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Report No.
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Investigation of the interaction between heat transport systems during the natural circulation decay heat removal in FBR; Transient thermohydraulics in core and primary and secondary heat transport systems

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

The complicated thermal hydraulics which was not observed in forced circulation occurs in the core during natural circulation decay heat removal in Fast Breeder Reactors (FBRs). Especially, in a FBR which has the auxiliary cooling system of DRACS (Direct Reactor Auxiliary Cooling System) type with direct heat exchangers (DHXs) immersed in the reactor plenum, cold sodium provided by the DHXs penetrates into inter-wrapper gaps in the core. This phenomena called Inter-wrapper Flow (IWF) will influence the thermal hydraulics in the core. Further, thermal-interaction between the cooling systems makes the natural circulation flow rate in the primary loop change and will influence the thermal hydraulics in the core. The purpose of this study is to grasp the interaction between the cooling systems and its influence on the thermal hydraulics in the core during natural circulation decay heat removal in FBRs. Transient Sodium experiments which simulated transitions from forced to natural circulation in reactors were performed under several kinds of operating conditions of the IHX secondary system and of the auxiliary cooling system using PLANDTL-DHX test facility with DRACS and PRACS (Primary Reactor Auxiliary Cooling System). In the DRACS, the heat removed by natural circulation in the IHX secondary system influenced the natural circulation flow rate in the primary loop more than the operation conditions of DRACS. When natural circulation in the IHX secondary loop was stopped, the IHX outlet temperature rapidly increased and the natural circulation flow rate in the primary loop decreased. In addition, reverse flows were detected in the outer subassemblies. The temperature rise in the heated length of the center subassembly was reduced by the cooling effect of the inter-subassembly heat transfer to the reverse flow subassemblies and IWF. When the natural circulation in the IHX secondary loop was continued, the natural circulation flow rate of 1% level of rated ...

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