AbstractA new ultrasafe type of nuclear power plant is described that has a complete“walk-away-from”characteristic. That is, the reactor can safely dissipate its shutdown heat even if its power and water supplies are cut off. The reactor is steam cooled and is designed to operate at one fixed steam density. Its reactivity characteristics are such that if the power level increases, the steam becomes less dense than the optimum and tends to shut the reactor off. Similarly, if the reactor is flooded with water, the reactivity greatly decreases and also shuts the reactor down. The reactor can be operated as a burner, a high-efficiency converter, or a breeder, depending on the isotopic content of the fuel. The plant operates at low pressure and relatively high efficiency with an example given at 1000 psia and 35% efficiency. The reactor is enclosed in a conventional steel vessel resembling a boiling water reactor. The vessel is connected to a large atmospheric pressure pool of water, and shutdown consists of passively coupling the pool to the reactor through the loss of steam flow. Shutdown cooling is provided by forced air and natural draft convection cooling of the pressure vessel. Sufficient water and passive cooling are provided by the pool for many months of shutdown water cooling. The plant piping is double walled, and all paths of radiation escape, including pressure-vessel cracking, are channeled through an on-line cleanup system.