The Gas-cooled Fast Reactor (GFR) is one of the concepts under investigation for possible future deployment as part of the Generation-IV program. The reference GFR design is a high-temperature (850°C core outlet) helium-cooled design. A 2400 MW_(th) direct supercritical CO_2 (S-CO_2)-cooled plant is under investigation at MIT supported by a DOE NERI grant which would serve as a lower-temperature alternative to the reference GFR. This design offers the same high thermodynamic efficiency (～48%) as the helium-cooled design, but at a much lower core outlet temperature of 650°C, thus circumventing the materials problems involved in operating at very high temperatures. The plant utilizes a Brayton recompression cycle at a maximum pressure of 20 MPa, which is very simple and compact with a single shaft and no intercoolers. The work presented in this paper deals primarily with evaluation of a Shutdown/Emergency Cooling System (SCS/ECS) for the MIT design. This system consists of 4×50% capable hybrid active/passive loops utilizing blowers to remove decay heat during shutdown or to provide emergency cooling in the event of a Loss of Coolant Accident (LOCA). Natural circulation is evaluated at various post-LOCA backup pressures. Two routes are identified by which coolant may bypass the core during post-LOCA decay heat removal; one is via duct breaks in the case of rupture of both hot and cold legs, and the other is through PCS loops once turbomachinery has stopped. Although the SCS/ECS performs well in natural circulation without accounting for core bypass, alternate coolant paths are shown to be significant enough to warrant implementation of a highly reliable active cooling systems for this design in future work.
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