The Integrated Reactor with Inherent Safety (IRIS) has been designed with a primary objective of ensuring fuel integrity by passive means only. The steam generator is a once-through helical coil type. The steel reactor pressure vessel is submerged in an outer pool contained in a prestressed concrete containment vessel. Thus, the coolant in the reactor containment vessel has a very low average specific enthalpy (243 kJ/kg), while its heat capacity is very large (42 GJ/°C). The primary flow path, which has a double syphon structure with the main coolant pumps located at the outlet of the steam generator, is formed by concentric annuli. The various components required for steady-state plant operation are driven by a turbine or by on-site power so that they can be automatically shut down. Due to these passive features, not only are various systems simplified or eliminated, but constraints on the plant layout are also reduced. Balance of mass, heat, and pressure are examined by computer calculations, and various geometric and thermal-hydraulic parameters are chosen. The reactor control logic is designed so that the IRIS can cope with a large loss of load. Safety analyses confirm that the reactor passively shuts itself down in accidents; for example, in a loss-of-coolant accident due to a break in the outer pool, the borated outer pool water is passively injected into the reactor pressure vessel through the break. A negative void coefficient is especially important in the IRIS since it does not have control rods. The atmosphere is used by heat pipes as the ultimate heat sink for decay heat removal; thus, the walkaway period is very long.