By using synchrotron radiation phase sensitive radiography we have examined the reactions of screw superdislocations or micropipes in silicon carbide single crystals: bundling, twisting, and splitting. To understand the nature of these reactions, a model of micropipe motion during crystal growth has been proposed. Based on this model, a computer simulation of the evolution of a random micropipe ensemble has been performed. The simulation demonstrates that the coalescence of micropipes with opposite-sign Burgers vectors may be realized in two ways: (i) their straightforward movement towards each other or (ii) their revolution about one another. Twisted dislocation dipoles arise when two micropipes are under strong influence of the stress fields from dense groups of other micropipes. The transformation of dipoles into semiloops as well as the annihilation of micropipes result in diminishing of their average density. (C) 2003 American Institute of Physics.