The importance of heat transfer in a narrow gap has been pointed out in relation to the in-vessel retention of molten core under a severe accident condition of a light water reactor. In this study, heat transfer experiment has been carried out using internally heated annuli with a narrow gap and closed bottom. Effects of gap clearances (1.0, 2.0, 4.0 mm) on the cooling process were examined. The inner wall initially heated up to 800 C was cooled down by water supplied to the upper plenum, the transient rod temperature being measured. From the temperature history, transient boiling curves were obtained, which were compared with predictions by existing heat transfer correlations for nucleate boiling critical heat flux (CHF) and film boiling. In narrow gap channels water penetration to the gap was restricted by counterflowing vapor, which lead to a decrease of heat transfer coefficients.

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The research related to thermal management has been significantly inreased, especially for NPP safety. The use of passive cooling systems both during the accident and operation become reliable in the advanced reactor safety systems. Therefore it should be enhanced through experimental studies to investigate heat transfer phenomenon of the heat decay in transient cooling condition. An investigation has been performed through experiment using an NC-Queen apparatus constructed with rectangular loop. Piping were consisting of tubes of SS316L with diameter, length, and width of 3/4 inch, 2.7 m, and 0.5 m respectively. The height between heater and cooler was 1.4 m. The experiment used initial water temperature at 70 o C, 80 o C, and 90 o C in heater area. Transient temperature was used as experimental data to calculate water mass flow rate. The results showed that the temperature in heater area and cooler area were decreasing of about 90.6% and 95.7% at initial temperatur of 80 o C, and of about 71.1% and 59.4% at initial temperature of 70 o C. Those results were at higher initial temperature of 90 o C compared with the initial temperature of 90 o C. The average of water mass flow rate increased 81.03% from initial temperatur of 70 o C. It was shown that the averages of removed heat in every second from water due to heat loss and cooler, were 3.51 watts, 5.06 watts and 6.85 watts respectively. The initial condition of heat stored in the water was quite different, but to the cooler heat removal capacity and heat loss was almost the same.

The heat pipe is one of the cooling media which is potential to be developed for the passive cooling system for nuclear reactors. To enhance the performance of the heat pipe, nanofluids have been used as the working fluid for the heat pipe. This paper studies the characteristics of nanofluids as the working fluid of heat pipe with screen mesh wick, which was the mixture of nano-sized particles (Al 2 O 3 and TiO 2 ) with water as the base fluid. The nanoparticles have average diameter of 20 nm, made with 1% to 5% volume fraction. The heat pipe thermal performance was tested using heater with different heat load. The experimental result shows the use of 5% Al 2 O 3 -water improve the thermal performance by reducing the temperature at evaporator side as much as 23.7% and the use of TiO 2 -water reduce the temperature at evaporator side as much as 20.2% compared to the use of water. The use of nanofluid also decreases the thermal resistance of heat pipe. As the use of nanofluid improves thermal performance of heat pipe, it has a potential for applications along with heat pipes at nuclear reactors.

SIMULATION OF OPERATIONAL CONDITIONS OF FASSIP-02 NATURAL CIRCULATION COOLING SYSTEM EXPERIMENTAL LOOP. The natural circulation is considered in the design of emergency passive core cooling system in a nuclear power plant. In that context, FASSIP-02 experimental loop is designed in order to investigate the characteristics of the natural circulation in a closed loop. This paper simulates the various operational conditions of FASSIP-02. The objective is to obtain the best operational conditions of FASSIP-02 once it is built. For that purpose, the simulation is done with different condition of the heater power, the pipe insulation and the expansion tank's valve using RELAP5 code. The simulation time is up to 50,000 s. The simulation results show that until 50,000 s the steady state condition could not be achieved yet when the heater power greater than 10 kW. The pipe insulation causes faster increase of the water temperature inside the pipe and the induced water flow rate, as well. While, if the expansion tank's valve is closed during the operation, the pressure inside the loop would increase, faster when the heater power is higher and could reach the critical pressure. It is concluded that in all cases to avoid the saturation condition, the heater power should be maintained lower than 10 kW, especially when the loop pipe is insulated.