Exotic Elementals

[Keltest]

uranium decay in over 4 billion years, so the rate of transformation into other elements is negligible. You may want to try other, less stable elements.
i'm not sure what the chosen isotope of plutonium would be, since it does not exist in nature (well, except one isotope of which there are a handful of atoms left from the earth formation, but three atoms per cubic meters is not much to say it exist in nature), but it is possible that a plutonium elemental would undergo spontaneous fission instantly after summoning.

That might be an interesting spell. Using a summon spell to make a bomb? I see abuse potential.