In order to simulate the boiling two-phase flow in a fuel assembly under earthquake conditions, it is necessary to consider the influence of structural oscillation of reactor equipment on boiling two-phase flow. If the coordinate system for an analysis is fixed to an oscillating fuel assembly under earthquake conditions, it can be seen that a fictitious force acts on the boiling two-phase flow in the fuel assembly. Therefore, a new external force

term, f, which simulates the acceleration of oscillation, was added to the momentum conservation equations (Eqs. (3) and (4)).

We assume that the analysis of boiling two-phase flow in a fuel assembly under earthquake conditions can be performed by using time-series data as an input if the time-series data of oscillation acceleration can be obtained from structural analysis results for a reactor (Yoshimura, et al., 2002) or if the measurement data of actual earthquakes can be obtained by seismographic observation.

In order to apply this improved method to the analysis of boiling two-phase flow in a fuel assembly under earthquake conditions, it is necessary to confirm that the simulation of boiling two-phase flow under oscillation conditions can be performed using the interface stress models shown in the preceding section; these stress models are empirical correlations and are based on experimental results under steady-state conditions. In the case of boiling two-phase flow analysis under oscillation conditions, these interface stress models may cause instability in simulation results.

In addition, it is necessary that large-scale analysis be performed within limited computable physical time and that it be consistent with the time-series data of oscillation acceleration obtained from the results of structural analysis in a reactor or with the measurement data from actual earthquakes. In structural analysis in a reactor (Yoshimura, et al., 2002), the minimum time interval of the analysis is limited to 0.01 s (100 Hz). Seismographic observation is also frequently performed with a sampling period of 100 Hz. If a high — frequency oscillation acceleration of over 100 Hz influences boiling two-phase flow in a fuel assembly, boiling two-phase flow analysis, which is consistent with the structural analysis in a reactor, cannot be performed. Therefore, it is necessary to evaluate the highest frequency necessary for this improved method to be consistent with the time-series data of oscillation acceleration.

A computable physical time of about 1 s is preferred for the boiling two-phase flow analysis in a fuel assembly because this analysis requires a large number of computational grids in order to simulate a large-scale domain such as a fuel assembly. If the results of the boiling two-phase flow analysis show quasi-steady time variation for long-period oscillation acceleration, it is not efficient to perform the analysis with a computable physical time span longer than the long period. Effective analysis can be performed if the analysis with a time span subequal to the shortest period of oscillation acceleration, for which the boiling two — phase flow shows quasi-steady time variation, by extracting earthquake motion at any time during the earthquake. Therefore, it is necessary to evaluate the shortest period of oscillation acceleration for which the boiling two-phase flow shows quasi-steady time variation.

The boiling two-phase flow was simulated in a heated parallel-plate channel, which is a simplification of a single subchannel in a fuel assembly. The channel was excited by vertical
and horizontal oscillation to simulate an earthquake in order to confirm that the boiling two- phase flow simulation can be performed under oscillation conditions.

In addition, the influence of the oscillation period on the boiling two-phase flow behavior in a fuel assembly was investigated in order to evaluate the highest frequency necessary for the improved method to be consistent with the time-series data of oscillation acceleration and the shortest period of oscillation acceleration for which the boiling two-phase flow shows quasi-steady time variation.