Which reactor type does not require enrichment for its fuel?

The Magnox reactor type is distinctive because it utilizes natural uranium as its fuel, which means that it does not require enrichment. Natural uranium contains about 0.7% of the fissile isotope U-235, which is sufficient for the reactor's operation due to the design and the use of graphite moderators that facilitate the fission process. This design allows the reactor to function effectively without the need for enriched uranium, making it unique among the options listed.

CANDU reactors, on the other hand, typically use slightly enriched uranium fuel but can also run on natural uranium. However, they often take advantage of the higher efficiency provided by some enrichment. AGRs (Advanced Gas-cooled Reactors) generally utilize enriched uranium fuel and are optimized for higher thermal efficiency. Fast-breeder reactors also rely on enriched fuel; they are designed to produce more fissile material than they consume, typically using a combination of enriched uranium and plutonium.

The ability of Magnox reactors to operate on natural uranium not only simplifies fuel management but also contributes to lower fuel costs and lends itself to a different approach to fuel cycle management compared to the other reactor types which do depend on fuel enrichment.