The Power Reactor Inherently Safe Module program is under way at General Electric Company Under U.S. Department of Energy sponsorship to develop a conceptual design for an advanced sodium-cooled liquid-metal reactor plant. This design is intended to provide significant reductions in plant construction and operating costs and reduced risk of construction delays while improving the already excellent level of plant safety achieved by the nuclear power industry. Design safety features are being developed that use inherent characteristics to passively respond to accident situations with high reliability and independence from human operator action. These features include seismic isolation of the reactor system, a shutdown heat removal system that relies only on naturally circulating atmospheric air to maintain safe temperatures even with a loss of coolant pumping, and a core designed to provide strong negative reactivity feedback with rising temperature.