Nuclear fuels and isotopes: in naturally occurring uranium, which isotope is present in the greatest abundance by percentage composition?

Introduction / Context:
Natural uranium is a mixture of several isotopes, chiefly U-238 and U-235, with traces of U-234. Understanding which isotope dominates is foundational for topics such as enrichment, reactor physics, and nuclear fuel cycles. This question checks awareness of natural isotopic abundance and why enrichment is required for many reactor types.

Given Data / Assumptions:

• “Natural uranium” means uranium as mined and processed without isotopic enrichment.
• Relevant isotopes: U-238, U-235, U-234 (trace); others like U-236 do not occur naturally in notable amounts.
• We focus on percentage composition by atom percent (or weight percent, nearly the same conclusion).

Concept / Approach:
Natural uranium is overwhelmingly U-238 (about 99.27% by atoms), with U-235 around 0.71% and U-234 about 0.005–0.01%. Because U-235 is the principal fissile isotope for thermal reactors, the low natural fraction typically necessitates enrichment for light-water reactors. Heavy-water or graphite-moderated designs can sometimes use natural uranium due to superior neutron economy.

Step-by-Step Solution:

Verification / Alternative check:
Textbook and data compilations consistently report ~99.27% U-238, ~0.711% U-235, and ~0.005–0.01% U-234. Any small variance in quoted figures does not change which isotope is dominant.

Why Other Options Are Wrong:

• U-235: only ~0.71% naturally.
• U-234: trace isotope (parts per ten thousand).
• U-236: not naturally abundant; generally produced in reactors.
• None of these: incorrect because U-238 is clearly correct.

Common Pitfalls:
Confusing “fissile importance” with “abundance”; U-235 is key for fission but is not the most abundant in nature. Assuming enrichment changes natural composition—enrichment is a separate industrial step.

Final Answer:
U-238