Answer: Fission products (including long-lived isotopes) and actinides.
- A The small amount of unfissioned U-235 remaining in the fuel rods
- B Fission products (including long-lived isotopes) and actinides
- C The cooling water that circulates through the reactor core
- D The structural metal materials used to build the fuel assembly
Correct answer: B. Fission products (including long-lived isotopes) and actinides
Explanation: Spent fuel: highly radioactive fission products (Sr-90, Cs-137, etc.) and minor actinides (Pu, Am, etc.) from neutron capture.
Binding energy per nucleon rises sharply for light nuclei, peaks around iron-56 (the most stable nucleus), then slowly declines for heavier nuclei - which is exactly why fusing light nuclei or splitting heavy nuclei both release energy: each moves the products toward this stability peak.
Concept context
Radioactivity, nuclear reactions, fission, fusion, and binding energy.