Answer:
The rods remain radioactive for thousands of years, so storage is a difficult problem.
Explanation:
Fuel rods represents the main element of the reactor core of a nuclear power plant.
They contain uranium-238 (U-238) enriched with uranium-235 (U-235), which is very unstable; when the nuclei of uranium absorb slow neutrons, they undergo nuclear fission, breaking apart and releasing huge amounts of energy.
In the process, several neutrons are released alongside with other products, and when they are slown down, they can be absorbed by other nuclei of uranium, further inducing more fissions.
The half-life of U-238 is 4.5 billion years, while the half-life of U-235 is approx. 700 million years: this means that the fuel rods remain highly radioactive for a very long time. Therefore, it is necessary to properly dispose them in a safe place where they do not represent a hazard. For instance, fuel rods: for example, they can be placed in sealed containers (built using concrete/steel to shield from the radiation) and buried underground.
