Thermal stratification tests in Monju upper plenum, 1; Temperature distributions under normal and scram conditions with 40% power operation

Doi, Yoshihiro; Muramatsu, Toshiharu; Kunogi, Kosuke

Thermal stratification phenomena are observed in an upper plenum of liquid metal fast breeder reactors (LMFBRs) under reactor scram conditions, which give rise to thermal stress on structural components. Therefore it is important to evaluate chracteristics of phenomena in the design of the internal structure in an LMFBR plenum. To evaluate the thermal stratification characteristics of the prototype fast breeder reactor, MONJU, axial temperature distributions were measured with a thermocouple tree installed into the upper plenum for normal and scram conditions with 40% power operation. The axial temperature distribution for the normal operation showed thermal stratification phenomena that temperatures were about 410C under the level of the fuel subassembly top, about 480C around the level of the flow holes and about 490C above the level of the upper flow holes. Temperature fluctuations were observed at the levels of the lower flow and upper flow holes. RMS (Route Means Square) of functuation at the lower and upper flow holes were 1.6 C and 1.5C, respectively. Based on the temperature histories of the upper plenum during scram conditions, temperature decreasing rate, temperature gradient and rising rate of stratification interface were evaluated. The temperature decreasing rates were about 5.0 C/sec at the core outlet, 2.0 C/sec around the level of flow holes and from 0.3 to 0.4C/sec at the top of the inner barrel. The temperature gradients were about 160 C/m, 90C/m, 45C/m and 170 C/m at the time of 120 sec, 180 sec, 600 sec and 4800 sec from the reactor scram, respectively. The rising rate of stratification interface is about 1.0 m/h for the period from 120 sec to 600 sec with the onset of reactor scram. On the other hand, the rising rate is about 0.6 m/h after 600 sec from the scram. The rising rate at the top of the inner barrel is 0.2 m/h ...