Investigation of interaction between heat transport systems during the natural circulation decay heat removal in FBRs; Influence of decay heat removal system type and the Secondary heat transport system

Hayashi, Kenji; Momoi, K.; Nishimura, Motohiko; Kamide, Hideki 

Steady state sodium experiments were performed to investigate interactions between the heat transport systems, i.c., the primary system, the secondary system, and the decay heat removal system, during the natural circulation decay heat removal in FBRs. The PLANDTL-DHX test rig was used for the experiments. The core model has seven subassemblies; the center assembly simulates pin bundle geometry of a core fuel subassembly in a large scale FBR and consists of 37 pins, six outer subassemblies consists of 7 pins. As the decay heat removal system, Dirtct Reactor Auxiliary Cooling System (DRACS) and Primary Reactor Auxiliary Cooling System (PRACS) can be selected. Experiments were carried out under natural circulation conditions in the primary loop and force convection conditions in the decay heat removal system. In cases using DRACS, natural circulation flow rate in the primary loop was smaller by 20% than that in cases using PRACS due to the low temperature in the upper plenum and also in the upper non-heated section of the core. When natural circulation was allowed in the secondary heat transport system, the natural circulation flow rate in the primary system increased in spite of the operation of DRACS. In cases using DRACS, inter-subassembly flow redistribution occurred; the center subassembly had larger flow rate than those in outer subassemblies due to the low natural circulation head in the outer subassemblies which were cooled by the inter-wrapper flow (IWF). The highest temperature in the core was reduced by IWF via not only the direct cooling effect but also the inter-subassembly flow redistribution. Temperature fluctuations around the PRACS cooling coil installed in the IHX were registered under the natural circulation conditions in the primary system. The amplitude of fluctuation was less than 20C and small on the points of structural integrity.