In this work, a thermal hydraulic analysis with RELAP5-3D/ver.40.3 of the Decay Heat Removal System (DHRS) for the Integral Inherently Safe Light Water Reactor (I2S-LWR) is discussed. The I2S-LWR is an integral reactor characterized by a high level of safety. The DHRS of the reactor is a passive safety system and it is made up of four independent trains, each composed of an Intermediate Heat Exchanger (IHX), placed in the reactor pressure vessel, an intermediate loop with pressurized water (at 70 bar) as working fluid and an Air Heat Exchanger (AHX), that rejects the residual power to the ambient air. The study consisted in modeling the I2S-LWR primary loop through the RELAP5-3D code in order to design the DHRS and analyze the shutdown transient under the Loss of Offsite Power Conditions.

The coupling system IHX-AHX has been optimized maximizing the rejected power to the ultimate heat sink under natural circulation, replacing the reactor core with two time dependent volumes with imposed temperature. The IHX has been designed limiting its dimensions to the available space in the reactor pressure vessel, while the required dimensions of the AHX, since it is not subject to space limitations, have been determined during the study.

The shutdown transient is described starting from the reactor core under nominal operating conditions and simulating the primary and secondary pump coast down with decreasing exponential functions. The analysis showed that the DHRS is capable of removing the residual power from the reactor core safely, accepting a failure of up to two trains of the DHRS. Furthermore, the presence of a fail-safe opening valve that isolates the IHX of the DHRS from the primary loop during nominal operation has been considered, showing a performance increase with the level of opening of the valve. Since the target of the DHRS is the removal of the decay power accepting the failure of one of the four trains, the AHX has been reduced in size to reach that target, in order to limit the cost of the heat exchanger.

An alternative working fluid for the intermediate loop, which consists of nanoparticles dispersed in water (nanofluid), has been evaluated. The use of nanoparticles improves the thermal properties of water through an increase in the heat transfer coefficient, but is characterized by a higher viscosity. The analysis confirmed that the bottleneck of the system is the heat transfer with ambient air in natural circulation, since the improvement in the exchanged power is not affected much by the enhancement of the thermal properties of the working fluid.