Nuclear decommissioning design and operational knowledge may remain relevant over many decades. Designing sustainable operations today requires meaningful simulation of future events, which in turn relies on the accurate representation of all relevant current knowledge. In decommissioning, human agents may be as important for maintaining operational safety as technical systems, so simulations must represent not just technical but tacit knowledge (embedded in practice) and the contextual and perceptual aspects of knowledge – i.e. how the meaning of data changes with the circumstances and viewpoint of stakeholders. We propose that this detail may be captured by ontological knowledge systems, which are augmenting traditional archives, databases or operational flowcharts in other highly interlinked, multidisciplinary, dynamic and highly specialised fields. Using the example of waste transport modelling, this study discusses how a coherent ontology unifying knowledge, physics and simulations could facilitate the design, assessment and communication of sustainable decommissioning.