Only lead 210 is radioactive. Lead 210 is only created by uranium. Lead 210 has a short half-life, uranium 235 has a long one. Thus, once uranium is removed, l

Only lead 210 is radioactive. Lead 210 is only created by uranium. Lead 210 has a short half-life, uranium 235 has a long one. Thus, once uranium is removed, lead 210 levels drop "quickly".