Embodiments are disclosed that relate to increasing a temperature in a low temperature zone in a steam reforming reactor via a radiative heating shunt. For example, one disclosed embodiment provides a steam reforming reactor comprising a reaction chamber having an interior surface, a packing material located within the reaction chamber, and a radiative heating shunt extending from the interior surface into the reaction chamber. The radiative heating shunt comprises a porous partition enclosing a sub-volume of the reaction chamber bounded by the porous partition and a portion of the interior surface, the sub-volume being at least partly free of packing material such that radiative heat has a path from the interior surface to a distal portion of the porous partition that is unobstructed by packing material.