How nuclear propulsion changed submarine strategy

Did nuclear submarine development improve defense or expand the strategic threat at sea?

It expanded the threat because nuclear boats were no longer constrained by fuel endurance and could remain at sea until food or human limits intervened.

Conventional submarines eventually had to refuel or return home, creating logistical and operational limits that an opponent could exploit. Nuclear propulsion removed fuel endurance as a routine constraint. Food stocks, maintenance, and human endurance then became the more immediate limits on a patrol. Submarines could remain deployed much longer and approach distant operating areas without the same refuelling requirement. The answer therefore judges nuclear propulsion primarily as an expansion of the strategic threat at sea, even though it also improved defensive submarine capability.