CFD-Simulation of a Boron Dilution Transient During Start-Up of the Coolant Pump in the 1:5 Scaled VVER-1000 Reactor Model

During so called boron dilution transients at pressurized water reactors too weakly borated water might enter the reactor core. This can be the case e.g. during re-start of the first main coolant pump after a deborated slug has formed for instance in the loop seal of the cold leg during stop of coolant circulation under Small Break Loss of Coolant Accident (SBLOCA) conditions. The deboration results in the insertion of positive reactivity and possibly leads to a power excursion. This scenario was studied in the 1:5 scaled VVER-1000 reactor model at OKB “Gidropress”.

The deborated slug was modeled by water with lower temperature entering during the start-up of the coolant pump the hot water inside the Reactor Pressure Vessel (RPV). The experiments were repeated 5-6 times to get statistically independent results. The mass flow rate was varied in a series of experiments studying Reynolds-number effects.

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 experiment with the final mass flow rate of 175 m³/h was taken for CFD-validation. The CFD-Code used was CFX-5.

The geometric details of the construction internals inside the RPV have a strong influence on the flow field and on the mixing. Therefore, an exact representation of the cold leg, the inlet region, the spacer in the downcomer, the elliptical perforated plate and the complicated structures in the lower plenum were modeled in detail. Some parts of the lower plenum internals were modeled using resistance coefficients. The computational grid contained 2.5 Mio. tetrahedral elements. The inlet boundary conditions (velocity, temperature) were set at the cold leg. As an initial guess of the turbulent kinetic energy and the dissipation rate the medium intensity was used. The outlet boundary conditions were pressure controlled. As in the experiment, the temperature differences for describing the boron dilution processes were used. A constant temperature at the RPV walls was assumed.

In the VVER-1000 reactor, the same characteristic flow pattern is observed in the case of the start-up of one pump like for Western type PWR. Due to a strong momentum insertion into the flow at the inlet nozzle, also a horizontal component of the flow is generated in the upper part of the downcomer. The injected flow is distributed into two main jets, the minimum boron concentration at the core inlet appears at two outer regions of the core inlet near the affected loop. The experiment and the CFX calculation showed that for the given slug volume a rather good mixing of the slug and the ambient coolant occur.

A comparison of measured and calculated degree of mixing is performed. Reasons for deviation and directions of further model improvements are discussed.