In practice, however, reactor-bred plutonium will invariably contain a certain amount of 240Pu due to the tendency of 239Pu to absorb an additional neutron during production. 240Pu has a high rate of spontaneous fission events (415,000 fission/s-kg), making it an undesirable contaminant. As a result, plutonium containing a significant fraction of 240Pu is not well-suited to use in nuclear weapons; it emits neutron radiation, making handling more difficult, and its presence can lead to a "fizzle" in which a small explosion occurs, destroying the weapon but not causing fission of a significant fraction of the fuel. (However, in modern nuclear weapons using neutron generators for initiation and fusion boosting to supply extra neutrons, fizzling is not an issue.) It is because of this limitation that plutonium-based weapons must be implosion-type, rather than gun-type. Moreover, 239Pu and 240Pu cannot be chemically distinguished, so expensive and difficult isotope separation would be necessary to separate them. Weapons-grade plutonium is defined as containing no more than 7% 240Pu; this is achieved by only exposing 238U to neutron sources for short periods of time to minimize the 240Pu produced.