Axial Power Distribution Oscillation

For a constant core power, the difference in average moderator temperature between upper and lower core region is constant and independent of axial power distribution. Hence, axial power distribution oscillation cannot be expected to be suppressed by the moderator temperature coefficient and the Doppler effect will be expected instead. Since the axial power distri­bution becomes flatter with burnup from an initial cosine distribution at BOC, the stability of the axial power distribution oscillation becomes lower and it may cause a little divergence near EOC. In this case, the axial power distribution can be controlled by the CAOC operation mentioned before and the axial power distribution oscillation can be sufficiently suppressed. The axial power distribution oscillation induced by axial Xe oscillation can also be suppressed as shown in Fig. 3.57. The axial power distribution can be continuously monitored by upper and lower separate ex-core neutron detectors and the oscillation can be detected and suppressed.