This is a common misconception. The longer the half-life, the less radioactive something is. By definition. Something with a half-life of "tens or hundreds of

This is a common misconception. The longer the half-life, the less radioactive something is. By definition. Something with a half-life of "tens or hundreds of millenia" is not very radioactive. Something with a half-life of infinity is not radioactive at all. The really dangerous stuff tends to be isotopes like iodine-131 (half-life 8 days, accumulates in the thyroid gland), strontium-90 (half-life of about 29 years, accumulates in the bones).