal facility and demonstrates how safety considerations can support decision making. The structured approach developed aims to provide assurance that developing and operating a geological disposal facility over several decades will not compromise its expected post-closure safety functions and the overall safety of that facility. To this end, this TECDOC defines and further develops the key concepts of the safety envelope, the design target, and the as-built state.