Many space-dependent dynamics codes<8,9) have a version for slow transient calcula­tions in which Xe and Sm equations are treated and delayed neutrons are neglected.

The compensation of the changes in Xe poisoning requires a movement of the control rods. If the rods used for this purpose are not uniformly distributed over the core, the resulting changes in flux distribution can only be properly treated with space-dependent codes. A typical example is given by the pebble-bed THTR reactor where a possible strategy of operation foresees that only reflector control rods are used for Xe override. Even in cases in which the flux distortion due to rod movement is kept to a minimum, there are discrepancies between zero and space-dependent calculations because the maximum change in Xe concentration occurs in the regions of maximum flux and hence of maximum importance.

An accurate value of the excess reactivity needed for Xe override can in many cases only be obtained with a two-dimensional calculation.19’