The invention relates to a collector assembly for a gaseous fluid for a radial reactor. The invention concerns a collector assembly (8) for a gaseous fluid suitable for being disposed in a reaction section with a moving bed of catalyst of a radial reactor. The collector assembly comprises a vertical cylindrical screen (9), permeable to gas and impermeable to particles of catalyst, and a vertical cylindrical tube (10) which is supported by this screen (9) and disposed in a concentric manner with respect to the screen. The tube (10) comprises one or more zones (17a, 17b) which are permeable to gaseous fluid, comprising a plurality of through holes and a plurality of zones (18a, 18b, 18c, 18d) with a reduced permeability to gaseous fluid compared with the permeable zone. Each zone with a reduced permeability has a porosity, defined as the ratio between total permeable surface area of the zone and total developed surface area of this zone, in the range 0 to 0.005.