As natural radioactivity progressively decays, the radioactivity emitted by a radioactive waste package decreases over time. Although glaringly obvious to physicists and engineers, the layman is often unaware of this fundamental fact. There is a popular belief that a waste package will be nearly as radioactive in 1,000 years as it is today! Despite it being a law of nature, too few people are aware that radioactivity decays.
Decay occurs slowly but surely. According to a study by the Finnish nuclear safety authority, upon removal from a reactor, spent fuel is 4 million times more radioactive than the natural uranium from which it was made. One year later, this factor has fallen to 60,000. After 500 years, the fuel is still 100 times more radioactive. It takes around 200,000 for the radioactivity from the waste package to match that of natural uranium.
Although this decrease is spectacular when measured in centuries and millennia, it appears very slow on human timescales! As the initial activity is very high, it will be a long time before the risk becomes negligible. The overall evolution is made more complicated by the variety of radioactive elements contained in the fuel. Each element decays according to its own half-life yielding daughter products that in some cases are also radioactive.
Around 10% of fission products have a radioactive half-life longer than 30 years. Some have half-lives of thousands or even millions of years. It should be noted that elements with very long half-lives disintegrate very slowly, and their radioactivity is correspondingly lower.
Fission products decay the most rapidly. The activity of such elements declines by a factor of 7 over 10 years, by 60 over 100 years, and by tens of thousands within 500 years. Caesium-137 and strontium-90 make the biggest contribution during the first hundred years or so. When these intermediate-lived fission products disappear, a few long-lived fission products (iodine-129, caesium-135 and technetium-99) remain. Note that activation products systematically represent only a small proportion of total radioactivity.
Plutonium and minor actinides, which are heavy nuclei, generally have long half-lives, as do their daughter products. Their radioactivity decays much more slowly. Over the very long term, the activity of spent fuel is largely determined by plutonium – if it has not been removed – and minor actinides. This explains why such importance is given to these products. It also explains the benefit of removing them from waste, if they can be destroyed by transmutation.
The presence of residual long-lived radioelements causes the rate of decay observed during the initial centuries to slow after around 500 years. By this point, the fuel's activity has already fallen by a factor of 40,000 since it was removed from the reactor. This residual amount is the focus of current research that seeks to further reduce it at millennial time-scales.
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