The neutron flux is not constant throughout the re­actor but decreases from the reactor centre outwards. As a result, the rate of irradiation of the fuel is not the same in all parts of the reactor.

To obtain the reactivity of the whole reactor, the average channel reactivities must be determined after first weighting the contribution from each channel by the square of the mean flux in that channel. A typical curve for the build up of reactivity in a virgin core is shown in Fig 3.36. It is assumed that refuelling is beina carried out continuously and at the rate re­quired by the fuel cde. Initially, the reactivity of the reactor follows a oune similar to that for an individual channel, i. e., after a small drop in the reactivitv (Samarium dip), it then rises to a maximum which is numerically lower in value than for a chan­nel. This is because some of the fuel in the core will have been replaced by new fuel which depresses the overall reactivity level. Following the peak, the re­activity falls and levels off at a value known as the equilibrium reactivity. The numerical value of this reactivity is approximately half that equivalent to the tareet irradiation at which the fuel is discharged. As the target irradiation is increased the numerical value of the equilibrium reactivity is reduced. There is therefore a limit to the target irradiation beyond which there will be insufficient reactivity in the core to sustain a chain reaction. The limiting channel aver­age target irradiation is of the order of 6000-6500 MVd/t, so that for reactivity reasons the potential of 14 000 MWd/t is not achievable.