As shown in Fig. 1, theMHTGR and OTSG of the NSSS are arranged side by side and connected to each other by a horizontal coaxial hot gas duct. Cold helium enters the main blower mounted on the top of OTSG and is pressurized before flowing into the cold gas duct. It enters the channels inside the reflector of the core and then passes the pebble-bed from top to bottom where it is heated to a high temperature.Hot heliumleaves the hot gas chamber at the bottom of reflector and flows into the primary side of the OTSG through the hot gas duct. The nodalization of the primary loop is shown in Fig. 2.