The IRIS reactor vessel (RV) [1] houses not only the nuclear fuel and control rods, but also all the major reactor coolant system (RCS) components (see Fig. XVI-1): eight small, spool type, reactor coolant pumps (RCPs); eight modular, helical coil, once through steam generators (SGs); a pressurizer located in the RV upper head; the control rod drive mechanisms (CRDMs); and, a steel reflector which surrounds the core and improves neutron economy, as well as it provides additional internal shielding. This integral RV arrangement eliminates the individual component pressure vessels and large connecting loop piping between them, resulting in a more compact configuration and in the elimination of the large loss-of-coolant accident as a design basis event.

As the IRIS integral vessel contains all the RCS components, it is larger than the RV of a traditional loop-type PWR. It has an inner diameter of 6.21 m and an overall height of 22.2 m including the closure head. Water flows upwards through the core and then through the riser region (defined by the extended core barrel). At the top of the riser, the coolant is directed into the upper part of the annular plenum between the extended core barrel and the RV inside wall, where the suction of the reactor coolant pumps is located. Eight coolant pumps are employed, and the flow from each pump is directed downward through its associated helical coil steam generator module. The primary flow path continues down through the annular downcomer region outside the core to the lower plenum and then back to the core completing the circuit. Additional details of the IRIS integral vessel can be found in IAEA-TECDOC-1451. [2]