These problems illustrate some important aspects of plutonium isotopes and plutonium metal pits (the core of a nuclear weapon). It takes 5 kilograms of plutonium-239 to make a nuclear weapon. What is the volume (or the size) of the plutonium? Express your answer in cubic centimeters. [As you can see in the Centerfold, the density of plutonium is 16 to 20 grams per cubic centimeter. For this problem, assume a density of 16 grams/cubic centimeter]. How many curies are there in 5 kilograms of plutonium-239? [see the Centerfold for specific activity. Remember that curies are a unit of radioactivity]. Based on the half-life given for plutonium isotopes in the Centerfold, how much of the initial 5 kilograms of plutonium-239 is left after 24,110 years? [Note the half-life is the time in which half the atoms of a radioactive substance will have decayed and formed atoms of a new element. Half of the original radioactive substance will decay after another half-life. Thus one-fourth the original amount is left after two half-lives, one-eighth is left after three half-lives, and so on. For instance, plutonium-239 decays into uranium-235. See diagram below] The Half-life of Plutonium-239

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Institute for Energy and Environmental Research

Spring 1994
Posted January 2002

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Answers The volume of a 5 kg plutonium pit is about 312.5 cubic centimeters. There are 315 curies in 5 kg of plutonium-239. 2.5 kg (or half) of the original Pu-239 remains after one half-life of 24,110 yrs. For a detailed explanation of the answers, please write to IEER.