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Safe Affordable Fission Engine (SAFE) are NASA's small experimental nuclear fission reactors for electricity production in space. Most known is the SAFE-400 reactor producing 400 kW thermal power, giving 100 kW of electricity using a Brayton cycle gas turbine. The fuel is uranium nitride in a core of 381 pins clad with rhenium. Three fuel pins surround a molybdenum-sodium heatpipe that transports the heat to a heatpipe-gas heat exchanger. This is called a Heatpipe Power System. The reactor is about 50 centimetres (20 in) tall, 30 centimetres (12 in) across and weighs about 512 kilograms (1,129 lb). It was developed at the Los Alamos National Laboratory and the Marshall Space Flight Center under the lead of Dave Poston. A smaller reactor called SAFE-30 was made first.
The working fluid used in the reactor is a Helium Xenon gas mixture.
The project is funded with discretionary money in the lab's budget and done mostly outside the researchers' normal work.