Spontaneous Nuclear Fission
The spontaneous nuclear fission rate is the probability per second that a given atom will fission spontaneously--that is, without any external intervention.
If a spontaneous fission occurs before the bomb is fully ready, it could fizzle. Plutonium 239 has a very high spontaneous fission rate compared to the spontaneous fission rate of uranium 235. Scientists had to consider the spontaneous fission rate of each material when designing nuclear weapons.