Simulation of mixing effects in a VVER1000 reactor

• Published in: Nuclear engineering and design Vol. 237
• Main Authors: Bieder, U., Fauchet, G., Betin, S., Kolev, N., Popov, D.
• Format: Journal Article
• Language: English
• Published: Elsevier 2007
• Subjects: Computer Science; Nuclear Experiment; Numerical Analysis; Physics; Trio_U; VVER1000; benchmark
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2007.02.015

This work has been performed in the framework of the OECD-NEA thermalhydraulic benchmark V1000CT-2. This benchmark is related to fluid mixing in the reactor vessel during a MSLB accident scenario in a VVER-1000 reactor. Coolant mixing in a VVER-1000 V320 reactor was investigated in plant experiments during the commissioning of the Unit 6 of the Kozloduy nuclear power plant. Non-uniform and asymmetric loop flow mixing in the reactor vessel has been observed in the event of symmetric main coolant pump operation. For certain flow conditions, the experimental evidence of an azimuthal shift of the main loop flows with respect to the cold leg axes (swirl) was found. Such asymmetric flow distribution was analyzed with the Trio_U code. Trio_U is a CFD code developed by the CEA Grenoble, aimed to supply an efficient computational tool to simulate transient thermalhydraulic turbulent flows encountered in nuclear systems. For the presented study, a LES approach was used. Therefore a very precise tetrahedral mesh with more than 10 million control volumes has been created. The Trio_U calculation has correctly reproduced the measured rotation of the flow when the CAD data of the constructed reactor pressure vessel where used. This is also true for the comparison of cold leg to assembly mixing coefficients. Using the conception data, the calculated swirl was significantly underestimated. Due to this result, it is possible to improve the lower plenum flow mixing matrices which are usually used in system codes.