A reactor core for use in a chemical reactor includes first and second reactor core sections. Each core section includes heat transfer fluid conduits extending through the core section in a first direction, the lengths of the conduits defining a core section width. Each core section further includes spaced apart parallel plates arranged into a plate stack, with the conduits extending through the stack. Surfaces of the plates have a catalyst coating applied to them. The plates are rectangular and define a core section depth and a core section length, the core section depth being substantially smaller than both the core section width and length. Channels are defined by the spaces between adjacent plates, and are open to flow through the reactor core section in both the core section depth and length directions. The first core section is adjacent to the second core section in the depth direction.