This paper presents some of the results obtained in the VRIMOR project (virtual reality for inspection, maintenance, operation and repair of nuclear power plants). The general aim was to integrate environmental scanning technologies with human modelling and radiological dose estimation tools, and to deliver an intuitive and cost-effective system for use by operators involved with interventions in radiologically controlled areas. The usability of the resulting products was one of the main success criteria. This paper describes the general approach and design mechanisms used in the HeSPI (HeSPI stands for the Spanish for Herramienta para la Simulacion y Planificacion de Intervenciones, or tool for the simulation and planning of interventions) tool that has been developed by one of the teams. The tool provides the designer of an intervention with a humanoid 3D model, or mannequin, that can be loaded into the desired environment and will be used by the designer as if he was manipulating a puppet, making it move around the environment and perform different kinds of actions, adopting varied postures, interacting with the objects in the environment and manipulating tools and equipment. A combination of a graphical user interface (GUI) and a voice recognition system, together with the selected design mechanisms, has proven to offer good enough interaction possibilities for this kind of desktop virtual environment.