A hydrogen generator that includes a solid fuel mixture, a liquid reactant, a liquid delivery medium (LDM), a movable boundary interface (MBI), a reaction zone, wherein the MBI provides constant contact between a reacting surface of the solid fuel mixture and the liquid reactant delivered by the LDM to form the reaction zone, and a product separation media, fluidly coupled to the reaction zone by a fluid junction, that degasses a product. The hydrogen generator may further include auxiliary LDMs disposed throughout the hydrogen generator, wherein said auxiliary LDMs may be operated based on a ratio of the liquid reactant flow rate to the hydrogen generation rate.