The institute of nuclear engineering and energy systems (IKE), University of Stuttgart, Germany has developed a new thermal hydraulic tool which can be used for three-dimensional thermal hydraulic analysis of pebble bed as well as block type HTRs. During nominal operation, the flow inside the gas-cooled High Temperature Reactor is essentially single-phase, compressible, and non-isothermal. So, at least one gas phase has to be considered beside the solid phase for thermal hydraulic analysis of HTRs. Each phase (e.g. solid, gas) is considered as a continuum which occupies only its respective fraction of the control volume. Thermal non-equilibrium is considered between phases and time dependent energy conservation equations for solid and gas phases are solved. Simplified momentum conservation equation for gas obtained from porous media approximation is solved along with the time dependent mass conservation equation. Provisions for simulating more than one gas component are available in this newly developed code TH3D which could be required for simulating some accident situations (e.g air / water ingress by pipes break). The interaction between phases is made by a set of constitutive equations which rely on semi-empirical correlations obtained from different experiments. Finite volume method with a staggered grid approach is used for spatial discretization and a fully implicit, time adaptive, multi step method is used for time-dependent discretization. A benchmark calculation which is oriented to the pebble type fuel reactor PBMR-400 and a 3D calculation were presented in HTR-2006 conference and will also be published in Nuclear Engineering and Design (NED) journal. In order to demonstrate the capabilities of TH3D for simulating all block type HTRs, a benchmark calculation which is proposed by IAEA CRP-3 and oriented to the Gas Turbine Modular Helium Reactor (GT-MHR) is performed. Calculations are performed for the steady state case (nominal operation) as well as for Loss of Forced Cooling (LOFC) with and without depressurization. The results obtained from TH3D are compared with the results obtained from several countries participated in this benchmark calculation program by using different code system. In this paper, results of this benchmark calculation and comparisons will be presented. A fuel model for pebble type fuel is implemented in TH3D where heterogeneity of heat production inside the fuel pebble is taken into account. The assumption of homogeneous heat production could be justified for steady state calculation or for slow transient but for fast transient calculation, the assumptions of homogeneous and heterogeneous heat production produce a huge difference for coupled thermal hydraulics and neutronics calculation. In order to show the capabilities of this newly developed code TH3D to couple with a neutronics system, it was coupled with a point kinetics model for a fast reactivity insertion case. In this case all control rods were withdrawn very quickly (with a velocity of 1 m/sec) to the end position. It was assumed that the scram signals were not activated when power or temperature was increased beyond a limiting value during this withdrawal process but the control rods system continued to be withdrawn up to the top position instead of getting down and the coolant flow was reduced by controlling the blowers. The neutronics feedback during this fast reactivity insertion case with homogeneous and heterogeneous fuel model will also be presented.

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