Abstract: A Modular Isolated Reactor Support System (MIRSS) assembly includes a cylindrical reactor support structure configured to structurally support a reactor enclosure system on seismic isolators, a collector cylinder configured to at least partially define a riser annulus between an inner cylindrical surface of the collector cylinder and an outer sidewall surface of the reactor enclosure system structurally supported by the cylindrical reactor support structure, and a divider wall configured to at least partially define a downcomer annulus between an outer cylindrical surface of the divider wall and a reactor building, and a plurality of exhaust ducts extending from the collector cylinder and through an interior of the cylindrical reactor support structure.