Improving Flux Tilt Control While Adjuster Control Rods are Removed from the Pickering NGS A Reactor

Abstract

Removal of adjuster control rods from the Pickering NGS A reactor core results in flux peaking and higher fuel powers in the centre region of the core. The present flux tilt control algorithm increases the level of the light water neutron absorber in the centre liquid zone controllers in an attempt to nullify flux peaking. However, due to the limited depth of the neutron absorption capability of the liquid zone controllers, the preremoval zone powers can not be achieved. This results in saturation of liquid zone controller levels and reduced flux tilt control. Recent operating experience as shown that in certain situations the reduced flux tilt control capability with adjusters removed results in uncorrected side to side azimuthal flux tilts. To increase tilt control in these situations an improved flux tilt control algorithm has been developed which switches the zone power flux tilt control targets to more realistic obtainable values as adjusters are removed. In this paper the computer simulations and analysis performed to develop and test the improved flux tilt algorithm is described. Also the improved performance of the new algorithm in one event will be demonstrated.