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 tall, across and weighs about . 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.