Natural uranium composition check: what percentage of the fissile isotope U-235 is present in natural uranium?

Introduction / Context:
Natural uranium is primarily U-238 with a small fraction of U-235, the fissile isotope that sustains chain reactions in many reactor types. Understanding this baseline composition is essential for topics such as enrichment, fuel fabrication, and heavy-water moderated reactor choices.

Given Data / Assumptions:

• Refers to naturally occurring isotopic abundance prior to any enrichment.
• Values rounded to two decimal places for ease.
• Fissile here specifically means U-235 (ignoring minor U-234 content).

Concept / Approach:
Natural uranium contains about 0.71% U-235, about 99.28% U-238, and a trace of U-234. Light-water reactors typically require enriched fuel (e.g., 3–5% U-235), whereas heavy-water reactors can operate with natural uranium because the moderator has very low neutron absorption, preserving reactivity.

Step-by-Step Solution:
Recall standard isotopic abundances of natural uranium.Identify the U-235 percentage approximately equal to 0.71%.Select the matching option.

Verification / Alternative check:
The widespread use of enrichment facilities and the design of CANDU-type heavy-water reactors both hinge on this small U-235 fraction, corroborating the remembered value.

Why Other Options Are Wrong:

• 6.31 and 12.73: Far above natural abundance; typical of enriched fuel ranges (though still low for some designs).
• 99.29: Closer to U-238 abundance, not U-235.
• 3.00: Represents low enrichment levels, not natural.

Common Pitfalls:
Confusing weight percent with atom percent; mixing up U-238 dominance with U-235 fraction.

Final Answer:
0.71