Experimental Study of Natural Circulation in Kartini Reactor Core: Effect of Primary Flow Rate and Power

The Kartini research reactor core cooling uses natural convection. Primary cooling system is used to cool coolant by circulating coolant in reactor tank to a heat exchanger dissipating the heat to the secondary cooling system and back to the reactor tank. The coolant is injected to the reactor tank through a diffuser located above the core in order to dilute the N-16 radionuclide. However, it turns out that the primary coolant flow affected the natural circulation in the reactor core . This study was conducted to study the effect of primary coolant flow on natural circulation in the reactor core at various reactor power, as well as to determine the characteristics of natural circulation in the core. The experimental study was focused in the hot channel (ring B). The reactor was operated with a power of 50 kW, 75 kW, and 100 kW. At each power, the flow rate of the primary coolant changed from 100 to 0 flow rate with decrement of 20. The instrumentation to measure coolant temperature were used to provide the axial coolant temperature distribution in the hot channel. From this temperature distribution, the flow pattern in the channel was predicted. An analytical sub-channel analysis was performed to calculate the natural circulation flow rate and DNBR. The experimental results showed that in all cases the primary flow influenced the flow in the core; the natural circulation was seemed balanced by forced circulation coming from primary flow. It was also observed that at low power, the temperature of the cooling fluid tends to be relatively stable compared to high power. When the primary flow rate is 0, natural convection occurs in the reactor core with the velocity (u) and DNBR of 9.22 cm/s and 9.02, 9.24 cm/s and 6.02, and 11.55 cm/s and 4.51 at the power of 50 kW, 75 kW and 100 kW, respectively. The values of DNBR are still larger than safety requirement. It is concluded that the flow in the core of Kartini reactor is mixed convection due to primary coolant forced flow and buoyancy of natural circulation. During the condition of fully natural circulation, the thermal-hydraulic safety limit is still kept. © 2024 American Institute of Physics Inc.. All rights reserved.