Nuclear marine/space propulsion: fuel form for compact propulsion reactors Reactors designed for propulsion applications (e.g., naval or space power units) require which fuel form to achieve a compact, long-life core at practical power density? Choose the correct option.

Given data

• Application: compact propulsion reactors (e.g., submarines, icebreakers).

Concept/Approach
To achieve high power density in a small core with long refueling intervals, the fuel must have a high fraction of fissile nuclei. Enriched uranium (typically enriched in U-235) provides sufficient reactivity in a compact geometry, unlike natural uranium which generally requires heavy moderation and large cores.

Why others are not best
“Any form” is too broad and incorrect; natural uranium would require impractically large cores for most propulsion uses; plutonium fuels pose different handling and design issues and are not the standard baseline choice.

Final Answer
Enriched uranium