"Radioactive" was written by Paul Rodgers, who also did the vocals.
Technically speaking, The Firm were one-hit wonders, and this is it. However, don't be too hasty to judge them, because they were a supergroup - with vocalist Paul Rodgers (Free, Bad Company), guitarist Jimmy Page (Led Zeppelin), drummer Chris Slade (Uriah Heep, Manfred Mann's Earth Band), and fretless bassist Tony Franklin. Franklin would later go on to be part of Whitesnake and Quiet Riot; Slade would later join AC/DC.
Page has since indicated that The Firm was never intended to last longer than two albums. Supergroups are notorious for being short-lived; there's often just too much talent (and accompanying egos) in one place for everyone to get along amicably.