XE-Prime was a nuclear reactor that was a focal point of the NERVA program. The XE-Prime reactor was incorporated in a complete mockup of a nuclear rocket engine and used to investigate engine start-up and shutdown characteristics, as well as the resulting engine performance. It was tested on June 11, 1969, in a vertical downward firing test of the engine in a simulated space vacuum (1.6 psia), achieving a power output of 1,140 MWt for 210 seconds. The other reactor series developed during the NERVA program was NRX.