NOVELTY - The hydrocarbon fuel reforming reactor device for generating a reformate, comprises four reactor panels functionally interconnected by three joint pieces at an angle of less than 180 degrees to form a reactor assembly (10) having W shape. The joint piece defines a mixing chamber for allowing functional passage of material between two panels, and two distinct passageways in the reactor device. The combination of the reactor panels and the joint pieces forms the reactor assembly. The two reactor assemblies are arranged in a serial stacked formation so that heating gas flows through W assemblies.    USE - The hydrocarbon fuel reforming reactor device is useful for generating a reformate, which is useful in fuel cell system.    ADVANTAGE - The hydrocarbon fuel reforming reactor device is more compact while retaining the low pressure drop characteristic of panel configuration of the steam reforming reactors, allows increased function of reformer panels in a decreased space, and allows increased efficiency of heat transfer and reaction within the device thus improving the performance of the reactor device.    DETAILED DESCRIPTION - The hydrocarbon fuel reforming reactor device for generating a reformate, comprises four reactor panels functionally interconnected by three joint pieces at an angle of less than 180 degrees to form a reactor assembly (10) having W shape. The joint piece defines a mixing chamber for allowing functional passage of material between two panels, and two distinct passageways in the reactor device. The combination of the reactor panels and the joint pieces forms the reactor assembly. The two reactor assemblies are arranged in a serial stacked formation so that the heating gas flows through the W assemblies in a first direction while reacting gases pass through the assemblies in a second direction. The first direction is perpendicular to the second direction. An INDEPENDENT CLAIM is included for a method for steam reforming a material in a reactor assembly.    DESCRIPTION OF DRAWING(S) - The diagram shows a top plan view of a fuel reforming reactor device.    Reactor assembly. (10)