The accumulation of corrosive and radioactive wastes in the primary system, including nuclear fuel assembly, significantly increases workers’ exposure to radiation. Zinc injection from 5 to 40 ppb into the Reactor Coolant System (RCS) of Pressurized Water Reactors (PWRs) has been known as an effective method to decrease the radiation fields and Primary Water Stress Corrosion Cracking (PWSCC). Zinc injection affects both corrosion product concentrations and characteristics of the deposited crud on oxide layers, because zinc is incorporated into the oxide films by displacing nickel, cobalt, and iron in primary systems. Radiation fields and corrosion might be mitigated as radioactive products, such as Co, which are removed by zinc injection. However, the zinc injection effects on fuel assembly in Nuclear Power Plants (NPPs) have not been much reported yet, even though some lab tests were carried out in USA and France. In this paper, we studied effects of zinc injection on the fuel assemblies in the Ulchin 1 NPP. The chemical and radiation analysis of radioactive corrosion products was performed to evaluate zinc injection effects on the fuel assembly in the Ulchin 1 NPP. Gamma spectroscopy was used to analyze crud samples for radioisotope contents. The Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) was used to analyze crud samples for elemental contents. The concentration of radioisotope Co-58 was decreased after zinc injection to 1/22 times that before the zinc injection. 1% - 2% wt% of zinc was incorporated through the substitution of Ni in the crud oxide layer. The Ni/Fe ratio was decreased to 0.69 from 1.12 after the injection, due to the Ni substitution by zinc. It was found that NiO and NiFe₂O₄ were converted to ZnO and ZnFe₂O₄, respectively. In conclusion, zinc injecttion was found to be an essential method to reduce the amount of radioactive Co-58 in the fuel assemblies of primary systems in NPPs.