Among the following nuclides, which is considered the best fissile nuclear fuel for thermal reactors under standard conditions?

Introduction / Context:
Fissile fuels sustain a chain reaction with thermal (slow) neutrons in conventional reactor designs. Recognizing which nuclides are truly fissile and practical is foundational for nuclear engineering exams and industry practice.

Given Data / Assumptions:

• Comparison is in the context of standard thermal reactor conditions.
• Focus on fissile character, fission cross section with thermal neutrons, and practical fuel-cycle use.

Concept / Approach:
Fissile nuclides include U-235, Pu-239, and U-233 (not in the list). Among the given options, Pu-239 is a well-known fissile isotope produced from neutron capture in U-238 followed by beta decays. It has a high thermal fission cross section and favorable neutron economy, making it a cornerstone of mixed-oxide (MOX) fuels and breeder strategies.

Step-by-Step Solution:
Evaluate each option’s fissility and practicality.Np-239: short-lived intermediate in Pu-239 production, not a practical fuel.U-234: not fissile under thermal neutrons; mainly a decay product and fertile in limited contexts.Pu-239: established fissile fuel with strong thermal fission characteristics.Th-236: not the key fissile; thorium breeds U-233 which is fissile.Therefore, Pu-239 is best among listed.

Verification / Alternative check:
Fuel-cycle literature confirms MOX fuels (UO2 + PuO2) leverage Pu-239’s fissile nature in LWRs and other designs, validating its selection here.

Why Other Options Are Wrong:
Np-239: transient nuclide, not used as reactor fuel.U-234: poor thermal fissility.Th-236: thorium’s valuable product is U-233, not Th-236.Am-241 (if considered): used in specialty sources; not principal thermal fuel.

Common Pitfalls:
Confusing fertile (U-238, Th-232) with fissile (U-235, Pu-239, U-233).Selecting intermediates that appear in breeding chains.

Final Answer:
Pu-239