For how long must technetium-99m generators not returned to the vendor be kept in storage?

The correct choice is based on the decay characteristics of technetium-99m, which undergoes a half-life of approximately 6 hours. To ensure safety and reduce potential radiation exposure, it is standard to allow radioactive materials to decay to a negligible level before disposal.

The guideline of keeping such generators for 25 half-lives is significant because, after this period, the remaining activity is typically considered to be at a safe level for handling or disposal. After 25 half-lives, the radiation level is reduced to about 1/335,544,320 of its original activity. This means that the risk of exposure from any residual radioactivity will be extremely low, meeting safety standards and regulations pertaining to the disposal of radioactive waste.

By adhering to this time frame, facilities can manage radioactive waste responsibly, ensuring compliance with safety protocols and regulations. This practice is crucial for protecting both personnel and the environment from unnecessary radiation exposure.