Why do radioactive materials like plutonium in nuclear waste continue to pose threat for many thousands of years?

The reason is that the nuclei of such materials are unstable, so the materials emit radioactive energy for a long period. In essence, they have long half-lives, which in turn depend on the number of nuclei that decay per second. A half-life is the time it takes for one-half of a sample of a specific radioactive material to decay. Half-lives range from as short as 10-21 seconds to billions of years.

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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