One solution to it from mid-1990s was sealed reactors that are refueled by sending them back to manufacturer, fitting on single railway car. Combined with being rather hard to extract the material due to coolant used (liquid metal solidifying when shut down or leaking) and various anti-proliferation poisons used, it makes a pretty good solution.
I am dubious. Something that fits on a single railway car can be stolen in its entirety, and then the question is “is it valuable enough someone might just do that?”
It’s the same reason that puts me off houseboats: “thieves stole my house”/“my nuclear reactor has gone missing” should be Onion headlines and Pratchett storylines respectively, not things that actually happen.
individual fuel assemblies are much smaller, and by this logic easier to steal.
There's a reason why logistics for certain materials include security services, and it's much harder to abscond with multiple-tens-of-tons container that can have tracking beacon than a single fuel assembly.
