Numerical analysis of experiments modeling LWR sump cooling by natural convection

G. Grotzbach, L. N. Carteciano, B. Dorr

Forschungszentrum Karlsruhe GmbH, Institut fur Reaktorsicherheit,

Germany

Abstract. An optional sump cooling concept for the European pressurized water reactor EPR was investigated at the Research Center Karlsruhe. This concept foresees to utilize single phase natural convection in water to remove the decay heat from the core melt. The natural convection was investigated by the SUCOS-2D and -3D scaled experiments. A numerical investigation and interpretation of these experiments was performed by means of the computer code FLUTAN. In this paper, the numerical investigation of SUCOS-3D is summarized. Fol­lowing the results of the former 2d experiments and the numerical analysis of both experiments, an unexpected temperature distribution is found in this 3d experiment. Basing on the experimental data it had to be postulated that one of the horizontal coolers was slightly tilted against the main flow direction. Additional numerical in­vestigations show that a slope of only one percent would explain the experimental flow field. Conclusions are also drawn on the limits of scalability and transferability of the experimental results to a reactor sump. A de­tailed transformation will only be possible by applying well validated CFD-codes and experienced code users. As the flow in the reactor sump will be turbulent and this flow is strongly three-dimensional and time — dependent, only the method of Large Eddy Simulation is considered of being an adequate tool for reliable trans­formation of the gained experience to analyses for the reactor sump at 1:1 scales.

1. INTRODUCTION

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The final safety barrier after a core melt down accident is the core catcher in the reactor sump. An optional cooling concept for the European Pressurized water Reactor EPR utilizes passive safety features to remove the decay heat from the sump. After the accident, a dry distribution and stabilization of the core melt in the sump region of the reactor (see Fig. 1) is foreseen. Then cooling of the core melt begins with the water from the in-containment refueling water storage tank. Water cooled heat exchangers and condensers are present in the reactor sump re­gion in order to remove the decay heat from inside the containment. The decay heat is trans­ferred from the core melt to the sump water by evaporation, natural convection, and conduc­tion. In the first days the convection of the sump water is in two-phase conditions; about ten days after the accident, single-phase natural circulation conditions are reached.

Подпись: FIG. 2. Schematics of the test geometry> SUCOS-2D. image251

The single phase natural convection is experimentally and numerically investigated in the Re­search Center Karlsruhe in the program SUCOS (SUmp COoling Small) (Knebel et al. 1995). The aim of this program is to obtain quantitative results to be transferred to the prototypic condition in order to make a statement on the feasibility of the single phase sump cooling. Two scaled test facilities (1:20) are applied in the program: SUCOS-2D (Fig. 2), which represents a two dimensional plane slab (580 x 275 x 235 mm) of a simplified reactor sump geometry, and SUCOS-3D (Fig. 3), which is a three dimensional scaled geometry (1298 x 580 x= 275mm) of the sump. Water was heated by a heated copper plate at the bottom of the pool simulating the core melt and cooled by horizontal and vertical heat exchangers in areas where they are pro­tected against vapor explosion consequences.

A numerical investigation of this sump cooling concept and the related model experiments is performed by using the FLUTAN code (Willerding at al. 1995). This thermal — and fluid — dynamical computer code is developed in the Research Center Karlsruhe for the numerical analysis of the passive heat decay removal in new reactor systems. It was already extensively validated and applied to analyses of model experiments for the decay heat removal in the fast breeder reactor SNR-300 (Weinberg et al. 1996). It is used here to investigate and interpret numerically the single-phase natural convection in the experiments SUCOS-2D and 3D. The aim of this numerical investigation is to confirm the feasibility of the sump cooling concept and to analyze in more detail the experiments.

The first step of this investigation consisted of the numerical simulation and interpretation of an SUCOS-2D experiment (Carteciano et al. 1999). The most important result was that SUCOS — 2D cannot be well reproduced with a two-dimensional calculation whereas three-dimensional calculations reproduce the experiment quite well. The simulations showed that significant
three-dimensional experiment specific phenomena are present in the experiments, but these 3d phenomena are of much less relevance in the reactor sump. Furthermore, the analysis of the calculation of SUCOS-2D gave information on the requirements for the modeling of geometry and boundary conditions for simulations of an experiment of SUCOS-3D which is the second and final step of the numerical investigation of the single-phase sump cooling concept. This step of analyzing the SUCOS-3D experiment in detail is documented in (Carteciano et al. 2000). Here, the most important results of this study are discussed to give at the end an out­look on the current status of methods to transfer all these results of the model experiments by CFD tools to EPR scales.