5.25. Understanding of the performance of the disposal system and its dependence on features, events and processes that are internal and external to the facility evolves as more data are accumulated and scientific knowledge is developed. “Early in the development of the concept, the data obtained and the level of understanding gained have to assure sufficient confidence to be able to commit resources for further investigations” [1] (see Appendix II, paras II.61?II.71). Before the start of construction, during emplacement and at closure of the facility, the level of understanding has to be adequate to support the safety case that can facilitate the process to make decisions and gain regulatory approvals to proceed.