RIFT (Reactor In Flight Testing) was a plan to implement NERVA (Nuclear Engine for Rocket Vehicle Application), otherwise known as Project Rover. (Ah, there's nothing like a sea of acronyms and code names in the morning.) The idea was to use a nuclear reactor as a rocket stage, ideally once you'd left the atmosphere. This wasn't anything like a nuclear weapon; you're basically pumping some kind of medium through a reactor and using the heat of the reactor to get it to a super-hot temperature and eject it out the back of the reactor, providing thrust. They were being looked at first as part of the Apollo project, and then for other deep-space exploration purposes, because compared to a chemical rocket you can get a lot more thrust, for a longer time, from a nuclear reactor rocket, at least in principle.

The RIFT project was cancelled when it became clear that they didn't need it for Apollo. The entire NERVA project was eventually shelved as the space program in general became gradually under strain from the ever-growing budget requirements imposed by the Vietnam War and the scaling back of American space exploration ambitions after Apollo. (If I were going to point out one thing that I think is under-appreciated by most Americans, it is the way that the Vietnam War ended up shutting down many other possible directions for American society. It truly was a "zero-sum game" in many ways, and plenty of things that were much more interesting/intriguing were abandoned in the face of trying to sustain a war of choice.)

Anyway, the implication of such a flight would have been a possibly deeper commitment to longer-range manned space travel. That was what these space reactors were being looked at for doing. There are no bans about putting nuclear reactors in space; there are only bans in putting nuclear weapons in space (the Outer Space Treaty, 1967) and the testing of nuclear weapons in space (the Partial Test Ban Treaty, 1963). Neither would apply to a NERVA rocket being used for peaceful purposes. The PTBT would apply to ban spaceflight schemes that used nuclear explosions (like Project Orion), but not nuclear reactors. So NERVA was not a casualty of any treaty; it was a casualty of budget cuts and mission changes.