Summary Lately, natural circulation has become an exciting topic because it has been proposed and applied in some advanced nuclear reactors as a passive safety system. The aim of the present study is to examine the effect of the loop geometry on the natural circulation based cooling system of a given power source. The experimental approach was achieved by the construction of two natural circulation loops with vertical heater and vertical cooler. Both circulation loops were built with a same vertical length of 100 cm and different horizontal lengths, each of 50 and 100 cm, respectively. The heater was realized with a stainless pipe wrapped by a Nichrome wire, whereas the cooling system was designed and built as a pipe in 30 cm long square tube. Thus, the cooling water will circulate around the pipe loop inside the rectangular cooling chamber. The temperature data acquisition was achieved via an Arduino‐based system, controlling four K‐type thermocouple sensors. The numerical simulation part in the present work, was carried out by the COMSOL Multiphysics software, using the dedicated heat transfer and fluid dynamic module, namely the computational fluid dynamic module. The experimental data were considered to parameterize the COMSOL input model. The experimental results showed a clear difference between two passive cooling loops according to the horizontal length. The doublings of the horizontal and vertical lengths of the loop have a direct effect on the fluid flow rate.

The passive nuclear reactor cooling system has become a fascinating topic, especially since the nuclear incident in Fukushima Daiichi, Japan. This study proposed to review a natural circulation-based passive cooling system in the geometry aspects of the loop. The numerical review was carried out using COMSOL Multiphysics software to study the geometry effect on the loop’s fluid characteristics. Moreover, related research data to the study of the effect of fluid flow rate on one of the advanced nuclear reactors, namely the Molten Salt Reactor and the effect of loop geometry experimentally, are presented in this study for enrichment purposes. This study clearly showed that geometry aspects need to be considered in designing a natural circulation-based passive cooling system of a nuclear reactor.

The natural circulation loop is one of the design concepts of a cooling system in new advanced reactors that has attracted many researchers to develop it. This study aimed to perceive the effect of horizontal width variation on the thermal behavior of a single-phase Natural Circulation Loop (NCL). NCL apparatus with a vertical heater and a vertical cooler was designed for experimental study. The height of the loop was 100 cm while the width of the loop was varied at 50 cm and 100 cm. The heater was designed using Nichrome wire on the outside of the stainless pipe while the cooler was designed using pipe-in-pipe with water flowing through the annulus. Arduino microcontroller and K-type thermocouple sensors were used in temperature data acquisition. XAMPP software was used in data recording. The results of this study indicated that the loop of a 100 cm width has a difference in the temperature of the fluid coming out of the heater and entering the heater that reaches 156,0% higher than the loop of a 50 cm width at the same input voltage. This study is supposed to be one of the references for a single-phase natural circulation loop.

The natural circulation systems have been widely applied to various technologies, for example, safety systems in molten salt reactors. Hence the instability of the natural circulation system became an exciting topic to study. This study aimed to find out the effect of horizontal width variation on the thermal behavior of a single-phase natural circulation loop (NCL) with vertical heater and vertical cooler. Three loops with horizontal widths of 50 cm, 100 cm, and 150 cm, respectively, were designed for experimental studies. Meanwhile, the vertical height of all three loops was 100 cm. The cooler was installed vertically on the upper arm of the loop to cool the fluid. The heater was installed vertically on the forearm on the other side of the loop to heat the fluid. K-type thermocouples integrated with the data acquisition system were used to measure the temperature distribution in several zones of the loop directly. The conclusion obtained in this study was a single-phase natural circulation loop with vertical heater and vertical cooler did not indicate a reversal of the direction of heat transfer at each operating power and geometry configuration. This study is expected to be one of the references to the design of a single-phase natural circulation loop as an advanced nuclear reactor passive safety system.