On the contrary, the well-funded, well-staffed Nazis found that process to be a bit more tricky. QED.
My point is that all nukes, regardless of size, require professional maintenance.
Nuclear triggers can't, due to atomic decay, last very long. That's just a fact of life.
As for nuclear cores/pits...they decay much more slowly (e.g. the Soviets were on a 7 year replacement cycle), but the isotopes that they decay into, though small in quantity early on, greatly inhibit chain reactions.
Furthermore, radioactive decay impacts electronics and even wiring (e.g. resistance, heat, capacitance, inductance). That decay also affects the conventional explosives (this is why the Soviets had to steal Britain's RDX). Even RDX has to be periodically replaced.
Contrary to urban myths, none of the above are easy to remedy in the field. Cutting and shaping fissionable metals is beyond the ability of your corner-store machine shop.
Obtaining trigger material itself (e.g. Po-210) requires full access to a nuclear reactor.
Larger atomic devices *can*, if desired, be built with slightly longer shelf-lives (e.g. using more shielding material), but the desired neutron radiation (which is, by definition, tied to short a half-life) to initiate even the large devices mandates rather constant maintenance cycles.
In short, you need State resources to build new or maintain old nukes (e.g. nuclear reactor for trigger replenishment, clean room labs for machining pits/cores, special conventional explosives, etc.).
Slight errors in shaping the replacement conventional explosives, or in the resistance or capacitance of the wiring, or in a mismatch in the electronics, or in the purity of the fissionable material, or in the decay rate of the chosen trigger materials...will all void a nuke's ability to go "boom."
Still be a "dirty nuke?" Sure. Re-visit Hiroshima? No.