NOVELTY - A reactor includes a distribution chamber (4) defined by an upstream surface (2) of a catalyst bed (1), a side surface (6), and a covering surface (7) formed such that its distance from the upstream surface of the catalyst bed is monotonically decreasing towards a central longitudinal axis (C) of the catalyst bed.    USE - For use in catalytic oxidation of a hydrocarbonaceous fuel (claimed), fuel cells..    ADVANTAGE - During normal operation, the gaseous mixture of the reactants flows in a swirling movement, tangentially with respect to the upstream surface of the catalyst bed, resulting in high strain rates or turbulence such that the risk of flame propagation is low. The monotonically decreasing height of the distribution chamber minimizes the occurrence of recirculation of the mixture away from the catalyst surface in the center of the swirl movement. Further, the required length of the mixing chamber may be achieved without the need for a height mixer, resulting to a compact reactor.    DETAILED DESCRIPTION - A reactor comprises an annular catalyst bed having an upstream surface perpendicular to a central longitudinal axis; and a distribution chamber for directing a gaseous mixture of reactants to the upstream surface of the catalyst bed. The distribution chamber has an inlet (5) for tangentially introducing the mixture into the chamber. It is defined by the upstream surface of the catalyst bed, a side surface having a central longitudinal axis that coincides with the central longitudinal axis of the catalyst bed, and a covering surface formed such that its distance from the upstream surface of the catalyst bed is monotonically decreasing towards the central longitudinal axis.    INDEPENDENT CLAIMS are also included for the following:    (A) a fuel cell system comprising the above reactor and a fuel cell; and    (B) a process for a catalytic oxidation, preferably for a catalytic partial oxidation, of a hydrocarbonaceous fuel, using the above reactor.    DESCRIPTION OF DRAWING(S) - The figure shows a longitudinal section of the catalyst bed and distribution chamber of the inventive reactor.    Catalyst bed (1)    Upstream surface (2)    Distribution chamber (4)    Inlet of distribution chamber (5)    Side surface (6)    Covering surface (7)    Cone half angle (8)    Mixing channel (9)    Inlet of mixing channel (10)    Outlet of mixing channel (11)