Virtual Reality (VR) technologies have potential for training applications, providing advantages in terms of cost reduction, safety and time. The success of a VR package for training relies greatly on its capacity to provide a real-time immersive effect to the user. For the sake of realism a simulated instrument must be able to compute the dose rate in the virtual environment with equivalent accuracy and in about the same time as the real instrument. Further, visual rendering must be smooth in order to provide a fluid scenario movement. This paper presents a VR based prototype to accurately model the dosimetry of radioactive and nuclear sources, to be selected from a wide library. Dose measurements should reflect local conditions, i.e., presence of shielding materials (with shape and type selected from a library) and accept sources irrespectively of their dimensions and shapes. The scope of the application is to train civil protection officers, including border / customs officers, fire-fighters, local police, etc. and let them acquire the skills necessary for Nuclear Security activities. These people have a wide professional profile and are not experts in the detection of nuclear and radioactive materials. A VR based dosimetry training application involves the creation of a 3D realistic scenario (e.g., a model of a customs station - including vehicle lanes and parking, accident area or even a nuclear site). This model can be populated with selected sources (i.e., the virtual sources). Trainees can practice on how to use an instrument, from a technical and procedural perspective, such that sources can be found and conclusions drawn. To be effective, two features are required: accuracy in the (virtual) radiation measurements and real time response. This challenge led to the development of methods and algorithms ensuring that these two, somehow contradictory, features can be met for any source (with any shape and dimension) and measurement distance.