ra ‧ di ‧ o ‧ ac ‧ tive /ˌreɪdiəʊˈæktɪv◂ $ -dioʊ-/ BrE AmE adjective
a radioactive substance is dangerous because it contains ↑ radiation (=a form of energy that can harm living things) :
the problem of how to dispose of radioactive waste
a consignment of highly radioactive plutonium