Abstract: A system for producing hydrogen gas from water decomposition using a thermochemical CuCl cycle, the improvement comprising the use of an insulated hydrogen production reactor comprising a reaction chamber and a separation chamber; the reaction chamber having a hydrogen chloride gas inlet and a solid copper inlet; one or more levels provided in the reaction chamber, the number of which is dependant on production scale and pressure drop; each level comprising a perforated plate with associated filter media, the perforations of the plate and media being of decreasing size from top to bottom of the reaction chamber, and being sized to permit downward flow of the hydrogen gas and molten CuCl products, as well as the HCL gas reactant, and to prevent entrainment of solid copper in the molten CuCl; the separation chamber being located below the reaction chamber and being of greater cross section than the reaction chamber and comprising a first hydrogen removal and entrained copper removal zone and a second molten CuC