IRIS employs simplified passive safety systems to mitigate the effects of all postulated design basis events. Shown schematically in Fig. II-3, these systems include the following innovative features:

• Pressure suppression system (PSS): located within the containment vessel, acts to condense steam released into the small spherical steel containment due to any postulated design basis LOCA or steam/feed line break. The IRIS PSS is designed to limit containment pressure to ~1.0 MPa, or only 65% of the containment vessel design pressure. The PSS also provides an elevated source of water that is available for gravity injection into the reactor vessel through the direct vessel injection (DVI) lines in the event of a LOCA;

• Emergency heat removal system: consists of four independent subsystems, each of which has a horizontal, U-tube heat exchanger connected to one of the four IRIS SG steam lines. These heat exchangers are immersed in the refuelling water storage tank (RWST) located outside the containment structure and act
as the heat sink for emergency heat removal system (EHRS) heat exchangers. The EHRS operates on natural circulation, removing heat from the primary system via the steam generators’ heat transfer surface, transferring the heat to the RWST water and condensing the steam, and returning the condensate back to the SG via the feedwater line. Following a LOCA, the EHRS heat removal function acts to depressurize the RCS by cooling the SGs, thus condensing the steam produced by the core directly inside the reactor vessel. The EHRS is designed so that only one of the four independent subsystems is needed to remove the decay heat, thus providing a very high degree of redundancy, important for both safety and security concerns;

• Long term gravity make-up system: combined with a small RCS depressurization system and containment layout, provides gravity driven make-up water to the reactor vessel to assure that the core remains covered indefinitely following a LOCA;

• Emergency boration system (EBT): Two full emergency boration systems provide a diverse means of reactor shutdown by delivering borated water to the reactor vessel (RV) through the DVI lines. By their operation, these tanks also provide limited gravity feed make-up water to the primary system;

• Automatic depressurization system (ADS): A small ADS from the pressurizer steam space assists the EHRS in depressurizing the reactor vessel if reactor vessel coolant inventory drops below a specified level. The ADS consists of two parallel lines, each with two normally closed valves. The ADS discharges into a quench tank through a sparger. This ADS function ensures that the reactor vessel and containment pressure are equalized in a timely manner, thus limiting the loss of coolant and preventing core uncovery following a postulated LOCA even at low reactor vessel elevation;

• Specially constructed lower containment volume: collects the liquid break flow, as well as any condensate from the containment, in a cavity where the reactor vessel is located. Following a LOCA, the cavity is flooded above core level, creating a gravity head of water sufficient to provide coolant make-up to the reactor vessel through the DVI lines. This cavity also assures that the lower outside portion of the reactor vessel surface is or can be wetted following postulated core damage events;

• Safety strategy of IRIS: provides a diverse means of core shutdown through make-up of borated water from the EBT in addition to the control rods; also, the EHRS provides a means of core cooling and heat removal to the environment in the event that normally available active systems are not available. In the event of a significant loss of primary-side water inventory, the primary line of defence for IRIS is represented by the large coolant inventory in the reactor vessel and the fact that EHRS operation limits the loss of mass, thus maintaining a sufficient inventory in the primary system and guaranteeing that the core will remain covered for all postulated events. The EBT is actually capable of providing some primary system injection at high pressure, but this is not necessary, since the IRIS strategy relies on ‘maintaining’ coolant inventory, rather than ‘injecting’ make-up water. This strategy is sufficient to ensure that the core remains covered with water for an extended period of time (days and possibly weeks). Thus, IRIS does not require and does not have the high capacity, safety grade, high pressure safety injection system characteristic of typical loop reactors.