Working from the lessons of the Fukushima Daiichi nuclear accident, we have been developing the following various safe technologies for boiling water reactors (BWRs), a passive water-cooling system, an infinite-time air-cooling system, a hydrogen explosion prevention system, and an operation support system for reactor accidents. The objective of the study reported here was development of the passive water-cooling system. The above technologies are referred to as ‘Inherently Safe Technology’. The passive water-cooling system works without electricity for the first 10 days after an event to remove a relatively large mount of decay heat from the core. The system consists of a condenser and a steam turbine-driven pump for transferring water from a suppression pool to the reactor. Steam from the reactor pressure vessel is condensed in the condensation tubes of the condenser, and the condensate flows out into the suppression pool in the primary containment vessel (PCV). The water temperature at the condensation tube outlet is lowered to less than the saturated temperature at the partial steam pressure of the maximum PCV design pressure to prevent the PCV failure. The condenser is located at a lower level, e.g., underground, for easier access and for supplying cooling water to a condenser pool without electricity during an event. The lower level condenser pool has an advantage that it can be seismically designed. To evaluate our concept of the water-cooling system, heat transfer tests were conducted using full-scale U-shaped single tubes with three diameter sizes under a wide range of pressure and inlet steam velocity conditions. The heat transfer data were obtained at system pressures of 0.2 to 3.0 MPa (absolute) and inlet steam velocities of 5 to 56 m/s. The heat transfer data with this wide range of pressure and inlet velocity conditions include thermal hydraulics conditions for a passive containment cooling system (PCCS) and some of the data can be extrapolated to isolation condenser (IC) conditions. We also confirmed thermal hydraulics conditions to determine the practicality of our new concept.

Two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase simulation method such as interface tracking method. Therefore, two-fluid model is useful for thermal hydraulic analysis in large-scale domain such as rod bundles. However, since two-fluid model include a lot of constitutive equations, applicability of these constitutive equations must be verified by use of experimental results, and the two-fluid model has problems the result of analyses depends on accuracy of constitutive equations. To solve these problems, an advanced two-fluid model has been developed in Japan Atomic Energy Agency. In the model, an interface tracking method is combined with the two-fluid model to predict large interface structure behavior accurately. Liquid clusters and bubbles larger than a computational cell are calculated using the interface tracking method, and those smaller than a cell are simulated by the two-fluid model. Constitutive equations to evaluate the effect of small bubbles or droplets on two-phase flow required in the advanced two-fluid model as same as a conventional two-fluid model. However, dependency of small bubbles and droplets on two-phase flow characteristic is relatively small, and the experimental results to verify the equations are not required much. The turbulent dispersion force term is one of the most important constitutive equations for the advanced two-fluid model. The turbulent dispersion force term has been modeled by many researchers for the conventional two-fluid model. However, the existing models include effects of large bubbles and deformation of bubbles implicitly, these models are not applicable to the advanced two-fluid model. In this study, we develop the new model for turbulent dispersion force term. In this model, effect of large bubbles and deformation of bubbles are neglected. The liquid phase turbulent kinetic energy and bubble-induced turbulent kinetic energy are considered to evaluate driving force in the turbulent diffusion of small bubbles. The bubble-induced turbulent kinetic energy is given by the function of bubble diameter and local relative velocity, and the liquid phase turbulent kinetic energy is similar to the single phase flow case. Furthermore, we considered energy transfer from the bubble-induced kinetic energy to the liquid phase turbulent kinetic energy. To verify the developed model, the advanced two-fluid model and the model for turbulent dispersion term were incorporated to the 3-dimensional two-fluid model code ACE-3D, and comparisons between the results of analyses and air-water two-phase flow experiments in 200 mm diameter vertical pipe were performed.

Two-fluid model can simulate two-phase flow by computational cost less than detailed two-phase flow simulation method such as interface tracking method. Therefore, two-fluid model is useful for thermal hydraulic analysis in large-scale domain such as rod bundles in nuclear reactors. However, two-fluid model include a lot of constitutive equations. Then, applicability of these constitutive equations must be verified by use of experimental results, and the two-fluid model has problems that the results of analyses depend on accuracy of constitutive equations. To solve these problems, we have been developing an advanced two-fluid model. In this model, an interface tracking method is combined with the two-fluid model to predict large interface structure behavior accurately. Interfacial structures larger than a computational cells, such as large droplets and bubbles, are calculated using the interface tracking method. And droplets and bubbles that are smaller than cells are simulated by the two-fluid model. Constitutive equations to evaluate the effects of small bubbles or droplets on two-phase flow are required in the advanced two-fluid model as same as a conventional two-fluid model. However, dependency of small bubbles and droplets on two-phase flow characteristic is relatively small, and the experimental results to verify the equations are not required much. In this study, we modified the advanced two-fluid model to improve the stability of the numerical simulation and reduce the computational time. Moreover, the modified model was incorporated to the 3-dimensional two-fluid model code ACE-3D. In this paper, we describe the outline of this model and the modification performed in this study. Moreover, the numerical results of two-phase flow in various flow conditions.