Investigation of Flow and Temperature Distributions Around Neutron Detector Housing in Control Plug
R. Gajapathya, K. Velusamy, P. Selvaraj, P. Chellapandi and S. C. Chetal
Thermal Hydraulics Section, Nuclear Engineering Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, India.
agaja@igcar.gov.in
ABSTRACT
In Liquid metal cooled Fast Breeder Reactor (LMFBR), the Control Plug (CP) which houses the control rods and Neutron Detectors (NDs) in shroud tubes is kept just above the core. Different streams of hot and cold sodium jets issuing out of the core subassemblies mix outside the shroud tubes and flows over them. A few streams exactly below the shroud tubes enter into them at their bottom. The CP shell and the shroud tubes are provided with perforations to allow sodium flow from them into the hot pool. A 60 deg sector of the CP comprising two shroud tubes, two NDs, core subassemblies below this sector and the hot pool has been modeled in 3-Dimensions using the Computational Flow Dynamics (CFD) code STAR-CD. Using this computational flow model, the steady-state flow and temperature distribution around the detector shroud tubes has been found out and the thermal loads experienced by detector have been estimated. It is seen that significant amount of sodium enters the shroud tubes at their bottom and flows up through the annular space which is 14% of the total flow entering into the CP from the core. These flow rates are in very good agreement with the experimentally measured flow rates. There is good mixing of hot and cold sodium streams outside and inside the shroud tubes and the axial temperature of the shroud tube averaged over its circumference at each elevation changes marginally. For all practical purposes, the axial temperature can be taken as constant. Due to the difference in sodium temperatures of subassemblies which feed these two detectors, it is seen that one of the detectors is having higher temperature than the other. Hence, the highest mean sodium temperature experienced by the detector has been recommended for the thermal design testing of the same. The details of the thermal hydraulic analysis carried out and interesting results obtained are presented in this paper.
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