What is the purpose of enriching U-235 in AGR fuel but not in Magnox reactors?

The purpose of enriching U-235 in Advanced Gas-cooled Reactors (AGRs) but not in Magnox reactors primarily relates to the different neutron balance requirements of these reactor types.

AGRs are designed to optimize the use of nuclear fuel efficiently by achieving a higher neutron economy. The enrichment of U-235 in AGRs facilitates a higher probability of fission, which helps ensure that the reactor achieves a sustainable chain reaction with fewer initial fissile materials. This is crucial for maintaining the efficiency and flexibility of the fuel cycle in AGRs, where the balance between neutron production and absorption must be carefully managed to sustain the required power output.

In contrast, Magnox reactors, which use natural uranium fuel, operate effectively without the need for enriched U-235 due to their design and the specific types of coolant and moderation they employ. These reactors have a different approach to managing neutrons, which allows them to utilize unenriched fuel.

Therefore, enriching U-235 specifically addresses the demands for neutron economy and balance in AGRs, which is distinct from the operational requirements of Magnox reactors.