He, of all people, a trained and experienced nuclear engineer, should have had a better idea than most of what was really at stake and what was available to handle the problem.

The official version is that he was concerned about nuclear proliferation, and even though enriching Uranium was beyond the capabilities of most small countries, recovering Plutonium from spent fuel was not, especially if you didn't care about taking a few risks. A widespread nuclear waste reprocessing industry would indeed flood the world with Plutonium, and some would eventually get into the wrong hands.

But the theory doesn't really hold up, even back then. Plutonium can be recovered from small production reactors, just like we did in WWII, with hot-water-heater technology. These exist all over the planet for industrial and medical use, and could easily be copied and modified by the bad guys. And it was only a matter of time before Uranium enrichment became easy enough to do that any moderately well-funded nation could make it happen. Proliferation could be slowed down, maybe postponed for a few years, but it could not be stopped.

I think his reasoning went something like this...The problems encountered with building breeder reactors as safe and dependable as pressurized water reactors were proving harder than anticipated (he no doubt worked on the Sea Wolf program when he was with Rickover) and with a widespread reprocessing industry the only source of large amounts of Plutonium, the 50s dream of a fully renewable long term nuclear economy was just not working outside the laboratory. No doubt the problems they were having at Hanford Works may have contributed to this point of view, although I'm not sure how the timeline on that fits into his administration. Also, neither the Congress nor the industry seemed eager to devote any serious funding to fuel reprocessing or breeder development. As a techie, he may have overestimated our progress and the future success of fusion, which has turned out to be a much tougher nut to crack than the physics community thought in the 70s.

My speculation is that feeling proliferation would be a problem, breeders and reprocessing just not ready in time to operate on an industrial scale, and fusion probably just around the corner anyway, he felt the risks were just too big to continue with fission as a fossil alternative. It appears, techie or not, he guessed wrong.

But what do I know?