Uranium enrichment basics: enrichment is performed to increase the concentration of which isotope in natural uranium for use as nuclear fuel?

Introduction / Context:
Natural uranium consists primarily of U-238 with a small fraction of U-235, the fissile isotope that readily undergoes fission with thermal neutrons. Many reactor designs require an increased proportion of U-235 to achieve criticality and practical performance, motivating enrichment technologies.

Given Data / Assumptions:

• Natural uranium composition: ~99.3% U-238 and ~0.711% U-235 (trace U-234).
• Goal of enrichment: improve fissile content for reactor fuel.
• Choices include fertile isotopes and non-uranium options.

Concept / Approach:
Enrichment processes (gaseous diffusion, centrifuges, laser-based methods) separate isotopes based on small mass differences to increase the U-235 content. U-233 is bred from thorium, and Pu-239 is bred from U-238; neither is increased by “uranium enrichment” of natural uranium feedstock.

Step-by-Step Solution:
Identify the fissile isotope in natural uranium: U-235.Recognize enrichment increases U-235 fraction while decreasing U-238 fraction.Select U-235 as the target of enrichment.

Verification / Alternative check:
Fuel specifications for light-water reactors cite low-enriched uranium (LEU) with a few percent U-235, directly reflecting enrichment objectives.

Why Other Options Are Wrong:
U-238: the dominant fertile isotope; its fraction is reduced during enrichment.U-233: not present in natural uranium; obtained via thorium fuel cycle.Pu-239: produced by neutron capture in U-238 followed by beta decays, not by isotope enrichment of uranium.

Common Pitfalls:
Confusing enrichment (isotopic separation) with breeding (neutron-induced transmutation); assuming all fissile isotopes are increased by the same process.

Final Answer:
U-235