Improved Thermal Hydraulic Analysis of Nuclear Reactor and Containment (THARE)34

2.4.2 Improved Thermal Hydraulic Analysis of Nuclear Reactor and Containment

Wall condensation model implemented in the FLUENT CFD code as user subroutine was further tested with SARNET condensation benchmark data. Discrepancy found between two alternate approaches needs still clarification. The newly implemented condensation model (based on partial pressure) shows higher condensation rate than the previous one (based on diffusion across the wall layer) especially on lower steam volume fractions.

OECD/SETH2/PANDA experiment with containment heat exchanger was modelled with FLUENT. The pre-test CFD analysis showed very effective condensation inside the heat exchanger. Steam condensation resulted in high concentration of helium (simulating hydrogen) and complex stratification inside the test vessels. Calculations will be compared with measurement when the measured data will be available.

Figure 2.4.2.2. CFD model of the PANDA vessels and vertical gas injection line in the left vessel.

Figure 2.4.2.3. Heat exchanger in the PANDA experiment and calculated heat flux in the tube array.

Figure 2.4.2.4. Calculated helium volume fraction in PANDA experiment.

PPOOLEX experiments STR-1, STR-4 and WLL-5-2 were calculated with APROS containment. The model was very simple. Only two nodes dry-well and wet-well, and walls divided according the experimental setup taking in account the condensate collectors. With simple model it was fast and easy to do parameter variations witch helped to understand the experiment and supported more detailed analysis in NUMPOOL project and in Northnet go- operation with KTH (Kungliga Tekniska Högskolan).

Figure 2.4.2.5. PPOOLEX test facility and APROS model.

Dry well

Wet well DN200 Blowdown pipe DN300 windows

for visual observation

Intermediate floor Relief valve

DN100 connection line between the dry well and wet well

DN200 Inlet plenum

Figure 2.4.2.6. Measured and calculated condensation in PPOOLEX test with different assumptions.

2.4.3 CFD modelling of NPP horizontal and vertical steam generators (SGEN)

The objective of the project is to develop a simulation methodology and tool for the modelling of horizontal and vertical steam generators of NPPs taking into account the multidimensional effects and the two-phase flow phenomena. The model developed in the project includes the essential physical phenomena occurring in steam generators, such as heat transfer from the primary to the secondary side and the pressure loss of the two-phase flow in the tube banks of the secondary side.

Three-dimensional simplified model is made for a horizontal steam generator of VVER-440 NPP and for the vertical steam generator of the PWR PACTEL test facility (Riikonen et al., 2009). The models of the secondary sides are implemented in the commercial Fluent CFD code.

The primary circuit is modelled with Apros.

Specific goals in 2009

In 2009, the porous media model for steam generators has been modified based on the experiences on the simulations performed in 2008. In particular, the fluid inventory and enthalpy behaviour have been tested with the physical velocity and superficial porous media models of the new Fluent version 12.0. The symmetric interfacial drag model has been replaced with experimental correlations which do not depend on bubble or droplet size. New correlations for the drag caused by the tube banks in cross-flow direction have been implemented for liquid water and vapour phases. The steam generator model has been tested in Fluent 12.0.

Available validation data on the horizontal steam generator of VVER-440 has been collected, and some geometry details have been added. In particular, the old feed water manifold has been added in the model because it is needed in the validation simulations. A series of validation simulations have been performed and the results on the void fraction, swell level and flow velocity of liquid water have been compared to the available experimental data. A test simulation on the transport of magnetite particles has been performed.

In Figure 2.4.3.1, the vertical position of the 70% mixture level above the tube banks is shown for a VVER-440 horizontal steam generator. The position of the mixture level is shown both on the cold side and on the hot side of the steam generator versus the axial position coordinate. As is

Figure 2.4.3.1. Calculated vertical position of the 70 % mixture layer versus axial coordinate.

The position of the mixture level is the distance from the top of the tube banks.

(a) (b) (c)

Figure 2.4.3.2. (a) Surface mesh of the CFD model for the PWR-PACTEL steam generator.

(b) Outer surface temperature (C) of the primary tubes. (c) Void fraction.

expected the position of the mixture level is higher on the hot side, where the boiling heat transfer is strong and the void fraction is large.

A model of the vertical steam generator of PWR-PACTEL has been constructed. The model consists of a detailed Apros model of the primary side and of a Fluent model of the secondary side. Steady state test simulations have been performed for the case with a pressure of 40 bars and a power of about 105 kW. The long time needed for achieving a stationary state was found to be an issue, which makes comparison with the experiments challenging.

In Figure 2.4.3.2(a), the surface mesh of the PWR-PACTEL model is shown. In the bottom part of the steam generator, a vertical plate separates the hot and the cold sides of the steam generator. In Figure 2.4.3.2(b), the outer wall temperature of the vertical U-tubes of the primary circuit is shown. The temperature has been interpolated from the Apros model to the porous media model describing the U-tubes in the CFD model. In the shortest U-tubes the flow is found to be reversed. In Figure 2.4.3.2(c), the void fraction obtained in a test simulation is shown. The circular cross-section of the feed water manifold can be seen in the downcomer on the cold side.

Strong boiling occurs on the hot side of the riser, where the void fraction is large.

Deliverables in 2009

The porous media models for the horizontal and vertical steam generators are described in: T. Pättikangas, V. Hovi, and J. Niemi, “Implementation of a three-dimensional porosity model for horizontal and vertical steam generators”.

The validation simulations performed for the VVER-440 horizontal steam generator are described in: T. Rämä, “Validation of the horizontal VVER-440 steam generator model”.