Cadmium II was a by-product of the propulsion system of Red Dwarf. When the inept Second Technician Arnold Rimmer of Z Shift was assigned to repair the ship's drive plate, he did a less than efficient job. Shortly afterward the Cadmium II leaked out explosively, exposing all the inhabited decks to a lethal dose of radiation. Only Dave Lister was spared, since he had been placed in a stasis booth as punishment for breaking quarantine regulations smuggling a cat on board.
This was such a massive dose that all aboard were killed almost instantly, with the radiation remaining in the background aboard ship for the next three million years. Holly managed to react in time to seal the cargo decks and send the ship out into deep space where it would not present a hazard. The radiation reduced the crew's bodies to a fine white powder, but left the inorganic matter aboard intact.
Lister's cat, Frankenstein, was safely sealed in the hold. Her kittens bred with each other and eventually evolved into Felis sapiens, a humanoid cat species. In the ensuing three million years, Holly presumably maintained the ship with the skutters and the garbage droids.
Once the background radiation had reached a safe level, Holly was able to release Lister and explain what had happened. Lister found it hard to grasp and idly started tasting the white powder. Holly explained that it was the dead crew, which caused Lister to sweep them up and bury them in space. One member of Felis sapiens, the Cat, joined him, as did the newly revived hologram of Rimmer.
Rimmer blamed Lister for the accident, arguing that if Lister hadn't kept "that stupid cat" and hadn't gone into stasis, he would have help mending the drive plate. ("RD: The End")
Lister later watched a recording of the radiation leak that Rimmer had Holly make him, recording his own death. He saw Rimmer in the Drive Room apologising to Captain Frank Hollister for not resealing the drive plate properly, and accepting full responsibility for any consequences. It was here Lister learned that Rimmer's last words were "Gazpacho Soup". ("RD: Me²")
Three million years later, Rimmer's hologram subconsciously accepted responsibility for the accident, so he was surprised when the Justice Computer found him guilty of 1167 counts of manslaughter. Kryten was able to prove that Rimmer was too incompetent to have repaired the drive plate correctly, and that it was his superiors who were guilty for giving him the assignment. Rimmer's conviction was overturned. ("RD: Justice")
When the ship and its' crew were later recreated by the nanobots, the restored Rimmer was able to access the confidential files of the old Red Dwarf. One of the brilliant insights he was able to bring to Captain Hollister's attention was the potential for a lethal radiation leak caused by damaging the drive plate. Although he stated that " Anybody who misrepaired one would have to have a brain the size of a leprechaun's testicle", he still suggested new safety procedures. ("RD: Back in the Red II")
Rimmer would later blame failures in the ship's health and safety procedures for the accident. ("RD: Entangled")
In the first Red Dwarf novel, the accident occurs very differently. In the novel, Cadmium II is described as part of the engine coolant system. Most notably, it was not Rimmer's fault, but the cause of both a mechanical failure - which got worse due to a lack of vigilance from the officers - and a tragic coincidence. In the Drive Room, French navigation officer Henri Dubois spilled a cup of coffee over his console, just as three red warning blips appeared on it. Dubois wrongly considered them the result of his spillage. Rimmer was actually on his way to the stasis chambers at the time, which he occasionally stayed in overnight in an attempt to preserve his youth. However, Rimmer stopped by a mirror to comb his hair, and in the extra minute he was out of the stasis booth, was caught in the blast and killed. Although it was the power of a neutron bomb, turning biological matter into dust, the ship remained structurally undamaged. Holly shot the ship out of the solar system to not contaminate Earth with the radiation. When Lister got out of stasis, three million years later, Rimmer was revived as a hologram as in the TV series.
In the novel, it is also stated that a tiny dose of radiation leaked into the cargo hold where Frankenstein was hiding. This minute amount was enough to accelerate the mutation of her kittens and was why Felis Sapiens evolved in such a relatively short time. (Infinity Welcomes Careful Drivers)
The element Cadmium has many industrial uses involving different isotopes and alloys, some radioactive, but due to the definition of Cadmium and its isotopes, Cadmium-2 cannot exist. However, the name Cadmium II could still be used.
Cadmium at Wikipedia