As we have seen in both cases of continuous or discontinuous reloading, after some years of reactor operation an equilibrium condition is reached in which all burn-up stages of the fuel are present at the same time in the reactor. The average reactor composition remains then constant (in the case of continuous refuelling) or has periodical variations (in the case of discontinuous refuelling, in which case the period is given by the refuelling interval). As only fresh fuel is present in the first core, a transition period (running-in) is necessary before equilibrium is reached. This period is economically very important because its fuel-cycle costs, usually higher than in equilibrium, can strongly influence the mean costs averaged over the reactor lifetime. Besides the maximum temperatures and control-rod requirements during running-in can pose limiting values for the reactor layout. The optimum running-in strategy is the one incurring the least economic penalty for operating off the optimum point during the first periods of the reactor life/73 All the constraints about temperature, dose, gradients, etc., are obviously valid for this phase as well as for the equilibrium condition.

For economical reasons it is necessary to limit the fissile inventory of the first core, but a lower limit is given by the power peaking in the fresh elements of the following loadings, which gets too high if the average fission cross-section in the core is low. The value of these power peaks should not be higher than in the equilibrium condition.

An ideal running-in strategy could consist in simulating the equilibrium core in the first loading. This would require a very high number of fuel element types with different fuel loadings, simulating the various burn-up stages of the equilibrium core. Fission products could be simulated by means of poisons (B, Gd, etc.). Normal refuelling operations would then immediately start. This would mean that some fuel would be discharged at a very low irradiation level, with the consequence of a high economic penalty. Besides the fabrication of so many different elements is hardly conceivable.

The opposite extreme would consist in loading entirely the reactor with fresh fuel elements of the type used in equilibrium. In this case the fuel loading and reactivity would be too high. A very high quantity of poison (or a high number of control rods) would be required, and the high fuel loading would also mean a high initial investment.

In pebble-bed reactors it is possible to use for the first loading the same type of fresh fuel elements used for the equilibrium condition.’81 The in-core fissile inventory is limited by mixing these elements with dummy graphite elements. In this case it is not possible to have the same ratio of fertile to fissile material as in equilibrium because the fertile material burns very slowly and in equilibrium is present in a considerable amount in the depleted elements, which are not present in the first core. The missing fertile material and the missing fission products are compensated with burnable poisons concentrated in absorber balls. Considering the passage time through the core, the rate of burn-up of the poisons should be chosen in such a way as not to disturb the axial flux distribution. Besides this poison burn-up should locally compensate the fuel burn-up as nearly as possible. From the beginning of the power operation dummy elements start being discharged and substituted with fuel elements.

The use of dummy elements has been also analysed in the case of reactors with prismatic fuel, but engineering problems are posed by non-power-producing channels and large flux tilts can occur with high local power peaks, so that this solution does not appear to be attractive. The best solution in this case appears to be the use of a certain number of fuel element types (four or more) with different fissile loading (or enrichment) in the first core.’7’ The excess reactivity is controlled by means of burnable poisons.

There is usually an initial non-refuelling period. The loading of equilibrium fresh fuel directly after this period may still give rise to too high power peaks, so that often fuel elements with intermediate fissile loading are needed for this intermediate phase.

The safety characteristics and control-rod requirements of the running-in phase must be carefully analysed. In particular the use of burnable poisons can have a strong effect on the temperature coefficient. Also the load-following capability (Xe overriding) during running-in can considerably differ from the equilibrium case.