English: The first ground experimental nuclear rocket engine (XE) assembly, in a "cold flow" configuration, is shown being installed in 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.

Functionally, the XECF (Experimental Engine Cold Flow) is similar to the breadboard nuclear engine system (NERVA Reactor Experiment/Engine System Test or NRX/EST) tested in 1966, except that the experimental engine more closely resembles flight configuration. In addition to the nozzle-reactor assembly, the XCEF has two major subassemblies: an "upper thrust module" (attached to test stand) and a "lower thrust module" containing propellant feed system components. This arrangement is used to facilitate remote removal and replacement of major subassemblies in the event of a malfunction. The cold flow experiential 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 operating procedures not previously demonstrated.

The XECF engine was 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 a nuclear 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 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, A3, EST, 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.