Pre-test CFD simulations of Gidropress Mixing Facility Experiments using ANSYS CFX

The main objective for the quantification of the fluid mixing in the downcomer and the lower plenum is the demonstration of the safety of the nuclear plant during non-symmetrical transients. This concerns two main topics: The risk of fragile brittle of the Reactor Pressure Vessel (RPV) during Pressurized Thermal Shock (PTS) transients and the risk of core reactivity excursion during non-symmetrical transient such as Main Steam Line Breaks (MSLB) or Boron Dilution Transients (BDT). These scenarios are studied in the 1:5 scaled VVER-1000 reactor model at OKB “Gidropress” in the framework of a TACIS project: ““Development of safety analysis capabilities for VVER-1000 transients involving spatial variations of coolant properties (temperature or boron concentration) at core inlet”.

The 3-D computational fluid dynamics (CFD) codes provide an effective tool for mixing calculations. In recent years, the rapid development of both the software and the computers has made it feasible to study the coolant mixing in sufficient detail and to perform the calculations for transient conditions. The CFD-Code used was ANSYS CFX. The geometric details of the construction internals inside the RPV have a strong influence on the flow field and on the mixing. Therefore, a detailed representation of the inlet region, the spacer in the downcomer, the elliptical perforated plate and the complicated structures in the lower plenum was necessary. All parts of the lower plenum structures were modeled in detail. The computational grid contained 4.3 Million nodes. In the VVER-1000 reactor, similar characteristic flow and mixing pattern are observed in the case of nominal flow conditions like for Western type PWR. Sensitivity analyses were performed following recommendations included in the ECORA Best Practice Guidelines.

Regarding the flow field and mixing in the downcomer during four loop operation at nominal flow rates, it has been shown that a sharp sector formation like in western 4-loop reactors appears, the flow field is inhomogeneous; in fact high velocity values occur beside the loop positions, and not below the inlet nozzles, which indicates the presence of recirculation areas or stagnant zones. Regarding the flow field and mixing at the core inlet, it has been shown that the mass flow rate distribution is more or less homogenous over the core diameter due to the lower plenum internals, the perturbed sector covers more or less one fourth of the core; a sharp sector formation like in western 4-loop reactors appears, weak mixing zones appear (around 97% of the unperturbed concentration). In most cases, the sensitivity analyses performed did not show any appreciable dependence of the results with respect to the addressed parameters. A three loop operation was chosen to show the differences of the flow and mixing behavior compared to the four loop operation.

An extensive experimental program is now running, studying different flow conditions in the reactor mock up, like the start of the 1st coolant pump or natural circulation conditions with density differences of the primary coolant. Pre and post test CFD simulations are carried out for code validation and for a deeper understanding of the flow and mixing behavior in the VVER-1000 reactor also in the future of the project.