Which isotope is fissile?

Which isotope is fissile?

• A. U-235
• B. U-238
• C. Pu-239
• D. Th-232

Answer: (A)

Fissile materials are those that can sustain a chain reaction when struck by neutrons, especially slow (thermal) neutrons. Uranium-235 readily fissions with thermal neutrons, releasing energy and extra neutrons that keep the reaction going. Uranium-238, while common, does not fission readily with thermal neutrons and is considered fertile—it can capture a neutron and eventually become a fissile isotope in a reactor. Plutonium-239 is also fissile and can sustain a chain reaction. Thorium-232 is fertile as well, breeding uranium-233, but it is not fissile by itself with thermal neutrons. So, among the options, uranium-235 is a well-known fissile isotope.

Fissile materials are those that can sustain a chain reaction when struck by neutrons, especially slow (thermal) neutrons. Uranium-235 readily fissions with thermal neutrons, releasing energy and extra neutrons that keep the reaction going. Uranium-238, while common, does not fission readily with thermal neutrons and is considered fertile—it can capture a neutron and eventually become a fissile isotope in a reactor. Plutonium-239 is also fissile and can sustain a chain reaction. Thorium-232 is fertile as well, breeding uranium-233, but it is not fissile by itself with thermal neutrons. So, among the options, uranium-235 is a well-known fissile isotope.