fissionable material
Technical term in nuclear physics for materials capable of sustaining a nuclear fission chain reaction, such as uranium-235 or plutonium-239.
The management of fissionable materials is strictly regulated internationally.
核燃料 (nuclear fuel) is a broader term that includes materials used in nuclear reactors, which may contain fissionable substances, but 核分裂物質 specifically refers to materials that can undergo fission.
Compound of 核分裂 (nuclear fission) and 物質 (material, substance).