Boiling two-phase flow in a simulated fuel assembly excited by oscillation acceleration was performed by the improved ACE-3D in order to investigate how the three-dimensional behavior of boiling two-phase flow in a fuel assembly under oscillation conditions is evaluated by the improved ACE-3D.

1.3 Computational condition

In this analysis, a 7 x 7 fuel assembly in a current BWR core is simulated, as shown in Fig. 9. Fuel rod diameter is 10.8 mm; the narrowest gap between fuel rods is 4.4 mm, and the axial heat length is 3.66 m.

Four subchannels surrounded by nine fuel rods without channel boxes are adopted as the computational domain shown in Fig. 9; this is the smallest domain that can describe the
three-dimensional behavior of boiling two-phase flow. This computational domain was determined to reflect the basic thermal-hydraulic characteristics in fuel assemblies under earthquake conditions.

In this domain, single-phase water flows in from the bottom of the channel with a mass velocity of 1673 kg/m2s and inlet temperature of 549.15 K. At the exit of the computational domain, pressure was fixed at 7.1 MPa. The mass velocity, inlet temperature, and exit pressure reflect the operating conditions in a current BWR core. The core thermal power is 351.9 W. The axial power distribution of the fuel-rod surfaces is shown in Fig. 9 and it simulates the power distribution in a current BWR core.

Figure 10 shows the boundary conditions and the computational block divisions. Here, the non-slip condition is set for each fuel-rod surface, and the slip condition is set for each symmetric boundary. In this analysis, the computational domain was divided into 9 blocks. The computational grids in each block have 10 and 256 grids in the radial and axial directions, respectively. The number of grids in the peripheral direction is as follows: 30 in block 1, block 3, block 7, and block 9; 60 in block 2, block 4, block 6, and block 8; and 120 in block 5.

In this study, the boiling two-phase flow analysis was performed under steady-state conditions to obtain a steady boiling two-phase flow. Subsequently, oscillation acceleration was applied. The time when the oscillation acceleration was applied is regarded as t = 0 s.

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Fig. 9. Computational domain and axial power ratio

Fig. 11. Time variation in oscillation acceleration

In this analysis, in-phase sine wave acceleration was applied in the X and Y directions as shown by the black arrow in Fig. 10. The magnitude and oscillation period of the oscillation acceleration in the X and Y directions were 400 Gal and 0.2 s, respectively, as shown in Fig. 11; these values are based on actual earthquake data measured in the Kashiwazaki-Kariwa nuclear power plant. The computable physical time in this analysis after applying the oscillation acceleration was 1 s based on the results described in section 2.