Natural uranium composition: approximately what percentage of uranium-238 (U-238) is present in natural uranium (the remainder being mostly U-235 with trace U-234)?

Introduction / Context:
Understanding the isotopic composition of natural uranium is foundational for nuclear engineering, fuel-cycle calculations, and reactor physics. Natural uranium is not a single nuclide; rather, it is a mixture dominated by uranium-238 (U-238), with a small fraction of uranium-235 (U-235) and a trace amount of uranium-234 (U-234). Knowing these proportions helps explain why enrichment is required for many thermal reactors and why fertile U-238 plays a major role in breeding reactions.

Given Data / Assumptions:

• Natural uranium isotopes: mainly U-238 and U-235; U-234 present in trace amounts.
• We assume standard terrestrial isotopic abundances.
• Percentages are by atom (or close to by mass for quick estimates).

Concept / Approach:
The accepted textbook composition of natural uranium is approximately U-238 ≈ 99.274–99.284%, U-235 ≈ 0.711–0.720%, and U-234 ≈ 0.005–0.006%. Rounded values often used in entrance exams are 99.29% for U-238 and 0.71% for U-235. These small differences in atomic mass and abundance drive the need for enrichment processes (e.g., gaseous diffusion, centrifugation) to raise the fissile U-235 fraction for most light-water reactors.

Step-by-Step Solution:
Identify typical abundances: U-238 ~ 99.29%, U-235 ~ 0.71%, U-234 ~ 0.005%.Match to options: 99.29% corresponds to U-238's share.Therefore, the correct percentage of U-238 in natural uranium is about 99.29%.

Verification / Alternative check:
Chemical handbooks and reactor physics texts consistently report U-238 at roughly 99.28–99.29% in natural uranium, validating the rounded exam figure of 99.29% as accurate for problem solving and quick recall.

Why Other Options Are Wrong:
0.71%: that is the common figure for U-235, not U-238.0.015%: closer to the trace U-234 content, not U-238.29.71% or 97.00%: inconsistent with well-established natural abundances.

Common Pitfalls:
Swapping the U-235 and U-238 percentages.Assuming mass percentages differ dramatically from atomic percentages; for these isotopes the difference is small.

Final Answer:
99.29%