Another type of radioactive decay is spontaneous fission. In this decay process, the nucleus will split into two nearly equal fragments and several free neutrons. A large amount of energy is also released. Most elements do not decay in this manner unless their mass number is greater than 230.
The stray neutrons released by a spontaneous fission can prematurely initiate a chain reaction. This means that the assembly time to reach a critical mass has to be less than the rate of spontaneous fission. Scientists have to consider the spontaneous fission rate of each material when designing nuclear weapons.
For example, the spontaneous fission rate of plutonium 239 is about 300 times larger than that of uranium 235. This forced scientists working on the Manhattan Project to abandon work on a gun-type design that used plutonium.