4.44. Assessment of the post-closure radiological impact forms the core of the safety case for a disposal facility. In addition to qualitative assessments, this involves a comprehensive quantitative analysis of the evolution of the disposal system and its environment, possible challenges to the safety functions and the resulting potential radiological impacts. A systematic approach to radiological impact assessment for the period after closure has been developed and described in IAEA ISAM (2004)[26]. In this approach, scenarios are used to describe possible evolutions of the disposal system and its environment. The potential migration of radioactive substances from the disposal facility, their movement in the environment and resulting radiation risks are quantitatively analysed by means of conceptual and mathematical models. Detailed guidance on this approach is provided in Section 5.