Research contents

3.2.1 37-rod bundles test

An Innovative Water Reactor for Flexible Fuel Cycle (FLWR) aims at the achievement of a high conversion ratio of plutonium mixed oxide (MOX) fuel, based on well-tested BWR technology. Since the FLWR makes plutonium multi-recycling possible, the reactor fills the need for effective utilization of the uranium resources and a long-term energy supply. Fig. 2.1.1 shows a comparison of the specifications of rod bundles between a boiling water reactor (BWR) and the FLWR.

The FLWR core is made by the tight-lattice bundle structure, and it is operated under low mass velocity and high void fraction conditions. These conditions are difficult for core cooling, and the FLWR thermal-hydraulic characteristics under such conditions are not known well. The confirmation of thermal-hydraulic characteristics is, therefore, one of the most important R&D requirements for the FLWR design.

We investigated the thermal-hydraulic performance of the FLWR core using a test section with 37-rod bundles under high pressure conditions simulating the FLWR operating conditions. Fig. 2.1.2 shows a photograph of the test section. We measured critical power and pressure drop under steady state and transient conditions in the tight-lattice bundles.

Fig. 2.1.3 shows a typical result of the thermal margin under the FLWR operating condition. The result obtains that the FLWR has enough thermal margins for cooling of the core.


Fig. 2.1.1　 Comparison of the specifications of two rod bundles

An FLWR core has a triangular tight-lattice configuration to reduce the moderation of neutrons. Since an amount of coolant through the core is quite smaller than that through a conventional BWR core, the confirmation of thermal-hydraulic feasibility is one of the most important R&D items for the FLWR.


Fig. 2.1.2　 Appearance of a test section which performed large-scale experiment

We investigated the thermal-hydraulic characteristics of the FLWR core using a test section with 37-rod bundles under high pressure conditions simulating the FLWR operating conditions.


Fig. 2.1.3　 Experimental results on thermal margin under the FLWR operating condition

We demonstrated that there are enough thermal margins to cool the FLWR core.