For pebble bed reactors, the problem exists that an in-core instrumentation is not possible. As a flux mapping method, we have developed the three-dimensional code ZELT-3D, which reconstructs the flux distribution in the core using the detector signals of the side reflector instrumentation as input. The results of a calculation utilizing this code and its associated theory for perturbation of flux distributions by absorber rods show that a three-dimensional flux mapping of perturbed fluxes is possible and positions of absorber rod tips can be detected well. We think that this flux mapping method can serve to locate xenon oscillations, misloaded core areas, and broken parts of absorber rods.