Complex sentence: The 5.27 year half life of the 60 Co is long enough to allow it to settle out before significant decay has occurred, and for it to be impractical to wait in shelters for it to decay, yet short enough that intense radiation is produced.
Split into simpler sentences: The 5.27 year half life of the 60 Co is long enough to allow it to settle out before significant decay has occurred. The half-life is long enough for it to be impractical to wait in shelters for it to decay, yet short enough that intense radiation is produced.