When Reactron encountered Supergirl for the first time, he claimed that his costume is made of a region of space. Supergirl is intrigued by how Reactron works but is instructed by Superman just to punch him. Reactron uses this curiosity until Supergirl compromises his power source and he is defeated. Later on, during the Who is Superwoman? story arc, he reveals that his old suit, which was destroyed by Supergirl, was not really a region of space, but a suit to save his life as he was dying from radiation sickness.
As a sergeant in the US Army, Ben Krullen was accidentally exposed to nuclear radiation that manifested itself years later. After returning from Vietnam, Krullen drifted from job to job only to be lured in to an alliance with the criminal group known as the Council. Its scientists transformed him into a human reactor and he did many tasks for new employers, battling the 2nd incarnation of the Doom Patrol and Supergirl in the process.
In one encounter with Supergirl, the pair were in the middle of a huge atomic explosion over Chicago which the maid of might managed to contain. However, Reactron appeared to overload on radiation and explode - although his body was never found.
Reactron joined the Suicide Squad for one mission in Russia. The Squad met up against The Doom Patrol. Reactron was believed to have blown up when Deadshot shot his containment suit full of holes. Reactron would survive his encounter with Deadshot. Reactron, Neutron, Geiger, Professor Radium, Mister Nitro and Nuclear would be sent to Bludhaven by Luthor's Society to discern a chemical leak. They would come into conflict with the government team of SHADE and Geiger would die. This group called themselves The Nuclear Legion. Reactron would once again die in battle, but this would be short lived.
Reactron has recently resurfaced now powered by a gold kryptonite heart similar to Metallo, who has a green kryptonite heart. He is able to focus his radioactive energies through the heart in the form a gold kryptonite blast. He used this tactic on the current Superwoman only to have her smirk at his futile attempts to hurt her. Lex Luthor sent a robot duplicate to infiltrate New Krypton. The robot duplicate changes the body chemistry of Reactron. Reactron causes a chain reaction killing himself and a handful of the 10,000 New Kryptonians. Alura, Supergirl's mother, was among the casualties.
In Superman/Batman, Supergirl and Robin are trying to find a murder,and there trail leads to Scarecrow. When Supergirl gets hit with fear toxin, she sees Reactron as a Black Lantern. He almost kills her, but lucky Robin convinces Supergirl its all in her head.