[en] Resin in the water treatment system of the Merah-Putih Gamma Irradiator (IGMP) is used as ion exchange.The pool containing demineralized water with low conductivity, less than 10 μS, to prevent metal components installed at the bottom part of the irradiator from corrosion. However, evaporation mainly due to heat from radiation source causes water level is reduced. Raw water from other resources must be added to maintain a demineralized water level. Theoretically, the conductivity depends on the contact area and contact time between the raw water with resin particles. Calculation from such a situation is difficult to be realized. Therefore, the conductivity and pH of the raw water is measured after being the raw water is reacted with the resin particles in the water treatment system. The exchange process is influenced by several factors, including the water flow rate. The purpose of this research is to find out the effect of water flow rate against conductivity so that the right water flow rate caused the ion exchange process runs optimally can be determined. The water treatment system is operated in filling mode. In the filling process, the water filling flow rate is varied in six, 100 liters/hour, 200 liters/hour, 300 liters/hour, 400 liters/hour, 500 liters/hour, and 600 liters/hour, respectively. Each water filling flow rate was observed for conductivity and water pH after passing through cation resin and anion resin. Data is collected every 3 minutes. Based on the data sheet, the optimal water flow rate through the cation resin is 120 m³/hour and the optimal water flow through the anion resin is 60 m³/hour. This research shows if the amount of water flow rate pass resin is closer to the value recommended by the data sheet, it can cause the ion exchange process runs optimally and the expected low conductivity is achieved when the pump was operated at the speed 600 liters/hour which produces 64.3 m³/hour at the point after passing through the resin tank. However, the pump is recommended to be operated at a maximum speed of 300 liters/hour, otherwise the PVC pipe break or burst. To overcome this situation, design engineering is needed, so that the pump can be operated at the recommended speed. (author)