A fuel loading scheme employing different enrich­ments is usfd in the AGR as a means of ‘flattening’ the radial power shape. This arranges for fuel situated near the edge of the core to produce power levels closer to that achieved by fuel in central positions, thereby making the overall shape of the power distri­bution more even. Higher enrichments are used nearer the core periphery in order to offset the effects of the greater neutron leakage. Usually two or three different radial enrichment zones in both initial and replacement charges are therefore used. The number of zones adopted varies between stations.

A useful criterion for measurement of the degree of power flattening in existence at any time is the ‘radial form factor’, defined quite simply as the ratio of peak to mean channel power over the whole re­actor. The total reactor power will always equal the sum of the individual stringer powers, but since there will be a limit on peak channel power in normal op­eration, total reactor power will depend upon the form factor and hence the constant incentive to keep this as close to unity as possible.

The concept of ‘axial form factor’, the ratio of peak to mean pow-er within a single channel, is also

commonly used. It exhibits wide variations from chan­nel to channel, since individual values are influenced by local regulating rod penetrations, which in turn influence the axial distributions of power. During the initial loading of AGR cores, two different enrich­ments are often used within the same channel, with the higher enrichments in certain top and bottom elements to provide some axial power flattening.

The in ilia l core and approach to equilibrium Initially all the fuel is fresh and unirradiated, but as soon as operation begins reactivity is reduced. Although this can be counteracted to a very small degree by withdrawing absorber (i. e., control rods), refuelling eventually becomes essential. Each of the initial charge fuel stringers is then progressively re­placed by feed fuel until, after about five years, all the initial fuel will have been replaced. The core will then consist of a mix of fuel with irradiations rang­ing from zero (i. e., fresh fuel just loaded) to the discharge limit itself (i. e., fuel about to be replaced). This signifies the attainment of ‘equilibrium’ — a phase in which a uniform spread of fuel irradiations is maintained by refuelling stringers as they reach their irradiation limit. This condition will remain for the rest of the working life of the reactor and can only be disturbed by further changes to the irradia­tion discharge limit.

Since the AGR will spend most of its life at fuel cycle equilibrium it follows that its core size, chosen by design and the refuelling strategy eventual­ly adopted, will be largely influenced by fuel cycle economics at this condition rather than at ‘start of life’ (SOL). Adequate consideration must nevertheless be given to the cost of assembling the initial charge, and accordingly certain schemes are designed to also improve fuel cycle economics at SOL. The financial outlay associated with the initial charge is usually included within the overall capital cost of the in­stallation,