English: The first ground experimental nuclear rocket engine (XE) assembly, (left), is shown here in "cold flow" configuration, as it makes a late evening arrival at Engine Test Stand No. 1 at the Nuclear Rocket Development Station, in Jackass Flats, Nevada. Cold flow experiments are conducted using an assembly identical to the design used in power tests except that the cold assembly does not contain any fissionable material nor produce a nuclear reaction. Therefore, no fission power is generated.

The large object at the right is one-half of an aluminum cylindrical closure that can be sealed about the engine, forming an airtight compartment, thereby permitting testing in a simulated space environment. The "cold flow" experimental engine underwent a series of tests designed to verify that the initial test stand was ready for "hot" engine testing, as well as to investigate engine start-up under simulated altitude conditions, and to check operation procedures not previously demonstrated.

The XECF (Experimental Engine Cold Flow) experimental nuclear rocket engine was a part of project Rover/NERVA. The main objective of Rover/NERVA (Nuclear Engine for Rocket Vehicle Application) was to develop a flight rated thermodynamic nuclear rocket engine with 75,000 pounds of thrust. The Rover portion of the program began in 1955 when the U.S. Atomic Energy Commission's Los Alamos Scientific Laboratory and the Air Force initially wanted the engine for missile applications. However, in 1958, the newly created NASA inherited the Air Force responsibilities, with an engine slated for use in advanced, long-term space missions. The NERVA portion did not originate until 1960 and the industrial team of Aerojet General Corporation and Westinghouse Electric had the responsibility to develop it.

In 1960, NASA and the Atomic Energy Commission (AEC) created the Space Nuclear Propulsion Office to manage project Rover/NERVA. In the following decade, it oversaw a series of reactor tests: Kiwi-A, Kiwi-B, Phoebus, Pewee, and the Nuclear Furnace, all conducted by Los Alamos to prove concepts and test advanced ideas. Aerojet and Westinghouse tested their own series: NRX-A2 (NERVA Reactor Experiment), A3, EST (Engine System Test), A5, A6, and XE-Prime (Experimental Engine). All were tested at the Nuclear Rocket Development Station at the AEC's Nevada Test Site, in Jackass Flats, Nevada, about 100 miles west of Las Vegas.

In the late 1960's and early 1970's, the Nixon Administration cut NASA and NERVA funding dramatically. The cutbacks were made in response to a lack of public interest in human spaceflight, the end of the space race after the Apollo Moon landing, and the growing use of low-cost unmanned, robotic space probes. Eventually NERVA lost its funding, and the project ended in 1973.