Operator support is essential for making decisions involving ageing infrastructure. In particular, ageing plants may have deviated from their original condition into a new state with a less predictable set of possible actions and outcomes. Standard procedures and practices may be insufficient to handle the risks and vulnerabilities of ageing nuclear infrastructure. Operator support must be designed with an understanding of how operators make decisions under uncertainty and a view to supporting unexpected situations. Prospect theory (PT) and ecological interface design (EID) are proposed as two complementary approaches for guiding operator support. PT describes how people make decisions under uncertainty and EID is an interface design approach for aiding operators with the problem-solving process in unanticipated situations. We suggest that these two approaches can be integrated to improve complex decision making in ageing nuclear plants.