It would seem the program was sort of cancelled in 1972. See more on EMPIRE and MORL below...

Here is, I would think, the definative reference to NERVA's Cancellation...near the bottom of the page http://history.nasa.gov/SP-4012/vol3/ch5.htm

During 1969, NASA conducted NERVA tests from March through August, with 28 successful engine startups. The engine operated for a total of 2.8 hours, including 3.5 minutes at full thrust (55 000 pounds). The next year saw the preliminary design of the NERVA flight engine, with a preliminary design review being initiated in October. Studies called for reusable stages, 11 meters in diameter. In 1971, the engine baseline design was completed and engine component detailed design was initiated. Fiscal year 1972 funding restrictions allowed NASA to support only selected critical engine hardware development; other aspects of the program were put on hold. In 1972, NERVA was officially cancelled. NASA's space nuclear program was reduced to investigating ways to use atomic energy on a much smaller scale than NERVA. The next year, the joint NASA-AEC Space Nuclear Systems Office was abolished since there were no plans to use a nuclear rocket during the next 10 to 15 years. NASA's interest turned to using atomic energy for auxiliary onboard power systems.

MORL: Manned Orbiting Research Laboratory...this study lead to Skylab.

Langley's next round of studies, called the Manned Orbiting Research Laboratory (MORL), rejected the rotating wheel once and for all. Late in 1963, Douglas Aircraft won this study contract and went on to build the Skylab workshop. In many ways, MORL illustrated what Apollo Applications might have accomplished if it had been given high priority and ample funding.

Rather than seeking to support large crews in the comfort of artificial gravity, MORL emphasized small crews that would live in weightlessness in versatile, compact stations. The basic station was to fly atop a Saturn I-B and hence had that rocket's diameter of 22 feet. Weighing 30,000 pounds at launch, MORL would enclose 9000 cubic feet of internal volume, with a crew of six. Each astronaut would serve a six-month tour of duty. A modified Apollo spacecraft, riding its own Saturn I-B, would carry supplies along with new three crew members to the space station.

EMPIRE: Early Manned Planetary-Interplanetary Roundtrip Expeditions

From its conception in 1958, NASA had been preoccupied with the still monumental task of putting men in space. Plans for going to the moon were in place, but were only roughly sketched out. Then in 1962, NASA ordered three companies, Ford, Lockheed, and General Dynamics, to do studies for the next phase in manned space exploration, studies which would eventually become project EMPIRE (Early Manned Planetary-Interplanetary Round-trip Expeditions), NASA's first major plan to put a man on Mars (1). Other plans had been dreamed up before, notably by Werner von Braun, but never before had NASA made a major formal study of just what it would take to travel to another world within a decade.But EMPIRE was not the first government study on interplanetary travel. General Atomic, a division of General Dynamics, had been forming its own plans throughout the 1950's (2). Then shrouded in secrecy, Project ORION was a revolutionary form of propulsion which had been studied with support from the Pentagon. Wanting to rid itself of ORION, the Pentagon had unsuccessfully tried to pass off the project to the newly created space agency (3). Eventually forces in NASA pressured it to agree to fund a study in 1963, what ended up being the last report General Atomic would publish on their brainchild.

http://www.nuclearspace.com/a_orion_and_empire.htm