Definition: A transuranium artificial radioactive element, atomic no. 94, atomic wt. 244.064. The best-known α-emitting isotope is 239Pu (half-life 24,110 years) which, like 235U, is fissionable and can be used in atomic bombs and nuclear power plants; 238Pu (half-life 87.74 years) is used as an energy source in pacemakers. Pu ions are bone-seekers; ingestion is a radiation hazard, as with radium and radiostrontium.