In thermal nuclear reactors that use enriched uranium fuel, the typical maximum concentration of fissile U-235 is approximately what percentage (order-of-magnitude used in many textbook examples)?

Introduction / Context:
Thermal power reactors moderated by light water generally require fuel enriched in the fissile isotope U-235 to maintain criticality and achieve practical core sizes and cycle lengths. Educational problems often quote a rounded “few percent” enrichment for simple estimates. Knowing the common enrichment range helps distinguish between natural-uranium, low-enriched, and highly enriched fuels.

Given Data / Assumptions:

• “Thermal nuclear reactors” refers to common light-water reactors (PWR/BWR) in introductory contexts.
• Question seeks a representative maximum value used in basic problem statements, not the entire industrial range.
• Answer choices are 1, 2, 3, and 7 percent.

Concept / Approach:
Natural uranium contains about 0.711% U-235. Light-water reactors typically operate with low-enriched uranium, historically around 2% to 3% in many textbook examples, though modern cores may use 3%–5% or slightly higher depending on design and regulations. Within the options provided, 3% is the standard “typical maximum” value taught in foundational problems.

Step-by-Step Solution:
Recognize that 1%–2% may be insufficient for standard LWRs in classic examples.Identify the commonly cited educational figure: about 3% enrichment.Select 3 as the best representative maximum from the given choices.

Verification / Alternative check:
Introductory reactor engineering texts frequently compare natural uranium (0.711%) with “a few percent” enrichment for LWRs; 3% is a canonical round number in problems and MCQs.

Why Other Options Are Wrong:
1 or 2 percent: below typical LWR practice in textbook problems.7 percent: exceeds the classic low-enrichment figures emphasized in entry-level questions and approaches ranges outside many historical baselines.

Common Pitfalls:
Confusing current industry practice (often >3%) with the simplified values used in examinations; mixing thermal-reactor enrichment with research or naval-reactor enrichments.

Final Answer:
3