i thought of a new radioactive element astatine (ASTN), decays into neutrons very quickly. when under pressure explodes violently with a similar reaction to bomb. created when uranium decays. when touched by STKM or STK2 they gain the ability to shoot out neutrons killing fighters and other STKM. when on contact with WATR stops decaying.
Uranium decaying into another element would break way too many saves. Also, "on contact with water" probably won't work the way you expect it to. If you added a small pile to a tank of water, the particles immediately touching the water wouldn't decay, but particles underneath would.