Fuel terminology: “enriched uranium” refers to uranium in which the U-235 content has been increased to what relative level compared with natural uranium?

Introduction / Context:
Natural uranium contains only about 0.71% U-235, the primary fissile isotope for most thermal reactors. Many reactor types require a higher fraction of U-235, hence the term “enriched uranium.”

Given Data / Assumptions:

• Natural uranium composition: ~99.27% U-238, ~0.71% U-235, trace U-234.
• Enrichment processes increase the U-235 assay above its natural level.
• We use the common definitional threshold relative to natural abundance.

Concept / Approach:
Any uranium whose U-235 content exceeds the natural 0.71% is called “enriched.” Low-enriched uranium (LEU) generally means ≤ 20% U-235 (typical power reactors use 3–5%); high-enriched uranium (HEU) is > 20% U-235. Statements implying “only fissile” or “only fertile” are incorrect, since mixtures always remain.

Step-by-Step Solution:

Verification / Alternative check:
Fuel specifications for light-water reactors list assays around 3–5% U-235, clearly above natural levels.

Why Other Options Are Wrong:

• Only fertile or only fissile: unrealistic—mixtures persist.
• Impurity-free: enrichment is isotopic, not chemical purification per se.
• Exactly 0.71%: that is natural, not enriched.

Common Pitfalls:
Confusing chemical purity with isotopic composition; equating enrichment with removal of all non-fissile isotopes.

Final Answer:
More than 0.71% U-235