**5. Conclusions**

connected to the upper annular pump suction plenum of the reactor vessel. IRIS is designed to limit the loss of coolant from the vessel rather than relying on systems to inject water into the reactor vessel. It has a compact, small diameter, high-design pressure containment that assists in limiting the blowdown from the reactor vessel by providing a higher back pressure in the initial stage of the accident. Furthermore, four trains of passive emergency heat removal systems (EHRS) help to depressurize the primary system by condensing steam, coming out of the reactor core, on the steam generators tubes (depressurization without the loss of mass), and to remove the decay heat. Finally, it features automatic depressurization system to condense steam, released from the top of the pressurizer, in the pressure suppression pool located inside the containment. Following the initiating event, the LOCA mitigation signal is rapidly actuated, the reactor and reactor coolant pumps are tripped and the four EHRS subsystems are actuated by closing the main feed and steam isolation valves and by opening the fail-open valves in the EHRS return lines connected to the SG feed lines. The EHRS is composed of pipes, valves and heat exchangers submerged in the water tank outside the containment. After the initial decrease of the coolant inventory in steam generators caused by the isolation of the feedwater flow, the EHRS operation restores the SG secondary fluid mass (**Figure 16**) and enables heat removal out of the steam generators (**Figure 17**). It does not take long for the situation to stabilize to ensure safe reactor conditions. Equalization of reactor vessel and containment pressures marks the end of the blowdown phase and start of a long-term cooling phase by the continued operation

188 Heat Exchangers– Advanced Features and Applications

of the EHRS, with the pressure being slowly reduced as the core decay heat decreases.

**Figure 16.** SG secondary-side fluid mass in IRIS NPP.

Steam generators (SGs) are nuclear power plant components where the steam, which drives the turbine, is produced. They also represent barrier that prevents radioactive fission products to escape outside the containment building.

In order to ensure safe operation of a nuclear power plant, SG parameters, the steam flow, steam pressure and temperature, feedwater temperature, circulation ratio and total inventory mass, have to be maintained within prescribed values. These values depend on the operating window and the thermal load. The pressure is controlled by the relief and safety valves and the inventory by the feedwater flow.

The inverted U-tube steam generators have quite large water mass in the secondary side which is important during accidents of loss of the secondary heat sink. In that design, primary system water which is at a higher pressure than the secondary fluid flows inside the tubes, while the secondary fluid is on the outer side. In the helical-coil SGs, the situation is opposite: the primary water is on the tubes' outside surface keeping the tubes in the state of compression. The loss of the secondary flow is here more critical due to limited amount of water, but this is compensated with passive auxiliary safety systems.

In the event of losing the primary coolant pumps, the natural circulation between the reactor core and the SGs can ensure safe cooling of the reactor core by establishing the two-phase flow inside the tubes, with only minimal operator actions required. Thus, the design of the nuclear steam generators and the auxiliary systems enables safe NPP operation, not only during the normal plant operation but also during the accident conditions.
