Alma links the place of fire and brimstone to the chains of hell. The torments of the unrepentant “shall be as a lake of fire and brimstone,” clarifying that he means these descriptions as metaphors rather than as literal places, and that is probably how his audience understood them. His previously use of “the chains of hell” is likewise an eternal emotional condition, not a physical locale.
Reference: The allusion to a lake of fire and brimstone draws on language used in both the Old and New Testaments. For the meaning of the allusion, see the commentary accompanying 1 Nephi 15:30 and 2 Nephi 9:10–12.