Nuclear missiles, fissile material, reactor cores, etc are transported on public roads - where else would they go? We haven't invented transporters yet so transportation is still roads, rail, sea, air - these things don't just magically appear at the launch site or at the power station. Someone needed to drive the semi that gets it there.

There is absolutely a chance of fissile materials being stolen in transport - that's why there's multiple levels of security and multilateral treaties with most countries around the world about this kind of issue.

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