Yudov Yu.V, Danilov I.G., and Chepilko S.S. Alexandrov Research Institute of Technology (NITI), Russia

24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)
Nuclear applications of computational fluid dynamics (CFD)


Yudov Yu.V, Danilov I.G., and Chepilko S.S.
Alexandrov Research Institute of Technology (NITI), 188540 Sosnovy Bor, Leningrad region,Russia
Phone: (813-69) 22667, Fax: (813-69) 23672, E-mail:
The Russian KORSAR/GP computer code was developed by a joint team from Alexandrov NITI and OKB “Gidropress” for VVER safety analysis and certified by the Rostechnadzor of Russia in 2009. The code functionality is based on a 1D two-fluid model for calculation of two-phase flows. Unsteady conservation equations are numerically solved using the semi-implicit scheme. The KORSAR/GP code includes a 3D reactor kinetics module. The code capability is intended for simulation of thermalhydraulic phenomena in circulation circuits of reactor systems and also for 3D modeling of coupled thermal-hydraulic (complete- core approximation) and neutronic processes in the reactor core.
A 3D CFD module is being developed by Alexandrov NITI for representing 3D effects in the downcomer and lower plenum during asymmetrical loop operation with the KORSAR/GP computer code. The CFD module uses Cartesian grid method with cut cell approach. Grids are generated using the adaptive refinement approach. The grid structure data is stored in Fully Threaded Tree (FTT) format. Small cut cells are merged with neighboring coarse cells. Convective terms are approximated using SDPUS-C1 high-resolution continuously differentiable scheme. Time integration of conservation equations is performed by the Kim-Choi second-order accurate implicit method. The Crank-Nicolson scheme is used together with linearization of non-linear terms. The Poisson equation for pressure at a new time level is solved by the multigrid method with Jacobi relaxation.
The paper presents a numerical algorithm for coupling 1D and 3D thermal-hydraulic modules in the KORSAR/GP code. To ensure mass and energy balances at the interface, convective fluxes for 1D module are calculated as a sum of fluxes at corresponding boundary edges of 3D module. Mass balance condition is used to couple pressures in the modules. A combined pressure field should be calculated at each time level. This is the most time- consuming operation. Cell sizes and, consequently, coefficients in discrete Poisson equations for pressure are different between 1D and 3D modeling domains by orders of magnitude causing slow convergence of iterative solutions. To increase the convergence rate, 1D module cells are included in the multigrid algorithm and considered as leaf cells on coarsest grid. Prolongation and restriction operators for these cells are trivial injection operators (values are merely copied as the level of the multigrid method changes). The relaxation procedure for 1D also uses simple Jacobi iterations.
The performance efficiency of the algorithm for coupling 1D and 3D modules was demonstrated by solving the benchmark problem of mixing cold and hot flows in a T- junction. The T-junction in that problem was modeled by CFD with prescribed inlet flow rates and fluid temperatures. The outlet part of the main channel downstream of the mixing region is modeled by the KORSAR/GP 1D module with given outlet pressures.

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