Experimental study on thermal stratification phenomena in compact reactor vessel of sodium cooled fast reactor; Evaluation on stratification interface behavior under natural circulation condition

Hagiwara, Hiroyuki; Kimura, Nobuyuki*; Onojima, Takamitsu; Nagasawa, Kazuyoshi*; Kamide, Hideki ; Tanaka, Masaaki  

Thermal stratification in the upper plenum is one of the most important issues of a reactor vessel in sodium cooled fast reactor. The steep temperature gradient across the stratification interface may cause the thermal load against the reactor vessel wall. In this study, the water experiment was carried out using the 1/11 scale upper plenum model of the Japan sodium-cooled fast reactor (JSFR) in order to evaluate the thermal stratification under the natural circulation condition and a direct heat exchanger (DHX) operation condition. The temperature gradient under the natural circulation condition was approximately 1/3 times smaller than that under the forced circulation condition. In the DHX operation case, the steep temperature gradient occurred in the lower region of upper plenum due to the cold fluid from the outlet of DHX.