This work has been performe
d in the framework of the OECD/NEA thermalhy
draulic benchmark
V1000CT-2. This benchmark is relate
d to flui
d mixing in the reactor vessel
during a MSLB acci
dent scenario in a VVER-1000 reactor. Coolant mixing in a VVER-1000 V320 reactor was investigate
d in plant experiments
during the commissioning of the Unit 6 of the Kozlo
duy nuclear power plant. Non-uniform an
d asymmetric loop flow mixing in the reactor vessel has been observe
d in the event of symmetric main coolant pump operation. For certain flow con
ditions, the experimental evi
dence of an azimuthal shift of the main loop flows with respect to the col
d leg axes (swirl) was foun
d.
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 to simulate turbulent mixing. 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 design data, the calculated swirl was significantly underestimated. Due to this result, it might be possible to improve with CFD calculations the lower plenum flow mixing matrices which are usually used in system codes.