The Outage and Maintenance Services Team at a nuclear power company is tasked with repositioning garter spring spacers. These spacers maintain the gap between the fuel carrying pressure tube and the insulating calandria tube. The repositioning process is essential for the safe operation of the nuclear reactor and life extension activities, but can be challenging due to possible tube deformations. We have developed a decision support tool to address this challenge. We outline how both empirical and knowledge based modelling methods are integrated to support both the planning and conduct of the repositioning process. Visualisation allows exploration of empirical data from past outages to assess successfulness (chance of and size of movements) and underpins prediction of the relative success of actions. Knowledge representation of the reasoning by experienced operators provides a set of decision rules about possible actions when there are repositioning difficulties. We explain our methodology for creating the rulebased knowledge model embedded in the decision support tool. We reflect on the validation and verification of our modelling given the journey from conceptualisation to use in a scheduled outage.