From Halopedia, the Halo wiki
Facility A-331 was one of the large Orbital Defense Generators that powered the Orbital Defense Stations in orbit around Reach. The facility was to be protected by the Spartan-II team arriving on Reach, but when the Covenant broke through the UNSC lines, the station was bombarded with heavy plasma, and all Spartans and Marines guarding the facility are assumed dead.
Many of the marines were killed when Longsword bombers accidentally bombed the site due to an error. When the Spartan Red Team came to the site, only four Charlie Company Marines remained there, all of which were terribly shell shocked.
There are at least seven reactor complexes. During the Fall of Reach, the Covenant compromised Complex Three, and the Spartans fell back to Complex Seven. That is the last that was heard from them.
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