Forerunner secondary reactor
From Halopedia, the Halo wiki
The Forerunner Bonus Reactor is a building found in Halo Wars which increases the tech level of any player that stations infantry within it by 1.The Forerunner Bonus Reactor itself cannot be destroyed but the infantry inside can be. Once taken it gives protection to those inside like a Deployable Lookout Tower.
History
They are reactors built and used by the Forerunners in their ground based attacks. In some battles, they had time to retreat and detonate them. On occasion they were overwhelmed by the Flood and were not able to destroy them[1]. This may be why flood can often be found stationed outside of them.
