As stated above, the neutron flux is not constant throughout the reactor. The flux distribution in a cylin­drical reactor w’ithout a reflector layer is shown in Fig 3.37. It can be seen that the decrease in flux level from the centre to the reactor edge is considerable. Since the reactor reactivity is a flux squared mean value, power production from such a reactor is non-uniform over the reactor, For this type of reactor the ratio of mean flux to peak flux is 0.275. This ratio can be improved by the incorporation of a neutron reflector on all sides of the reactor, this being normal practice

■’ 1 »nr ■ v■ гг. rciU’or ге;кчііі> build-up

for power reactors. A further improvement is obtained by the placement of extra absorption in the form of steel bars or absorbers in the centre of the reactor. This depresses the flux at the expense of an increase in critical size. The overall effect is illustrated in Fig 3.37 and the procedure is known as flux flattening. The resultant flux shape is characteristic of power reactors and for obvious reasons provides regions known as the flattened zone and the unflattened zone.

Returning to the channel reactivity curve, it will be appreciated that at the start of reactor life, the overall reactivity is low so that only a small quantity of ab — ~ sorber may be loaded to the centre. The flattened zone radius will be approximately half that obtainable at equilibrium, although within a few months of op­eration the full flattened radius is obtained when the overall reactivity first reaches that equal to the equi­librium reactivity (see Fig 3.36).

As irradiation proceeds the reactor reactivity builds up and additional absorbers will have to be loaded to further flatten the flux. This enables the control rods, which will have been gradually inserted to bal­ance the reactivity build-up to be restored to their normal operation position. At the same time, the Rattened radius will have been increased towards the equilibrium flattened radius. Since it is only the chan­nels in the Rattened region that operate at maximum power, the power output of the reactor is a function of the Rattened radius and will therefore be a mini­mum at start-up. The reactor channel Rows are ad­justed initially to match the larger Rattened radius Rux distribution at equilibrium and channels in the unflattened region will be overcooled. They will be running below design temperature, reducing bulk out­let gas temperature and causing a slight loss in power

because of reduced thermal efficiency. This is a penal­ty which must be accepted in a design where gag settings are not readily adjustable during operation. Overcooiing of the unflattened zone will persist, al­though decreasing in magnitude until the ‘guarantee condition’ is reached. This is the condition of the build-up curse where the reactivity of the core first reaches the equilibrium value and consequently the flattened radius becomes equal to the equilibrium flattened radius. At this time it is possible to test the guarantee of full design output against design condi­tions. The loading/unloading of the absorber must be regarded as an essential component of the fuel cycle since it contributes the means by which the utilisation of fuel and reactor power output is maximised.