NOVELTY - The reactor (100) has a reaction chamber (102), a packing material e.g. nickel, located within the chamber and an annular-shaped radiative heating shunt (124) extending from an interior surface of a wall e.g. outer wall (104), into the chamber. The shunt has a porous partition enclosing a sub-volume (126) of the chamber bounded by the porous partition and a portion of the surface. The sub-volume is free from the packing material such that radiative energy has a path formed from the surface to a distal portion of the porous partition that is unobstructed by the packing material.    USE - Cylindrical-shaped steam reforming reactor i.e. methane steam reforming reactor, for converting a mixture of steam and a feed gas i.e. methane, into hydrogen, carbon monoxide and/or carbon dioxide in a steam reformer system (claimed) for a fuel cell. Can also be used for converting alcohol e.g. methanol and ethanol, or propane to produce hydrogen, carbon monoxide and/or carbon dioxide.    ADVANTAGE - The reactor has the shunt for allowing a desired intensity of radiative heat to reach a farther distance from the reactor. The radiative heat is provided to the packing material through radiative heat transfer from the wall, where a packing material temperature can be cooled in multiple regions spaced from the wall due to reduction in intensity of radiative heat as a distance from the wall increases, thus improving radiative heat transfer to a set of locations spaced from a radiative heat source, and hence driving a reaction in vicinity of the shunt toward formation of more hydrogen, carbon monoxide and/or carbon dioxide. The shunt in reduced presence of the packing material in the sub-volume allows radiative heat to travel to the distal portion of the porous partition, thus allowing a greater amount of heat to be transferred to a low temperature zone near an inner wall of the reactor, and hence driving the reaction toward completion near the inner wall. The shunt reduces a cold zone existing near the inner wall to reduce feed gas slip, thus increasing efficiency of the reactor.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a steam reformer system comprising a heater.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a cylindrical-shaped steam reforming reactor including an annular-shaped radiative heating shunt.    Cylindrical-shaped steam reforming reactor (100)    Reaction chamber (102)    Outer wall (104)    Annular-shaped radiative heating shunt (124)    Sub-volume of reaction chamber (126)