Some reactors don't have this risk, but I'm not read on it.
Spent fuel from nuclear power plants-- probably the more vulnerable point in the supply chain-- is nonetheless difficult to the point of extreme expense & impractical to refine to weapons grade. You would need such large quantities that it would be extremely difficult to steal enough without notice: Someone is going to notice the 200,000 lb cask leaving a storage facility. Then the refinement process is difficult, requiring immense sums of money to built the significant infrastructure required for refinement. North Korea, the newest nuclear power, got there with significant help from the Soviets extending back in the 60's and it still took them decades to get there.
This is all on top of the fact that any modern plant designs use feedstock fuel that is even harder to refine into anything weapons grade, and is much more efficient to the point that spend fuel refinement difficulties dwarfs the above roadblocks for fuel from older designs.
Dirty bombs are more problematic, but also significantly less destructive, and suffer from many of the same logistical hurdles, notably that the material most useful for a dirty bomb is also material that requires massively robust precautions to ensure the thieves don't kill themselves with radiation exposure long before any chance of making use of it. Less useful material that requires fewer precautions is not practical for dirty bombs without significant refinement capacity.
All of this at a time when real measurable deaths from fossil fuel powerplants, their pollution, and their industrial accidents, are themselves more dangerous, right now, than theoretical & difficult to achieve usages of fuel from new nuclear power plants.
Some reactors don't generate weapons-grade isotopes when operated as intended, but if you control the reactor, my understanding is that it is easy to modify it so that it does.
There is no technical safeguard against nuclear proliferation - all of them are political.
