Takeharu Misawa, Hiroyuki Yoshida and Kazuyuki Takase

Japan Atomic Energy Agency Japan

1. Introduction

Safe operation of nuclear reactors under earthquake conditions cannot be guaranteed because the behavior of thermal fluids under such conditions is not yet known. For instance, the behavior of gas-liquid two-phase flow during earthquakes is unknown. In particular, fluctuation in the void fraction is an important consideration for the safe operation of a nuclear reactor, especially for a boiling water reactor (BWR). The void fraction in the coolant is one of the physical parameters important in determining the thermal power of the reactor core, and fluctuations in the void fraction are expected to affect the power of the plant.

To evaluate fluctuation in the void fraction, numerical simulation is the most effective and realistic approach. In this study, we have developed a numerical simulation technique to predict boiling two-phase flow behavior, including fluctuation in the void fraction, in a fuel assembly under earthquake conditions.

In developing this simulation technique, we selected a three-dimensional two-fluid model as an analytical method to simulate boiling two-phase flow in a fuel assembly because this model can calculate the three-dimensional time variation in boiling two-phase flow in a large-scale channel such as a fuel assembly while incurring only a realistic computational cost. In addition, this model has been used to successfully predict the void fraction for a steady-state boiling two-phase flow simulation (Misawa, et al., 2008). We expect that the development of the boiling two-phase flow analysis method for a fuel assembly under earthquake conditions can be achieved by improving the three-dimensional two-fluid model analysis code ACE-3D (Ohnuki, et al., 2001; Misawa, et al., 2008), which has been developed by the Japan Atomic Energy Agency.

This paper describes an analytical method for boiling two-phase flow in a fuel assembly under earthquake conditions by improving ACE-3D and shows how the three-dimensional behavior of boiling two-phase flow under these conditions is evaluated by the improved ACE-3D.