The concept of tuned mass dampers is used to reduce the vibrations of buildings due of dynamic loads. Basic idea of this method is to attach a pendulum to the construction which is designed in a way to minimize the dynamic deformations. Goal of this thesis is to compare two possible designs of a reactor construction in Croatia. Within a first approach the reactor is placed directly on the supporting steel structure. Alternatively, the reactor is suspended at a higher construction level by means of hangers. Advantage of the second solution is, that one can use the reactor directly as pendulum in terms of the concept of tuned mass dampers. It is state of the art to use a pendulum with a small mass, compared to the building, to reduce the vibrations. However, by using the reactor as a pendulum to minimize the deformations, the mass of the pendulum is larger than the modal mass of the spporting structure. This thesis illustrates if it is possible to achieve the same positive effect using the heavy reactor as tuned mass damper compared to a pendulum of small mass.