Although the basic phenomena of fuel irradiation are similar in PWRs to AGRs and magnox reactors, there

are some important differences due principally to different operating conditions. One difference arises from the batch refuelling in PWRs, which necessitates a high level of reactivity control to balance the re­activity change throughout a fuel cycle, and results in a substantial change to power shape at the end of each cycle. Another factor is that reactor control is effected by uniform boron poisoning of the coolant to accommodate slow reactivity changes and by move­ments of a control assembly bank to provide rapid control. There is no individual rod assembly control system, PWR also achieves higher power density and hence higher xenon levels than in AGRs.