atw 2018-09v3

inforum

atw Vol. 63 (2018) | Issue 8/9 ı August/September

Development and Validation of a CFD

Wash-Off Model for Fission Products

on Containment Walls

Katharina Amend and Markus Klein

The research project aims to develop a CFD model to describe the run down behavior of liquids (wall films, transition

of film flow into a discrete number of rivulets, droplets) and the resulting wash-down of fission products on surfaces in

the reactor containment. Numerical experiments allow for a deeper physical understanding, which is the basis for an

improved semi-empirical modeling.

This paper presents a three-dimensional

numerical simulation for water

running down inclined surfaces

coupled with an aerosol wash-off

model and the resulting particle transport

using the software package

OpenFOAM. The wash-off model is

based on the procedure used in AULA

(German: Abwaschmodell für unlösliche

Aerosole, wash-off of insoluble

aerosol particles) in the lumped

parameter code COCOSYS [1]. A

parameter variation was conducted

and the simulation results are compared

to the laboratory experiments

performed by Becker Technologies

[2].

1 Introduction

The desired goal is the prevention of

environmental contamination with

radioactive particles after a core

meltdown in a light water reactor. The

containment in a nuclear reactor

building prevents high pressure radioactive

steam from escaping in the

event of an emergency. During such a

critical accident in a light water

­reactor, most of the fission products

enter the containment building in the

form of soluble and insoluble aerosols.

These particles might deposit

on walls and installation surfaces.

Condensing steam that is also released

into the containment can wash down

even insoluble particles into the

containment sump.

In previous studies [3, 4] the understanding

of the run down behavior

of water, the formation of film flow,

rivulets or droplets, was the main

subject of interest. This study investigates

the wash-off of insoluble

particles based on the run down behavior

of water on inclined plates and

the developing flow patterns using

CFD simulations.

2 Laboratory experiments

The laboratory tests are part of the

THAI AW3 test program [5]. They

investigate the aerosol wash-down

behavior of non-soluble silver from

inclined walls by steam condensate.

Trapezoidal plates (plain stainless

steel or decontamination paint coating)

with different inclinations

are loaded with dry silver aerosol. At

the uppermost part water enters

the plate via a tubular distributor

with a given flow rate. The water

flows down the plate, washes off

part of the particles and is finally

collected in cups, which get exchanged

after a specified time period.

The samples are put into a cabinet

dryer and the remaining aerosol mass

is weighed to quantify the wash-off.

Pictures taken during the experiments

show the flow patterns and run

down behavior of the water on the

plates, see Figure 1.

Two kinds of silver aerosol particles

are used: a fine silver powder

and coarse silver powder. The fine

­silver powder is specified with a particle

diameter of 0.7-1.2 μm for 99.9 %

of the particles and as averaged

par ticle diameter of the undisturbed

powder d p = 1 μm can be assumed.

It has a bulk density of ρ bulk =

1.1 g/(cm 3 ) and a specific surface of

A sp = 2.5 m 2 /g. For the coarse silver

powder the specification of particle

diameter is 1.5 – 2.5 μm (99.9 %).

Here the averaged particle diameter is

d p =2 μm, the bulk density is also

ρ bulk = 1.1 g/(cm 3 ) and the specific

surface A sp = 1.21 m 2 /g.

3 Simulation of the water

field

In previous studies the simulation

of the flow field with three different

inclinations, namely 2°, 10° and 20°,

and with empirical contact angle field

and filtered randomized initial contact

angle distribution (FRICAF) were

presented [4, 6]. The computational

domain is a trapezoidal geometry

(length = 1.215 m, small base =

0.09 m, large base = 0.475 m, Figure

2), as used in the laboratory

experiments [5].

| | Fig. 1.

Pictures of lab tests [5] with 2° inclination and

a mass flow rate of 11 g/s after 2 min and

15 min.

| | Fig. 2.

Schematic of computational domain of

inclined trapezoidal plate, dimensions in mm.

The simulations are carried out

with the software package Open-

FOAM using the standard two-phase

solver InterFoam. The Navier-Stokes

equations for isothermal and incompressible

multiphase flow are solved

and the phase interface is captured

by the Volume-of-Fluid method. The

time step is adjusted such that the

maximum Courant number is below

0.4 to ensure a sufficient level of accuracy.

The time is discretized via Euler

implicit. The inlet is extending over

the entire upper boundary with a

Young

Scientists'

Workshop

Awarded

Katharina Amend

was awarded with the

2 nd price of the 49 th

Annual Meeting on

Nuclear Technology

(AMNT 2018) Young

Scientists' Workshop.

469

AMNT 2018 | YOUNG SCIENTISTS' WORKSHOP

AMNT 2018 | Young Scientists' Workshop

Development and Validation of a CFD Wash-Off Model for Fission Products on Containment Walls ı Katharina Amend and Markus Klein

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