A basic decision in the design of any nuclear reactor is whether to remove and insert fuel while the reactor is operating (on-load refueling) or when the reac­tor is shut down (off-load refueling).

The choice between on-load and off-load refueling is dictated primarily by economic factors. On-load refueling is much more complex, and the cost of the equipment is high. On the other hand, total shutdown of the reactor system for a significant period to allow off-load refueling leads to a loss of electrical power output, and this in itself is very expensive. In general, reactors that have a large throughput of fuel, such as natural uranium reactors (Magnox and CANDU) op­erate with on-load refueling, whereas those with a lower throughput of en­riched fuel (e. g., P^^ and B^WR tend to use off-load refueling. The advanced gas-cooled reactor (AGR) is intermediate between these two cases; it has the ca­pability of on-load refueling, though this is only just being introduced into rou­tine operations Qenkins et al., 1995). For fast reactors, off-load refueling is necessary because of the veiy large changes in reactivity that would occur with any fuel movements during operation.