[en] In this work, methods to evaluate and apply interface discontinuity factors for the reactor simulator DYN3D are discussed. Both Generalized Equivalence Theory and Black-Box Homogenization interface discontinuity factors were considered. As DYN3D is based on the nodal expansion approach, it has been extended to allow for generating interface discontinuity factors for both its nodal and pin level diffusion solution. Selected verification cases utilizing the lattice codes SCALE 6.1/TRITON and Serpent 2.1.15 are presented. Applying interface discontinuity factors, DYN3D reproduces given reference solutions in almost all cases. If strong absorbers dominate at least one of the cells of the geometry, small residual errors attributable to DYN3D's solution algorithm are observed. Producing interface discontinuity factors for a problem as a whole typically proves to be computationally prohibitive. Hence, PWR application cases in which interface discontinuity factors were generated for a number of super-cells of the geometry were analyzed. The potential of the interface discontinuity factors to improve the DYN3D solution was found to depend heavily on the suitable choice of super-cells. (authors)

[en] Highlights: • The pin-by-pin reactor simulator DYNSUB is an improved multi-physics tool. • The simulation of LWR cores with pin-by-pin/sub-channel resolution is possible. • DYNSUB pin-by-pin was successfully applied to the MOX/UO_2 core transient benchmark. • The applicability of DYNSUB pin-by-pin for LWR safety analysis proven in principle. - Abstract: The evolutionary multi-physics tool developed at the Karlsruhe Institute of Technology is the homogeneous pin-by-pin reactor simulator DYNSUB, an internal coupling of the 3D neutron kinetics code DYN3D developed by Helmholtz Zentrum Dresden Rossendorf and the in-house sub-channel code SUBCHANFLOW. The ultimate goal of the on-going efforts concerning DYNSUB is to provide a cost-effective improved description of light water reactor core behavior with pin-by-pin resolution for both static and transient safety relevant scenarios. A cost-effective computer code is defined to be executable on commodity computing clusters which users/customers commonly have access to. Efforts undertaken to improve DYNSUB’s numerical performance and parallelize the code system are presented in this work. Moreover, the coupled code system has been extended in terms of fuel pin level homogenization corrections and flexible mapping schemes. After optimization and extension DYNSUB is successfully applied to study the OECD/NEA and U.S. NRC PWR MOX/UO_2 core transient benchmark with both fuel assembly/channel and pin level/sub-channel model resolution. Even though further improvements in terms of numerical performance and accuracy of physical models are required, the applicability of DYNSUB pin-by-pin simulations for light water reactor safety analysis is proven in principle in this work

[en] Highlights: • An internal coupling between Serpent 2 and SUBCHANFLOW is developed and verified. • Temperature and density variations are treated with the on-the-fly TMS method. • Limitation of TMS regarding thermal scattering is evaded by stochastic mixing. • Successful benchmark with MCNP5 and TRIPOLI based multi-physics solutions. • Serpent 2/SUBCHANFLOW applied to full PWR core under hot full power conditions. - Abstract: Efforts to develop high-fidelity, in silico or ab initio, high performance multi-physics tools are undertaken by many groups due to the availability of relatively cheap, large-scale parallel computers. To this end, an internal coupling between the Monte Carlo reactor physics code Serpent 2 and the sub-channel code SUBCHANFLOW has been developed. The coupled code system is intended to serve as reference for deterministic reactor dynamics code developments in the future. It exploits the fact that Serpent was conceived as a lattice code for such deterministic tools. The coupling utilizes Serpent’s recently introduced universal multi-physics interface. With the multi-physics interface enabled, Serpent treats temperature dependence of nuclear data using the target motion sampling method. Since the target motion sampling methodology cannot be applied to thermal bound-atom scattering or unresolved resonances, a stochastic mixing fall back algorithm to enable the simulation of thermal reactors has been implemented. The developed coupled code is verified by code-to-code comparison with an external coupling of the Monte Carlo tool TRIPOLI4 and SUBCHANFLOW as well as the internally coupled code MCNP5/SUBCHANFLOW. Simulation results of all code systems were found to be in good agreement. Thereafter, the second exercise of the OECD/NEA and U.S. NRC PWR MOX/UO_2 core transient benchmark is studied to demonstrate that Serpent 2/SUBCHANFLOW may be employed to analyze realistic, industry-like cases such as a full PWR core under hot full power conditions in a reasonable amount of time. The obtained simulation results are compared to known benchmark solutions and the numerical performance of Serpent 2/SUBCHANFLOW is analyzed to assess the feasibility of routine application. While Serpent 2/SUBCHANFLOW’s performance in terms of physics and numerical efficiency is found to be generally satisfactory, options to further improve the coupled tool concerning both aspects are discussed. Afterwards, first efforts to validate Serpent 2/SUBCHANFLOW using the hot zero power state of the cycle 1 of the BEAVRS benchmark are presented