Experimental Study on Thermal Stratification in a Reactor Vessel of Innovative Sodium-Cooled Fast Reactor —Mitigation Approach of Temperature Gradient across Stratification Interface—

概要

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An innovative sodium-cooled fast reactor has been investigated as part of the fast reactor cycle technology development project (FaCT). Thermal stratification after a scram is one of the main thermal loads of a reactor vessel (R/V). R/V has an upper inner structure (UIS), which consists of perforated horizontal plates and control rod guide tubes, and has a slit in the radial direction for fuel handling. The UIS slit causes an asymmetric flow pattern in R/V. A water experiment using a 1/10-scale model was carried out. A steep temperature gradient across the stratification interface was observed at the neighborhood of the UIS slit in the experiment. This means that the jet through the UIS slit entrains the bottom of the stratification interface. In order to mitigate the temperature gradient across the stratification interface, the height of a plug, which was installed in the upper plenum to infill a hole at the dipped plates where a fuel handling machine was inserted during a fuel exchange operation, was changed as the parameter in the experiment. The experimental result shows that the temperature gradient near the R/V wall in the case of the lower plug position was 21% smaller than that in the case of the higher plug position. This reduction of the temperature gradient was sufficient to maintain the structural integrity of the R/V wall against the thermal stratification.