Lesser Australium

Lesser Australium (Tasmanite) is an isotope of Australium. Like it, it is fully stable and emits no radiation, despite being fissile and self-igniting.

Production
Tasmanite makes up ~83.3% of Australium and can only be extracted with a SILEX.

Uses
Tasmanite is used to make strong but "safe" Low Enriched Australium Fuel for the RBMK, as its function is quite predictable.

Trivia

 * Given its only fission products are stable Lead and Lead-209 (which is also quite unusual), we can estimate its atomic mass to be ~∓416.
 * This is a mass difference of ~∓21 compared to Greater Australium. Given this and the wildly different melting points of the two, it is possible they're actually different elements entirely.