Answer to Question #10835 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
It is generally quoted that spent (used) nuclear fuel must be protected for up to 2,000 years. But I have heard that the radioactivity of spent fuel is reduced by 70 percent in the first 30 to 40 years. Is that true and if not where do I find information on the natural reduction in radioactivity of spent fuel?
Radioactivity in spent (used) nuclear fuel decreases at a known rate by radioactive decay. Your estimate of 70 percent after 30 to 40 years is too low. It decreases by 99.9 percent after 40 to 50 years! This is because the dominant elements in the fuel, fission products, have half-lives of 30 years or less.
The common mistake people make is equating time for decay with time for protection. They are not the same. After 99.99 percent decay, the fuel will still be radioactive, but may no longer need protection. It depends on where it is and in what condition. A good discussion of disposal and protection options is found in The Nuclear Waste Primer, by the League of Women Voters Education Fund available on Amazon.
Information about radioactive decay can be found on the Internet. A place to start is the World Nuclear Association website. It has several graphs showing decay of spent fuel over time.
Joel I. Cehn