In the event of a radiological accident involving one or more victims and potential high doses, it is essential to know the dose distribution within the body, in order to sort the victims according to the severity of the irradiation, and then to take them to the most suitable medical structures. However, there are currently very a few techniques that can be rapidly deployed on field and capable of characterizing an irradiation. Therefore, a numerical simulation tool has been designed. Called SEED, this tool is integrated into a militarized and hardened device, which can dispense with any communication with a cluster of processors thanks to a powerful multi-core calculator. The user can navigate in three dimensions in the accident scene thanks to a graphical user interface including a “first person” camera. It allows a rapid modeling of this scene, and a visual exchange with the victims and witnesses of the event. In order to ensure the performance of the SEED tool, two dosimetric validations were performed. The first consisted in comparing the dose value provided by SEED to that given by a reference calculation code: MCNPX. The purpose of the second validation was to perform an experiment irradiating a physical dummy equipped with dosemeters and to reconstruct this irradiation using SEED. These two validation protocols have shown very satisfactory results. They confirm that this new tool is able to provide useful information to medical teams in charge of dosimetric triage in case of a major external exposure event.