The fast reactor core consists of the core fuel assemblies, blanket fuel assem­blies for breeding, control rod assemblies for regulating power and reactivity, and radial shieldings. The heat generation of those components largely depends on the assembly type. Among the common assembly types, i. e. the core fuel assembly or blanket fuel assembly, the heat generation also depends on the loading position. In the thermal-hydraulic design of fast reactors, the coolant is adequately allocated among the assemblies in order to efficiently utilize the coolant led to the reactor and to satisfy the thermal design limits. As described in the list [1] of Sect. 4.1.7, the allowable thermal design limits consists of those for the fuel centerline temperature and for the fuel cladding temperature. The fuel centerline temperature is significantly influenced by the linear heat rate, while the fuel cladding temperature strongly depends on the coolant tempera­ture. Thus, the flow allocation is designed with consideration of the maximum cladding temperature design limit so that this temperature is almost equal among the core fuel assemblies and blanket fuel assemblies.

The high pressure plenum and low pressure plenum are provided inside the reactor vessel, and coolant is supplied from the high pressure plenum to the core fuel assemblies with high heat generation and from the low pressure plenum to the blanket fuel assemblies with low heat generation. Since the coolant flow direction of sodium cooled reactors is from the lower part to the upper part of the reactor vessel, the flow allocation device is installed in the bottom of the fuel

Fig. 4.9 Flow allocation of Monju [6]

assemblies. For example, the flow allocation device is installed in the connecting tubes which are between the upper and lower core support-plates. The entrance nozzles of the fuel assemblies are inserted into the connecting tubes. The designed flow rate is allocated by the combination of the flow allocation holes of the connecting tubes and the orifice diameter at the entrance nozzles.

An example of flow allocation design is shown in Fig. 4.9. In the flow allocation design, the necessary flow rates of the core and blanket fuel assemblies for satisfying the thermal design limits are calculated based on the evaluation of the cladding temperatures using the designed power distribution. The number of flow allocation regions is suppressed so that the flow allocation device is not complicated. To do that, the fuel assemblies are categorized as several groups. Within each group, the required flow rate is similar among the assemblies. Then, the flow rate for each group is determined. The flow allocation device is designed so that the pressure drops from the lower plenum to the upper plenum are equalized among all the flow paths with the flow allocation which is required for satisfying the thermal design limits. In this process, the pressure drop char­acteristics of each core components and the bypass of coolant flow are consid­ered. The pressure drops at the flow allocation device and the core components are calculated using the pressure loss coefficients based on the experiments such as the water flow tests associated with the core internals, separate tests for the flow allocation device, and the simulated fuel assembly tests using water.