NOVELTY - Reactor (1) for generating a hydrogen-containing gas, comprises an inlet (2) for fuel, an inlet (3) for an oxidizing agent, an outlet (4) for the hydrogen-containing gas, and a catalyst unit (5) comprising an inlet- and an outlet side charged with the fuel and optionally the oxidizing agent during the operation of the reactor, from which the hydrogen-containing gas flows out during the operation of the reactor, at least one inlet channel which is open at the inlet side and closed at the outlet side, and at least one outlet channel which is closed at the inlet side and open at the outlet side.    USE - The reactor is useful for generating hydrogen-containing gas in an aircraft. The fuel cell system is useful in an aircraft (claimed).    ADVANTAGE - The reactor exhibits prolonged service life, and prevents the contamination of the fuel cell by carbon black particles formed in the reactor.    DETAILED DESCRIPTION - Reactor (1) for generating a hydrogen-containing gas, comprises an inlet (2) for fuel, an inlet (3) for an oxidizing agent, an outlet (4) for the hydrogen-containing gas, and a catalyst unit (5) comprising an inlet- and an outlet side charged with the fuel and optionally the oxidizing agent during the operation of the reactor, from which the hydrogen-containing gas flows out during the operation of the reactor, at least one inlet channel which is open at the inlet side and closed at the outlet side, and at least one outlet channel which is closed at the inlet side and open at the outlet side. The inlet- and the outlet channel are separated by a gas-permeable and porous wall which is coated with a catalyst which catalyzes the conversion of the fuel to the hydrogen-containing gas. An INDEPENDENT CLAIM is also included for an aircraft fuel cell system, comprising a fuel cell, a fuel source, the above reactor for reforming the fuel from a fuel tank, to obtain the hydrogen-containing gas, a supply unit which comprises an inlet for fuel, an inlet for oxidizing agent, an outlet for fuel, an outlet for oxidizing agent and optionally an ozone generator which is connected with the inlet for the oxidizing agent and the outlet for the oxidizing agent, and an oxidizing device which is connected with the oxidizing agent inlet of the supply unit, and is designed for providing the oxidizing agent, where the supply unit is arranged for conducting fuel to the reactor through the fuel outlet and optionally simultaneously a first oxidizing agent to the reactor through the oxidizing agent outlet during the operation of the reactor, and for conducting a second oxidizing agent to the reactor through the oxidizing agent outlet during regeneration phase of the reactor.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the reactor.    Reactor for generating a hydrogen-containing gas (1)    Inlet for fuel (2)    Inlet for oxidizing agent (3)    Outlet for the hydrogen-containing gas (4)    Catalyst unit (5)