The rings fire the pulse that destroys sentient life but do not destroy themselves, that only occurred in Halo 3, when the master chief purposely fired the premature Halo ring being made on the Ark because it would blow instead of firing, destroying all remaining flood. Also in Halo CE Instalation 04 was destroyed by the Pillar Of Autumns fusion reactors to also destroy all local flood infestation. The rings that are around now in the Halo universe have been fired once and that was by the ISO Didact as a last resort to destroy the flood 100 000 years ago, that firing killed all life and flood organisms in the galaxy, apart from the species, flood specimens and certain forerunners that were safe on the rings and forerunner shield worlds eg The Didact on Requiem.