The Tech Nuclear Plant is a neutral structure that, when captured by an Engineer, basically serve as a smaller and more secure version of Soviet Nuclear Reactors. While it only produces half as much as power as the Nuclear Reactor does, it does not explode as violently upon destruction and cannot be sabotaged like its conventional counterpart.
Tech Nuclear Plants provide even more for their owner. Just like the Tech Power Plants, they come with their own security features to prevent sabotage, making them secure sources of power. When destroyed, they explode just like the Soviet Nuclear Reactor, however the explosion is smaller.
Capturing the Tech Nuclear Plant early on will allow a commander to skip building power plants and instead construct technology buildings faster, since this structure will be able to cover for all their power hungry requirements. Also, the commander won't have to worry about sneaky infiltrators and Invaders to sabotage this structure, forcing the enemy to destroy it outright.
However, Tech Nuclear Plants should be well defended, as the player owning it will have a big disadvantage if it is destroyed, especially if it's the primary source of power. This is further proven by the fact that it's usually located outside a base.
In the skirmish game modes Tech Share, Unholy Alliance and Ultimate Alliance, Tech Nuclear Plants will replace any Tech Secret Labs normally present in a map.