Fission reactions and subsequent neutron escape happen very quickly; this is important for nuclear weapons, where the objective is to make a nuclear pit release as much energy as possible before it physically explodes. Most neutrons emitted by fission events are prompt : they are emitted effectively instantaneously. Once emitted, the average neutron lifetime ( τ τ ) in a typical core is on the order of a millisecond, so if the exponential factor α α is as small as 0.01, then in one second the reactor power will vary by a factor of (1 + 0.01), or more than ten thousand. Nuclear weapons are engineered to maximize the power growth rate, with lifetimes well under a millisecond and exponential factors close to 2; but such rapid variation would render it practically impossible to control the reaction rates in a nuclear reactor.