**2.4 Investigation of the effect of oscillation period on boiling two-phase flow behavior**

The computational domain and thermal hydraulic conditions are the same as those for boiling two-phase flow in the parallel-plate channel, as described in the preceding section. The oscillation acceleration was applied at *t* = 0 s, after steady boiling flow was obtained.

Nine cases of oscillation acceleration, as shown in Table 1, were applied in order to investigate the influence of the oscillation period of the oscillation acceleration upon the boiling two-phase flow behavior. As shown in the preceding section, the influence of the horizontal oscillation acceleration upon boiling flow was greater than the influence of the vertical oscillation acceleration. Therefore, only the horizontal oscillation acceleration was investigated in these analyses. The minimum oscillation period of 0.005 s, as listed in Table 1, is equal to half of the minimum time interval of structural analysis in a reactor. The maximum oscillation period of 1.2 s is almost equal to the computable physical time of about 1 s. In all cases, magnitude of the oscillation acceleration was set to 400 Gal. Case G in Table 1 is the same as the horizontal oscillation acceleration case shown in section 2.3.

Development of an Analytical Method on Water-Vapor Boiling Two-Phase

Fig. 8. Standard deviation distribution of void fraction distribution

Therefore, the influence of the oscillation acceleration is small in this range.

extracting an earthquake motion of about 1 s at any time during an earthquake.

**assembly excited by oscillation acceleration** 

evaluated by the improved ACE-3D.

**3.1 Computational condition** 

heat length is 3.66 m.

**3. Application to the boiling two-phase flow analysis in a simulated fuel** 

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

In this analysis, a 7 × 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

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

Flow Characteristics in BWR Fuel Assemblies Under Earthquake Condition 167

Case A Case B Case C Case D Case E Case F Case G Case H Case I

On the other hand, in cases where the oscillation period was greater than 0.30 s, the standard deviation distributions hardly varied with the variation in the oscillation period.

From the information above, it can be confirmed that the boiling two-phase flow analysis, which is consistent with the time-series data of oscillation acceleration and has a time period greater than 0.01 s, can be performed. This is because oscillation acceleration with an oscillation period of less than 0.01 s has very little influence on the boiling two-phase flow. In addition, the time variations in the void fraction in cases where the oscillation period is greater than 0.30 s are close to quasi-steady variation. This means that the computable physical time of about 1 s is enough to evaluate the response of the boiling two-phase flow to the oscillation acceleration. Therefore, it can be confirmed that effective analysis can be performed by


Table 1. Computational cases

Figure 7 shows the time-averaged distribution of the void fraction. In spite of the different oscillation periods, the time-averaged distribution of the void fraction hardly changed. Therefore, the influence of oscillation period upon the time-averaged void fraction could not be detected.

Figure 8 shows the standard deviation distribution of void fraction fluctuation. In cases where the oscillation period is less than 0.01 s, the influence of the oscillation acceleration is small because the magnitude of the void fraction fluctuation is very small compared to that in the cases where the oscillation period is greater than 0.02 s. When the oscillation period is greater than 0.02 s, although the magnitude of the void fraction fluctuation increases with elevation, it decreases near the top of the heated region.

In cases where the oscillation period is between 0.02 s and 0.30 s, the standard deviation distributions varied significantly with the variation in the oscillation period. In Case F, the magnitude of the void fraction fluctuation was highest locally. Therefore, the distribution of void fraction fluctuation was significantly dependent on the oscillation period in this range.

Figure 7 shows the time-averaged distribution of the void fraction. In spite of the different oscillation periods, the time-averaged distribution of the void fraction hardly changed. Therefore, the influence of oscillation period upon the time-averaged void fraction could not

Case A Case B Case C Case D Case E Case F Case G Case H Case I

Figure 8 shows the standard deviation distribution of void fraction fluctuation. In cases where the oscillation period is less than 0.01 s, the influence of the oscillation acceleration is small because the magnitude of the void fraction fluctuation is very small compared to that in the cases where the oscillation period is greater than 0.02 s. When the oscillation period is greater than 0.02 s, although the magnitude of the void fraction fluctuation increases with

In cases where the oscillation period is between 0.02 s and 0.30 s, the standard deviation distributions varied significantly with the variation in the oscillation period. In Case F, the magnitude of the void fraction fluctuation was highest locally. Therefore, the distribution of void fraction fluctuation was significantly dependent on the oscillation period in this range.

Table 1. Computational cases

Fig. 7. Time-averaged void fraction distribution

elevation, it decreases near the top of the heated region.

be detected.

Case Oscillation period A 0.005 s B 0.01 s C 0.02 s D 0.04 s E 0.08 s F 0.15 s G 0.3 s H 0.6 s I 1.2 s

Fig. 8. Standard deviation distribution of void fraction distribution

On the other hand, in cases where the oscillation period was greater than 0.30 s, the standard deviation distributions hardly varied with the variation in the oscillation period. Therefore, the influence of the oscillation acceleration is small in this range.

From the information above, it can be confirmed that the boiling two-phase flow analysis, which is consistent with the time-series data of oscillation acceleration and has a time period greater than 0.01 s, can be performed. This is because oscillation acceleration with an oscillation period of less than 0.01 s has very little influence on the boiling two-phase flow. In addition, the time variations in the void fraction in cases where the oscillation period is greater than 0.30 s are close to quasi-steady variation. This means that the computable physical time of about 1 s is enough to evaluate the response of the boiling two-phase flow to the oscillation acceleration. Therefore, it can be confirmed that effective analysis can be performed by extracting an earthquake motion of about 1 s at any time during an earthquake.
