Xenon-135

Xenon-135 is an extremely radioactive and dangerous isotope of Xenon responsible for poisoning nuclear reactors (specifically, the RBMK) when on low power. It is so radioactive that it is naturally pyrophoric.

Production
In an RBMK, when the fuel receives a low amount of neutrons/flux, Xenon-135 begins to build up faster than it is burned away. If this poisoned fuel is recycled in a SILEX, the Xenon may be extracted, albeit at a low chance of 1%.

Uses
Xenon-135 has no uses currently, but it is planned to be used in grenades to releases it into the environment, giving nearby entities severe radiation poisoning.

Trivia

 * Xenon is a gas IRL, not a solid.