radiation warning symbol; trefoil
Refers to the internationally recognized symbol for ionizing radiation, often a black trefoil on a yellow background.
病院の放射線室のドアには放射能マークが貼ってある。
A radiation warning symbol is affixed to the door of the hospital's radiology room.
Compound of 放射能 (radioactivity) + マーク (mark), a straightforward modern term.