The advantages of 3D CAD models cover the whole plant life-cycle-from layout and design to construction and maintenance. Change of workflow is the underlying reason for their success. Without a 3D model the spatial arrangement of components is uncertain, leading to ad hoc measures to deal with unforeseen collisions during the erection of new plant or unplanned work such as the erection of scaffolding or radiation shielding during maintenance work. Using a 3D CAD model makes these and other situations more predictable. The overall workload during original construction and outages can be optimised because competing options can be evaluated in good time, outside the time pressures of building schedules and outages.