Investigation of in-vessel thermal stratification characteristics for large scale liquid sodium cooled fast breeder reactors with general purpose multi-dimensional code AQUA

Hendro Tjahijono*; not registered

Thermal stratification phenomena is an important problem in liquid metal fast breeder reactor (LMFBR) vessel where it can induce thermal fatigue failure of structures. The phenomena is observed in the upper plenum zone under reactor scram conditions. Investigation on characteristics of this phenomena is performed using a general purpose multi-dimensional code AQUA. There are 3 sizes of FBR vessel having been investigated such as: large size with 9.6 m of diameter, medium size with 8.16 m of diameter and small size with 6.72 m of diameter. Two types of calculation have been applied in this investigation such as: steady state calculation with QUICK / FRAM and transient calculation with QUICK / FRAM and first order upwind. In this calculation, fully implicit method has been used for the beginning followed by semi-implicit method until steady state condition and then transient run during 1000 seconds simulating a scram condition from full power operation condion. For large size of FBR, three dimensional (3D) model and two dimensional (2D) model have been performed. Due to the very long CPU time in 3D model, for medium and small size, only 2D model has been applied. Thermal stratification is investigated after reactor scram in the both model. The average rising speed of the thermal stratification calculated using least square method in the period between 100s to 1000s after scram are 3.96 m/hr in small vessel, 8.28 m/hr in medium vessel, 6.84 m/hr in large vessel 2D model and 7.56 m/hr in large vessel 3D model calculated after 200s. The maximum of the maximum axial temperature gradients in all cases during transient are around : 800 C/m in small vesse1, 700C/m in medium vessel, 1000 C/m in large vessel 2D model and 400C/m in large vessel 3D model.