Maximum U-235 content in enriched-uranium fast breeder reactors: up to what percentage of U-235 may be used in such cores?

Introduction / Context:
Fast breeder reactors (FBRs) are designed to operate with a fast neutron spectrum and to convert fertile material (often U-238) into fissile material (such as Pu-239). Core fuel compositions vary by design and purpose (experimental, prototype, or commercial), but an understanding of the upper bounds used in enriched-uranium fast cores is a common exam topic.

Given Data / Assumptions:

• We refer to fast reactors that employ enriched uranium as fissile driver material.
• Enrichment denotes the percentage of U-235 in uranium.
• The question asks for an upper-end figure sometimes cited for such designs.

Concept / Approach:
Fast-spectrum reactors do not require moderation, and their neutronics can accommodate a wide range of fissile assays. While many practical fast reactors use lower enrichments (e.g., tens of percent or MOX fuels), educational references and MCQ compendia often cite that fast reactors using enriched uranium may contain up to very high enrichments on the order of several tens of percent and even approaching weapons-grade levels in certain specialized cores. The extreme upper bound commonly quoted in exam contexts is about 85% U-235.

Step-by-Step Solution:

Verification / Alternative check:
While many fast reactor cores today prefer MOX or lower-assay fuels, historical and theoretical treatments acknowledge designs that could utilize very high U-235 enrichments; hence, 85% is used as an accepted “maximum up to” figure in numerous exam banks.

Why Other Options Are Wrong:

• 5% and 15%: typical of light-water or some experimental ranges, not an upper bound for fast cores.
• 45% and 65%: possible in some contexts but do not represent the quoted “maximum up to” figure.

Common Pitfalls:
Confusing typical operating enrichments with exam-style maximum values; assuming the value implies common practice rather than a permissible upper bound.

Final Answer:
85%