Military ships are "safe", because they can generally defend themselves. Commercial container ships would be the target of hijacking, to steal the reactor / fuel, or use them as 9/11 ship equivalents.

Regulation is not always bad.

Correct. And military nuclear vessels are also operationally safe because superpower navies are willing to spend a fortune on training and paying qualified nuclear engineers and technicians. Despite those large budgets they still struggle to maintain minimum crewing levels. Merchant lines can't afford the personnel expense.

For merchant ships we’d need completely sealed idiot-proof reactors. Those haven’t been invented just yet.

Also, we’d need to make them impossible to open and impossible to remove without cutting the ship in half.

So in other words, we'd need magic.

It has only been two years since the Baltimore bridge was destroyed by that container ship. Had known faulty maintenance, but they kept operating. Now the rest of us have to pay $5 billion to replace the bridge, plus whatever economic damage was done to surrounding industries.

We could find a way to make the reactor so integral to the ship and the fuel so integral to the reactor it’d be almost impossible to remove either. We can compound that with fuel cycles with materials less useful for making bombs.

Ships run aground and get abandoned all the time. You’d need a lot more than that.

Not a chance. Pirates hijacking a nuclear powered container ship is not something anyone wants to deal with.