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       The JSC «EREC» Laboratory of thermo-hydrodynamics is involved in the development of computer thermo-hydraulic codes for investigation of NPP safety operation, calculation analysis of new structural solutions for NPP equipment, calculation justification of new NPP designs.
       Three-dimensional code REMIX (Reactor MIXing) that allows to simulating non-steady state space coolant flows with boric acid of different concentration in the VVER flow path in the area from inlet loop pipe branches to the core outlet, has been developed in the Laboratory of computer thermo-hydrodynamics. The code is based on three-dimensional Reinolds-averaged Navier-Stock's equation. In software complex REMIX, the equation numerical integration is performed by means of the finite volume method using unstructured computational grids of hexagonal cells, that allows to simulating of complicated geometry of VVER reactor downcomer. The computer complex REMIX was validated on the basis of experimental results gained in Russia (FSUE OKB «Gidropress») and abroad.
       The Laboratory of computer thermo-hydrodynamics, jointly with FSUE OKB «Gidropress», developed a three-dimensional thermo-hydraulic code STEG (STEam Generator) used for analysis of thermo-hydraulic processes proceeding in the second loop of a horizontal steam generator including distribution of admixtures and deposition of corrosion products. Water-steam mix flow in the steam generator secondary circuit is calculated on the basis of numerical integration of non-steady state three-dimensional equations of multi-phase medium mechanics. STEG code was validated on the basis of results from the experimental investigations on the full-scale steam generators of VVER NPPs. At present, the STEG code is widely used for the analysis of new designs of horizontal steam generators for the innovation project of NPP with VVER.
       The Laboratory of computer thermo-hydrodynamics has developed the software complex VAPEX (VAPor Explosion) used for modeling core melt/water interaction in the course of the NPP hypothetical severe accident. VAPEX code allows to simulating the process of corium jet splitting and mixing of large-dispersed melt drops with water as well as further propagation of thermo-detonation wave in case of explosive interaction of core melt with water. The calculation of the melt/water interaction process is based upon numerical integration of non-steady-state three-dimensional equations of multi-phase medium mechanics. VAPEX code was validated upon the results of experimental investigations executed on foreign test facilities. At present, VAPEX code is included in the branch through system of reactor codes used for justification of VVER NPP safety operation.

Distribution of relative boric concentration in the VVER reactor downcomer while passing of the coolant deborated plug (calculation by means of REMIX code)
Distribution of water velocities and true volumetric void fraction in cross section of PGV-1000M


       The picture demonstrates the results of calculations made by means of VAPEX code for one of the experiments carried out on QUEOS test facility (Germany). Here the process of melt drops/water mixing was investigated using metal balls fallen into the water.
       At present, VAPEX code is embedded in the system of the associated Russian severe accident codes RATEG/SVECHA/GEFEST as a special module describing core melt/water interaction. Availability of this module allows considering fully sophisticated thermo-physical processes that may occur in severe accidents.




Calculation of QUEOS experiment on investigation of melt drops/water mixing using VAPEX code.
Calculation results (colored lines) are applied to the experimental pictures.