[en] Calculation of reactivity changes caused by movement of reflector elements (control characteristic) is very complicated for compact reactors because of the large neutron leakage of the core and the neutron flow effects with reflector segments capable of eing deflected. These problems can be treated three-dimensionally by means of the Monte Carlo method or two-dimensionally with certain approximative assumptions by Ssub(N) or diffusion codes with boundary conditions for a cavity. The reactivity changes caused by reflector movements partly are so small (e.g. with fine control elements), that with independent Monte Carlo simulation - even if weighting techniques are applied - irresponsibly long computing times would be necessary. That is why for these problems a modified Monte Carlo techniqueis applied which is based on the principle of correlated simulations for the neutron traveling distances and by which very small as well as larger reactivity changes can be determined with relatively small effort in computing time. (orig./RW)