Nuclear power stations have a relatively high capital cost and low fuel cost, so that there is a tendency to use them for base load. On the other hand, it will be difficult to use
very big power stations only for base load. In general all power stations tend to be used for part load as they get older and more efficient plants become available.

At present the ability of following load variation, at least between 100% and 40% is a general condition posed by all customers. Sometimes it is even required that the station should be able to restart at any moment after shut-down (full Xe override).

Enough excess reactivity must be invested in the control rods in order to allow for Xe override. A typical curve of reactivity as a function of time for a 100%-40%-100% power variation is given in Fig. 12.8. The value Ak = 0 corresponds to the equilibrium Xe concentration for 100% power.

One can see that there is a decrease in reactivity about 5 h after a power reduction and an increase about 3 h after return to full power. The minimum of Ak has to be compensated by extraction of control rods which are normally inserted in the reactor in the equilibrium condition. This results in a neutron loss and in an increase of the fuel-cycle costs. It is therefore necessary to weight the increased plant flexibility against the resulting additional costs.

The excess reactivity required for Xe override is dependent on the core composition, and fuel rating. A softer spectrum tends to increase the effective Xe absorption cross-section, while a high rating tends to increase the number of Xe atoms relative to the number of fuel atoms.

The problem of the Xe override has different aspects depending on the fuel- management scheme. In reactors with batch or discontinuous loading excess reactivity is present for a great part of the fuel life. In this case, if one can tolerate limitations in Xe override toward the end of the refuelling period, the additional excess reactivity can be reduced, using for Xe override the excess reactivity available at the beginning of a cycle for burn-up compensation (e. g. full override for some part of the fuel life only, and reduction down to 40% at any moment). In reactors with continuous reloading no excess reactivity is available in the equilibrium condition for burn-up compensation.

The control-rod requirement for Xe override may vary during the running-in period and it is necessary to check the Xe overriding capability as a function of time during this

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phase. Theoretically instead of extracting control rods it could be possible also to insert fissionable material in the reactor, but the movement of active and heat-producing parts gives rise to great difficulties. This solution is not considered practicable at present.