The nuclear reactor is the Tier II version of power generation, it produces a moderately larger amount of power, however it is significantly more expensive. Like most other buildings the Nuclear Reactor can be combined into a 3 wide building, increasing production and allowing more workers. The nuclear reactor requires 60 dwellers to unlock.
To increase the speed at which power is generated place dwellers with better Strength in this room.
The nuclear reactor room, like other rooms, gains a production and upgrade cost bonus from being connected to another room of the same type and level. The rooms will gain more than x2 production bonus for each connected room. Upgrade costs will be 25% cheaper for a double room and about 33% cheaper when upgrading a triple room vs upgrading 3 single rooms.