The present life of magnox fuel is limited to 5000- 6000 MWd/t channel average irradiation or dwell time of 11 years, whichever is the shorter. For fuel cycle purposes, the reactor is divided into three radial zones:

• Flattened zone — irradiation limited.

• Unflattened zone — irradiation or’reactivity limited.

• Unflattened zone — dwell time limited.

In the case of the flattened zone, the rate of fuel changing is at a maximum and uniform over the whole zone.

The unflattened zone will have fuel changing rates which will vary with the radius from the rate em­ployed in the flattened zone at its inner radius to the rate in the dwell zone at its outer radius. For practical purposes, it is usual to sub-divide these zones into smaller radial regions such that the average channel irradiation rate in each of the smaller regions may be regarded as constant giving a uniform rate of re­fuelling. The outermost zone of the unflattened region is usually known as the dwell zone and contains fuel w^hose life is limited metallurgical^ to 11 years. The channel content of this zone is of the order of a few — percent at the most of the total core. A typical fuel cycle for the whole reactor is illustrated by Fig 3.38.

It should be noted that starting with a virgin re­actor, equilibrium is not reached until the whole of the first charge has been replaced. Under the present regime, this will be 11 years. However, equilibrium conditions approximate by the time the flattened zone and about half of the unflattened zone have been refuelled.

From the point of view of the mechanics of the fuel cycle, an increase in the target channel irradia­tion simply requires the same channels in a zone to be evenly spread over a greater length of time. Re­ferring to Fig 3.38, the flattened zone is shown as completing its cycle in five years. A further cycle is started bv returning to the channels first discharged at the start of the original e>cle and continuing se­quentially through the channel for another foe vearv. It’, however, conditions enabled an increase in target irradiation to give an equivalent dwell time of six years then the same channels would be discharged over a six year period instead of five years. There would be a resultant decrease in refuelling rate. Such a change is illustrated in Fig 3.38 and would normally be reflected in similar changes in the refuelling of the unflattened zones. As shown, changes may take place at any time and are not restricted to the end point of a cycle.