Nuclear Energy – Fissionable Uranium Which isotope of uranium is capable of sustaining a nuclear chain reaction with thermal (slow) neutrons?

Introduction / Context:
In reactor physics, only certain nuclides are “fissile,” meaning they readily undergo fission upon absorbing slow (thermal) neutrons and can sustain a chain reaction. Knowing which uranium isotope is fissile is essential for understanding nuclear power and weapons basics.

Given Data / Assumptions:

• We focus on uranium isotopes used in reactors.
• Thermal neutrons have low kinetic energy (~0.025 eV at room temperature).
• A self-sustaining chain reaction requires a fissile isotope.

Concept / Approach:
U-235 is fissile with thermal neutrons and is the key component in natural and enriched uranium fuels. U-238 is fertile (captures a neutron to eventually form Pu-239) but is not fissile with thermal neutrons. U-239 is a short-lived intermediate in plutonium breeding, and U-245 is not a standard naturally occurring or reactor-relevant isotope in basic curricula.

Step-by-Step Solution:

Verification / Alternative check:
Commercial light-water reactors use fuel enriched in U-235 (typically ~3–5%) precisely because U-235 sustains a thermal-neutron chain reaction. This operational fact corroborates the choice.

Why Other Options Are Wrong:

• U-238: Not fissile with thermal neutrons; requires fast neutrons or breeding to Pu-239.
• U-239: Short-lived beta emitter, not a fuel.
• U-245: Not a standard fissile isotope in basic reactor discussions.

Common Pitfalls:
Confusing “fissile” (readily fissions with slow neutrons) with “fertile” (can be converted into a fissile nuclide). U-238 is fertile, not fissile.

Final Answer:
U-235