If a 99mTc spill occurs and some material remains, how long will it take for the exposure rate to decline from 32 mR/hr to 2 mR/hr?

To determine how long it will take for the exposure rate of 99mTc to decline from 32 mR/hr to 2 mR/hr, one must consider the half-life of 99mTc, which is approximately 6 hours. The exposure rate decreases exponentially based on this half-life.

Starting at 32 mR/hr, the exposure can be calculated over successive half-lives. After one half-life (6 hours), the exposure rate would decrease to half of 32 mR/hr, which is 16 mR/hr. After two half-lives (12 hours), it would drop to 8 mR/hr. After three half-lives (18 hours), the exposure rate would reduce to 4 mR/hr. Finally, after four half-lives (24 hours), the rate would reach 2 mR/hr.

Thus, it takes a total of four half-lives, or 24 hours, for the exposure rate to decline from 32 mR/hr to 2 mR/hr. This makes the statement that it takes 24 hours the correct answer, as it accurately calculates the time based on the half-life of the radioactive material involved.