An event in November 2007 in Ascó-1 nuclear power plant (Spain) originated the release of a significant amount of hot metallic particles through the discharge stack. Particles were dispersed and deposited in roofs and neighbouring areas within the NPP controlled area. However, the event was not detected until March 2008. More than 1,300 hot points with radioactive particles were found, 94% located inside the double fenced controlled area and 6% within the exclusion area; 5 particles were out of the exclusion area, across the river. To provide additional insights on the potential consequences of the release, a computational fluid dynamics (CFD) code, Ansys-CFX-11, has been used to simulate the near-range atmospheric dispersion and deposition of the particles. The purpose of the analysis was to assess the distance travelled by particles of different sizes. A very detailed model of the site was built, taking into account the buildings and the terrain features including the river valley and the surrounding hills. The modelled domain was 3.2 x 5.2 km, with the atmospheric layer up to 4 km height. The atmospheric conditions recorded during different periods of time were classified into 37 representative categories. In general, the distribution of the particles found was adequately reproduced. Particles larger than 100 microns could not travel beyond the double fence. Particles between 50 and 100 microns could have been deposited mainly within the exclusion area, with a small probability of travelling farther. Smaller particles could have travelled beyond, but also should have been deposited in the nearby area, while the majority of particles found are larger, thus indicating that the size of the released particles should be above 50 microns. The detailed CFD simulation allowed answering relevant questions concerning the possibility of having an impacted region larger than the exclusion area
