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The time-dependent 3D fine-mesh few-group discrete ordinates (SN) neutron transport code TORT-TD is a GRS development for transient 3D core calculations. For thermal- hydraulic feedback, TORT-TD has been coupled with the system code ATHLET, the subchannel code COBRA-TF and the porous medium code ATTICA3D.

Whereas the code systems TORT-TD/ATHLET and, in particular, TORT-TD/COBRA-TF is primarily intended to high-fidelity transient fine mesh LWR simulations of both neutron kinetics and thermal-hydraulics phenomena, TORT-TD/ATTICA3D aims at the analysis of 3D issues in high temperature reactors of pebble bed type. The features of TORT-TD include:

• Direct solution of the time-dependent 3D SN equation for both Cartesian and cylindrical geometry without approximations like, e.g., a quasi-static approach;
• Unconditionally stable implicit time integration;
• 64 bit encoding to meet tight convergence criteria and to enable TORT-TD to be applied to large realistic problems exceeding 32 bit RAM limitations;
• Arbitrary number of prompt and delayed neutron groups; arbitrary Legendre scattering (Pl) and SN order;
• Fully integrated 3D fine-mesh few-group diffusion solver (steady state and time-dependent) in both Cartesian and cylindrical geometry for fast running scoping calculations to be used as preconditioner for subsequent transport calculations or future embedded transport-diffusion analyses:
• Movements of single control rods or control rod banks;
• Processing of parameterized tabulated cross section libraries for up to 5 state parameters; interpolation either linear or with cubic spline polynomials, thus allowing to study the impact of different interpolation schemes on cross section evaluation;
• Generalized Equivalence Theory (GET) at pin cell level to reduce homogenization errors;
• Time-dependent anisotropic distributed external source capability;
• Leakage and buckling calculation over larger spatial regions (e.g. spectral zones) using the neutron current density in discrete ordinates representation;
• Calculation of Xenon/Iodine equilibrium and transient distribution as a prerequisite for operational transients;
• Graphical pre- and post-processing tools for visualization of input data and output quantities.

Within the applied internal coupling approach, the respective thermal-hydraulic code models the entire fluid dynamics and heat transfer processes in the primary circuit including the core region. The exchange of spatial distributions (power density, thermal-hydraulic state) is managed by interface routines, e.g. the standard ATHLET interface. This allows maintaining each code individually whilst the coupled code is represented by a single executable.