The core of a power reactor must be designed to safely, reliably and economically produce heat and transfer it to produce steam. In addition to controlling the processes, it must be capable of being refuelled and maintained. The overall process is complex involving neutronics, thermal hydraulics and structural engineering while at all times satisfying the three fundamental safety principles discussed in Section 10.9. The application to reactor core design is discussed in an IAEA guide (IAEA, 2005).

The design of the core is also key to the economics of the plant. Efficient fuel utilisation helps reduce fuel costs while the ability to rapidly refuel the plant improves availability. Both PWRs and BWRs have to be shutdown to be refuelled. To facilitate this, the reactor vessel is set into a refuelling canal. This consists of a structure within the containment, which is steel lined and can be flooded once the RPV head is removed. This then provides both shielding and cooling and is connected to the fuel pool to allow the transfer of fuel assemblies to the fuel pond.

In some designs the fuel pond is in the containment building and is directly connected to the refuelling canal. In others it is in a separate building and so fuel is transferred through a tube between the buildings, remaining underwater during the whole process. The use of fuel ponds inside the containment is conducive to fast refuelling but requires a containment designed to be accessible during normal operation.