It can be considerably lowered in various ways. The amount of plutonium used in fission weapons is in the 3 to 5 kilograms range. The smallest theoretical critical mass of plutonium-239 is only a few hundred grams. In contrast to nuclear weapons, nuclear reactors are designed to release energy in a sustained fashion over a long period of time. This balance is achieved by ensuring that each fission produces exactly one other fission.
All isotopes of plutonium are radioactive, but they have widely varying half-lives.
The half-life is the time it takes for half the atoms of an element to decay. The various isotopes also have different principal decay modes. Table 2 shows a summary of the radiological properties of five plutonium isotopes. The isotopes of plutonium that are relevant to the nuclear and commercial industries decay by the emission of alpha particles, beta particles, or spontaneous fission. Gamma radiation , which is penetrating electromagnetic radiation, is often associated with alpha and beta decays .