Final answer:
The storage time for radioactive materials to decay until safe varies depending on the isotope's half-life, ranging from days for short-lived isotopes to millennia for the long-lived ones. The decay process is not affected by environmental factors and depends solely on the properties of the isotope.
Step-by-step explanation:
When storing radioactive materials for decay until they are safe, the time required for this process can range significantly. The time depends on the material's half-life, which is the period needed for half of the radioactive atoms to decay. This decay process occurs at different rates for different isotopes, and thus the length of time needed for safe storage can vary.
For isotopes with short half-lives like Iodine-131 (8.02 days), the storage time could be a few days to weeks. Conversely, isotopes with longer half-lives such as Strontium-90 (28.8 years) or Plutonium-239 (24,000 years), require much longer time frames, ranging from decades to centuries, or even thousands to millions of years.
Therefore, when considering how long radioactive materials must be stored for decay, the correct answer could be 'b) Decades to centuries', and depending on the isotope, even 'c) Thousands to millions of years'. Handling radioactive waste is a significant concern in nuclear power production due to the longevity of the hazard it poses.