Spent nuclear fuel — What does the spent fuel discharged from a thermal (fission) reactor contain?

Introduction:
Upon discharge, spent nuclear fuel is a complex mixture. Understanding its composition is essential for safety, storage, and reprocessing strategies in the back end of the fuel cycle.

Given Data / Assumptions:

• LWR or PHWR spent fuel after typical burnup.
• U-235 and U-238 initially present; neutron irradiation occurs.
• Reprocessing (if applied) separates usable isotopes from high-level waste.

Concept / Approach:
Fission of U-235 (and Pu isotopes once formed) yields a wide distribution of fission products (e.g., Cs-137, Sr-90). Some U-238 captures neutrons to form Pu-239 and higher plutonium isotopes. Not all initial fissile material is consumed; unburnt uranium remains, and plutonium is created.

Step-by-Step Solution:
Fission products: Present in significant fractions; principal contributors to heat and radioactivity.Plutonium: Bred from U-238 (U-238 → U-239 → Np-239 → Pu-239), with further isotopic evolution to Pu-240/241 etc.Unused fuel: Residual U-235 and abundant U-238 remain after discharge.Hence, spent fuel contains all three categories.

Verification / Alternative check:
Standard spent-fuel assays confirm approximate compositions: majority U (mostly U-238), a few percent fission products, and a percent-level plutonium inventory depending on burnup.

Why Other Options Are Wrong:

• (a), (b), (c) individually capture only part of the inventory.

Common Pitfalls:
Assuming spent fuel is 'used up' completely; ignoring plutonium formation during operation.

Final Answer:
all (a), (b) & (c)