Why doesn't uranium ore spontaneously undergo a chain reaction? Uranium ore must be "weapons' grade" in order for it to undergo chain reaction. Uranium ore does spontaneously undergo slow chain reaction over time finally resulting in it becoming lead. Uranium ore, mostly U-235, must be enriched with U-238 in order for it to undergo a chain reaction. Uranium ore is mostly non-fissionable isotope U-238 and undergoes a chain reaction only if mixed with a moderator to slow down the neutrons