The half-life of thorium-234 is about 24 days; but for uranium-238 is 4.5 billion years. It is plutonium-239, with a half-life of 24,100 years, which is the real cause for concern. Almost all plutonium-239 is formed as a waste product in nuclear reactors that use uranium as basic fuel. The disposal of this waste has become a serious problem because of its relatively long half-life, during which it gives off radiation in the form of dangerous alpha particles.
No matter how much of plutonium is there, the half-life stays the same. Twenty kilograms of plutonium-239 will decay in 24,100 years to produce 10 kilograms of plutonium-239, which will decay in another 24,100 years to leave 5 kilograms of it. This process would continue until no more of plutoniu-239 remains.
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