Zero chance?
How many things have happened in the last few years that you would have said had zero chance of happening in America just ten years ago?
I worked on one of the industry teams that lost the competition for ACTUV Phase II, and I currently work with a lot of former nucs and submarine captains.
Given that the USN is currently constrained from practicing with active sonar because it might hurt the whales, there is no possibility we’re going to have an unmanned nuclear reactor or radioisotope source just sailing around. I can’t conceive of circumstances that would change that.