Development of a fast 3D thermal-hydraulic tool for design and safety studies for HTRS

Abstract A three-dimensional thermal-hydraulic code called TH3D is being developed at the Institute of Nuclear Technology and Energy Systems (IKE), University of Stuttgart. The objective of this endeavor is to provide a tool which can be used to analyze, design, and safety related issues in high temperature reactors. The tool shall be generally applicable for modular HTRs. Operational conditions with forced cooling as well as accident situations with heat removal by conduction and natural circulation shall be covered. Coupling to a reactor physics code shall be provided to account for the feedback of neutronics and thermal-hydraulics. Emphasis is on capturing essential effects resulting from three-dimensional features (e.g. power distribution with block-type fuel elements) rather than on a high level of detail, in order to keep computation times reasonably low. In general, we strive for a quick-turn analysis that provides enough insight to make informed decisions that cannot wait for the extensive time it takes to conduct in-depth, detailed analyses, e.g. with large CFD models. The physical and numerical basics of the new code are given. The porous media approach is applied. The time dependent mass and energy conservation equations and simplified steady-state momentum conservation equations (dominance of friction) are solved for the cooling gas along with the time dependent energy conservation equation for the solid. An appropriate set of constitutive equations (e.g. effective heat conductivity of solid, pressure drop, heat transfer coefficient, etc.) is applied. A finite-volume method is used for the spatial discretisation. A fully implicit method with adaptive time step selection is applied for the temporal integration in transient problems. In this paper results of calculations for a benchmark design defined by OECD/NEA/NSC [Reitsma, F., 2004. PBMR Coupled Neutronics/Thermal Hydraulics Transient Benchmark the PBMR-400 Core Design. PBMR Company Ltd., South Africa] are presented. Here, the core is modeled as 2-D geometry by some simplified assumption made by benchmark definition. The calculations address steady state under nominal operation conditions and behavior under loss of cooling accidents with and without de-pressurization. The results of the new code TH3D are compared to those of the well know, well-established thermal-hydraulic code THERMIX/KONVEK [Banaschek, J., 1983. Berechnungsmethoden und Analysen zum Dynamischen Verhalten von Kraftwerksanlagen mit Hochtemperaturreaktor, Dissertation RWTH Aachen; Petersen, K., 1984. Zur Sicherheitskonzeption des Hochtemperaturreaktors mit naturlicher Warmeableitung aus dem Kern im Storfall, Dissertation RWTH Aachen]. The results generally show a good agreement except some regions described below. Finally, in order to demonstrate the capability of doing simulations for 3D geometry, a first example calculation is presented where a non-symmetric situation arising from withdrawal of single control rods is addressed.