A dictionary-entry caption: the colon introduces the definition, the semicolon separates two clauses. 放射性同位体 'radioactive isotope' and 放射性降下物 'radioactive fallout' share the 放射性 '(having the property of) radiation' prefix. 含まれ is the conjunctive masu-stem of the passive 含まれる 'to be included,' chained without ~て — a written/literary stylistic alternative to the te-form for connecting clauses.