Electrical demand and the threshold for nuclear power

What is the smallest surface combatant for which nuclear propulsion would make sense?

Probably a cruiser-sized ship whose lasers and railguns impose a large continuous electrical load, though mission demand matters more than a fixed displacement.

Cruiser size is the initial estimate, but the weapons fit matters more than a precise displacement. A ship relying heavily on lasers and railguns could need a substantial continuous electrical supply. Nuclear generation is proposed for that base load because of its power density. Gas, diesel and batteries could still supplement it. No particular reactor or tonnage threshold is specified.