As a U.S. Nuclear Regulatory Commission project manager, I took tours and inspected several nuclear power plants for how utilities handle spent fuel. The fuel rods are very hot and extremely radioactive after being used in the reactor. The U-235 fuel produced heat energy as well as radioactive isotopes (elements that have the same number of protons but different number of neutrons) which are called fission products. For example, water is well known by the chemical symbol H2O with two hydrogen protons. Tritium is called heavy water because of an extra neutron and designated as H3. Radioactive substances give off three types of radiation: alpha, beta, and gamma which can all be harmful without proper protection.
There are three ways to protect against radiation: time, distance, and shielding. Radiation decreases over time following half-lives of radionuclides. For example, tritium (also mentioned in the previous blog) has a half life of 12.4 years so concentrations dissipate quickly as compared to strontium-90 which takes about 30 years for amounts to decrease by half or plutonium-239 with a half-life of 24,000 years. The more distance provided will decrease exposure to radioactivity as well as using metal and concrete for shielding.
If you look at a periodic table, calcium and strontium are in the same group 2 alkaline earth metals. They behave in similar ways, so the big concern would be that Sr-90 substitutes for Ca and gets into milk products or bones! That is why testing of the environment around nuclear power plants is so important to prevent exposures.
At the power plant, spent fuel is commonly kept in steel and concrete lined pools in water about 40 feet deep for at least five years. Some utilities have transferred older fuel to “dry cask” storage located at the power plant. I understand that the spent fuel will never be useful for reprocessing as is done in France and must be stored or disposed in a geological repository which is the subject of the next blog.