[en] Measurements of worths of banks control rods by either the rod drop or the pulsed neutron methods can be seriously affected by the presence of spatial harmonics. In the work described in this paper, the method of modal analysis for determining transient behaviour is described and the results obtained by the method are illustrated in terms of experimental rod drop and pulsed source measurements. The experimental results were obtained in a heterogeneous core at the startup of the Prototype SGHWR at Winfrith and include an asymmetric rod pattern under both hot and cold conditions. It is shown that reasonable accuracy in the predicted flux transients and assessed reactivities can be obtained by computing the fundamental mode and its first seven harmonics in the horizontal plane for these experiments. Some comparisons are also made between the results of the modal method and those obtained from a more conventional finite difference solution has the attraction of providing a deeper insight into the basic characteristics of the transient behaviour, which can be useful in planning the experimental procedure. On the other hand the conventional finite difference method is probably more directly suitable for use in safety studies once the fit to experimental rod worths has been completed. (author)