Radioactive elements that last the longest are the least radioactive.

Spent nuclear fuel is not dangerously radioactive for thousands of years, as is the common notion.  After the first few hundred years the amount of remaining radioactivity exposure potential has dramatically decreased by magnitudes.

Short-lived radioactive elements in spent fuel tend to be highly radioactive but for a short time.  Radioactive Iodine 131is very radioactive but decays rapidly in around 30 days.

On the other hand, Plutonium 239 has a half-life of 24000 years and is minimally radioactive giving off its radioactivity very slowly over that long period of time.

Phil Carlson