Strontium-90

Strontium-90 is a very radioactive and dangerous isotope of Strontium that is a common fission product. Due to its half-life, it outputs a lot of decay heat and still lasts for a few decades. It is naturally pyrophoric from decay heat and also hydroreactive due to the chemically reactive nature of Strontium.

Production
Sr-90 is a common fission product, so it will have to be produced via recycling of Short-Lived Nuclear Waste (non-decayed). Short-lived waste contains on average ~5.5% Sr-90. It also comes from the fuel cycles of MOX and Highly Enriched Uranium-233 Fuel in the Research Reactor.

Uses
Although it has more planned uses, the most notable current use is a high-mid tier strength RTG pellet. They provide 15 power per tick. If RTG decay is enabled, each will have a lifespan of 208,800,000 ticks, computing to ~4 IRL months, with a stated in-game lifespan of 43 and a half years (assuming a 100 day year). They decay to Zirconium pellets. This also means it will make 3,132,000,000 power in total over that time.

It also indirectly decays to stable Zirconium, so allowing for it decaying would be a method to produce Zirconium.

Trivia

 * Strontium-90 isn't just hazardous due to its high radioactivity, but because of the chemical nature of the element which causes it to imitate Calcium in the body, resulting in Sr-90 accumulation in the bones, in turn causing serious health problems from the radiation.
 * Strontium-90 is produced in nuclear explosions, so it can be found in fallout, which can then be fall onto farmland, and eventually enter humans due to ingestion of milk from contaminated cows.
 * This is no longer an issue to worry about due to the banning of nuclear testing decades ago and the short half-life of this isotope.
 * Has both a nugget and tiny powder form, which is somewhat unusual.
 * You cannot shred a nugget into a tiny powder nor smelt a tiny powder to a nugget yet.