Numerical modeling of condensation pool (NUMPOOL)

Numerical methods for analyzing pressure suppression pools in boiling water reactors are developed. The numerical modelling work of the project has three objectives. First, supporting the CONDEX project at the Lappeenranta University of Technology (LUT), where experiments are performed with the pressurized PPOOLEX test facility. Second, improving understanding of the thermal hydraulic phenomena in the dry well and in the wet well compartments of the pressure suppression pool of BWRs. Third, developing methods for estimating pressure loads originating from the pressure suppression pool.

In the NUMPOOL project, CFD simulations have been performed for an experiment, where steam is blown into the dry well initially filled with air. When the temperature and the pressure in the dry well increase a mixture of air and steam starts flowing through a vent pipe into the water pool of the wet well compartment. The molar fraction of steam gradually increases in the air-steam mixture flowing into the water pool and strong direct-contact condensation occurs.

Fluid-structure interaction (FSI) analyses of the PPOOLEX experiments have also been performed. In the FSI simulations, the pool wall motion is accounted for in the calculation of pressure loads. Coupling between CFD and structural analysis codes has been realized with the MpCCI middleware. Mainly experiments with air blowdown have been analysed so far to remove uncertainty of the two-phase modelling.

Specific goals in 2009

In 2009, bulk condensation model has been added in the CFD model. Test simulations on the bulk and wall condensation models have been performed. Improvements on the wall condensation model have been made, which improve the stability of the model and make using longer time steps possible. Some improvements of the direct-contact condensation model were done, but more time than was initially planned was used in modelling wall condensation in the uninsulated PPOOLEX device.

A long test simulation of PPOOLEX steam experiment WLL-05-02 has been performed. The simulation has been repeated after making improvements in the wall condensation model. The experiment was also simulated by using different constant heat transfer coefficients for the direct-contact condensation. The condensation was found to be too weak in simulations longer than 100 s, where the mole fraction of vapour in the gas exceeded 80%.

FSI analyses have been carried out for the PPOOLEX experiment SLR-05-02 and comparison with experimental data has been made. Numerical instability of explicit FSI coupling has been circumvented by using the Linear Perturbation Method (LPM). LPM simulations have also been validated against full FSI calculation by using an axisymmetric model of the PPOOLEX facility.

An article of the FSI simulations has been written for the “20th International Conference on Structural Mechanics in Reactor Technology (SMiRT 20)”.

FSI calculations of a realistic BWR containment have been started by considering only the initial non-condensable phase of blowdown. The structural and the CFD models of the containment have been constructed. A one sixteenth sector of the concrete containment including one blowdown pipe has been modelled. FSI calculations by using one- and two-way coupling as well as the LPM have been made.

In Figure 2.4.1.1, an example of a CFD simulation result is shown, where steam is blow into the dry well initially filled with air. Molar fraction of steam increases in the dry well and wall condensation of steam occurs. When the pressure increases, the vent pipe is cleared and the mixture of air and steam flows into the wet well. Direct-contact condensation occurs when gas flows into the water pool.

t = 0 s 20 s 40 s 60 s Figure 2.4.1.1. CFD simulation of a PPOOLEX experiment, where steam is blown into the drywell. Molar fraction of steam in the gas phase is shown at different instants of time.

In Figure 2.4.1.2, experimental and simulation results are shown for a PPOOLEX experiment, where air was blown into the pool. The wall pressure can be separated into two components: the pressure caused by the blowdown and the pressure caused by wall motion. The calculation with rigid walls shows only the blowdown load. Amplitude and frequency of the pool motion obtained with LPM are fairly close to the experiment. The frequency is higher in the one-way coupled calculation due to the absence of water. The smaller displacements obtained with one- way coupling may be caused by the higher eigenfrequency of the empty pool. In addition, damping of the pool motion is faster when the mass of water is neglected.

1.20E+05 1.25E+05 1.30E+05 1.35E+05 1.40E+05

1.5 1.7 1.9 2.1 2.3 2.5

Time [s]

Pressure [Pa]

-4.0E-04 -2.0E-04 0.0E+00 2.0E-04 4.0E-04

1.5 1.7 1.9 2.1 2.3 2.5

Time [s]

Displacement [m]

LPM One-way Experiment

Figure 2.4.1.2. Wall pressure below pipe and wall displacement in PPOOLEX experiment and in calculations with LPM and with one-way coupling.

Deliverables in 2009

CFD simulation of PPOOLEX experiments on direct-contact condensation in the wet well.

Fluid-structure interaction calculation of one PPOOLEX experiment.

FEM modelling of a BWR sector.

The CFD and the FSI calculations are described in the report: T. Pättikangas, J. Niemi and A. Timperi, “CFD and FEM modeling of PPOOLEX experiments”.

2.4.2 Improved Thermal Hydraulic Analysis of Nuclear Reactor and Containment