The optimisation of radiological protection of the workers in nuclear industry is an important part of the safety culture The application of the ALARA concept (to keep exposures As Low As Reasonably Achievable) is not always straightforward as it is influenced by the site geometry, source distribution and work organization. A good ALARA pre-job study must be performed and should contain predicted doses in the work area and investigate the effects of geometry, material, source or work position changes. This information provides a quantitative basis to select between various alternative work scenario's for a specific operation. In order to handle this information SCK•CEN developed the VISIPLAN 3D ALARA planning tool. This PC-based tool makes it possible to create and edit work scenario’s taking into account worker positions and subsequent geometry and source distribution changes in a 3D environment.In this presentation we will show how the tool is successfully applied in the planning of maintenance and especially in the planning of decommissioning activities. Special attention will be given to methods used to build an adequate model of a site involving the analysis and interpretation of radiological measurements going from standard ambient dose measurements to the more advanced gamma scanning techniques.