Mixing of sodium flows above the core of fast reactors

Abstract

The subject of this paper involves monitoring the temperature of the heat transfer agent above the core and fluctuations in the temperature of the sodium, affecting the strength of the constructions under steady-state reactor operating conditions. Sources of temperature differences may be local superheating with deformations of the fuel element lattices, blocking of parts of the cross sections of the fuel assemblies, and other such factors. Some information about this is presented in [1][2][3].The experimental assembly consisted of seven models of fuel assemblies (Fig. 1). Nineteen fuel element simulators separated by wire coils were mounted in each model. Electrical heating of the simulators was provided only in the central model. The thermocouple probe (Fig. 2) consisted of 30 chromel-alumel thermocouples in stainless-steel capillaries of diameter 0.8 mm. The probe could be placed along the height from the middle of the caps (h = 0) to 200 mm. The placement of the thermocouples is shown in Fig. la. There were also thermocouples in the caps of the fuel assemblies in the cross section of the lower edges of their windows, at the outlet from the model and at the inlet to the "cold" fuel assemblies.There were two independent heat-transfer-agent feeds: into the central model and simultaneously into the six lateral models. We considered the case of superheating of the heat transfer agent in the central model, accomplished either by feeding the heat transfer agent at a higher temperature or by heating it with the electric heaters of the fuel element simulators. The basic parameters for the two series of experiments are given in Table 1. When feeding hotter heat-transfer agent, the possibility of generating additional thermal noise from the electric heaters is eliminated.The average temperature was measured by means of a scanning system. The temperature pulsations were measured and statistically treated using a system including an amplifier circuit, a magnetograph, and a computer. We entered 8100 numbers for each statistical treatment.The temperature distribution over the radius of the model, averaged over the thermocouple readings, is shown in Fig. 3. From this figure it follows that the "hot spot" from the central fuel assembly spreads out with an increase in height. There is some anomaly for low velocity of the "hot" flow (see Fig. 3a, b, c): at some distances from the end of the cap of the fuel assembly, a temperature minimum appears. The effect is possibly connected with cold sodium from adjacent cells entering this region.The flow of heat with an increase in distance from the caps of the models occurs differently for the central sodium flow and the flows from the lateral windows (at the radius rl) (Fig. 4). At a height of 200 mm, the temperature difference between the central flow (t 0) and the cold sodium tr4 (at the radius r4) is only 20% of the initial temperature difference (t h -tc) (the indices h, c mean hot, cold at the inlet to the model). Within the cap, the temperature of this flow decreases by 10%:tO-tr...