Power quality of any electrical system plays important role as it describes its performance. Power quality monitoring and investigation are necessary to maintain perfect and safe operation of sensitive equipment especially for nuclear facilities. In this work, a case study about harmonic analysis of an electrical power system at a Nuclear Research Reactor (NRR) has been done to evaluate the current state from the PQ point of view. This is a part of a research project where the purpose of this project is to improve the nuclear reactor electrical system performance and efficiency. This improvement will be done by renewing some of its components and using energy saving loads. . Firstly, a study is given for different types of linear and nonlinear loads in NRR and their impact on the PQ of the electrical system. Then, the paper investigates the practical measurement of the nuclear reactor electrical system parameters; and then building a model of the electrical system using Electrical Transient Analysis Program (ETAP) software to be used for simulations at different operating scenarios. The simulation results are compared with the measurements for credibility.