5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
Experimental Determination and Analysis of Iodine Mass Transfer Coefficients
from THAI Test Iod-23
K. FISCHER 1, G. WEBER 2, F. FUNKE 3, G. LANGROCK 3
1
Becker Technologies GmbH, Eschborn (GE)
3
AREVA GmbH, Erlangen (GE)
2
GRS, Garching (GE)
ABSTRACT
Mass transfer of molecular iodine (I2) at the water pool - gas interface has been investigated in the experiment Iod-23 at the THAI facility as function of water motion. I2 was
dissolved in a water volume, and the iodine released from the water was carried immediately to a filter by continuously flushing the air space. From the iodine accumulation on
the filter, the iodine mass flow and the mass transfer coefficient could be evaluated. Two
different states of water motion were repeatedly established: a stagnant pool, and a convecting pool with well defined flow distribution, in order to cover a wide range of conditions for I2 mass transfer. The measurements show that the mass transfer coefficient is
strongly dependent upon the water motion, with much higher values for the convecting
pool. The experimental data can be correlated by means of a water surface film renewal
model superimposed to the established two-film theory, where the water-side I2 mass
transfer coefficient kw is related to the I2 molecular diffusivity in water D and the airwater contact time tc according to
kw
D
.
tc
The contact time can be derived from the water flow distribution. The correlation is valid
for laminar and turbulent water flow conditions with non-breaking surface waves.
1
INTRODUCTION
Radioactive iodine isotopes have caused the major late health effects from the Chernobyl
accident [1]. The radiological importance of iodine has motivated intensive research on
the transport processes and chemical interactions in the containment of nuclear reactor
plants in order to assess and mitigate environmental releases during anticipated severe accidents [2]. The dominant volatile iodine compounds are molecular iodine (I2) and gaseous
organic iodides. In the containment of a light water reactor, much of the iodine released
from the damaged nuclear fuel is accumulated in the sump water pool, from where it can
be released as gaseous I2 or organic iodides to the containment atmosphere according to
the changing concentrations of these iodine species in water and air. The rate of iodine
mass exchange across the water surface is governed by the air-side and water-side boundary layers. The transport of volatile iodine in these layers depends upon molecular and turbulent diffusion and advective flow. Large-scale experiments on I2 mass transfer have been
conducted in the THAI facility in order to represent different thermal hydraulic conditions
that may be representative for containment accident transients, and to support related
model development and validation. The importance of large-scale tests is given by the
scale-dependency of turbulent transport processes.
In the SISYPHE experiments [3], the influence of steam evaporation upon the iodine mass
transfer was investigated. In these tests, oxygen was used instead of iodine as transported
Session 3“Source Term Issues”, paper 3.6
1/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
gas species in order to minimize reactions with surfaces, so the results had to be scaled
with respect to molecular diffusivity and weight of I2 and O2 which are differing significantly. The impact of the water phase motion upon the masss transfer was investigated by
changing the rate of water recirculation, and it was shown that the water-side mass transfer coefficient strongly increases when the water motion is enhanced. However, no quantitative relation between water motion and mass transfer coefficient was obtained in this
work. For different rates of water recirculation, different values of the water-side mass
transfer coefficient were determined, but it is not clear how these values can be transferred to reactor containment conditions.
In the THAI facility, large-scale iodine containment tests have been conducted for many
years by Becker Technologies in cooperation with AREVA and GRS. The experimental data
base established serves mainly COCOSYS and the incorporated iodine model AIM [8], but
also other codes, for model development and validation [4]. The availability of advanced
instrumentation to measure iodine concentrations in gaseous and liquid phases, as well as
the flexibility to establish controlled thermal-hydraulic conditions to investigate dedicated
transport phenomena, were considered as suitable to initiate a basic study of iodine mass
transfer at the water surface. Major results of this study (THAI test Iod-23) are given in the
present paper.
2
EXPERIMENTAL SETUP
A general view of the THAI test vessel is given in Figure 1. The stainless steel vessel has a
total height of 9.2 m and a diameter of 3.2 m. Thermal insulation and heating of all walls
allow to establish dedicated temperature levels.
Figure 1: THAI vessel with general thermal hydraulic instrumentation
Session 3“Source Term Issues”, paper 3.6
2/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
For the present investigation of iodine mass transfer, only the sump volume of the vessel
was utilized and equipped with all necessary instrumentation, see Figure 2. The sump volume was filled with water to an elevated level, leaving only a narrow air space on top. A
pH of 2 was established in the sump to suppress hydrolysis of I2, since hydrolysis would unneccesarily complicate the model analysis with respect to I2 mass transfer objectives. The
air space was separated from the main THAI vessel by a flat glass plate. The water surface
had a diameter of 1.368 m, the height of the air space was 0.09 m. The basic process realized in this geometry was the transfer of I2 dissolved in the water phase to the overlying air
space which was continuously flushed by a high rate of iodine-free air. The air was loaded
with I2 from the water surface and guided to an iodine filter where the accumulation of I2
mass was measured. A water recirculation loop allowed to investigate the I2 mass transfer
under stagnant or moving water conditions.
Figure 2: Configuration of THAI facility for I2 mass transfer test (including target
values of boundary conditions)
For high-precision iodine measurement, the dissolved iodine was marked by the radioactive
I-123 isotope. An aqueous injection line allowed fast release of the iodine mass to the water pool at the beginning of the test. During injection, the sump recirculation was operated to safely achieve a homogeneous I2 concentration in water; pretests showed homogenisation times of less than a minute. During the subsequent long-term experimental phases,
the air recirculation loop was operated with constant flow rate. Measurements were taken
as follows:
-
accumulated I2 mass on the filter by online radiation monitoring (OD) and by discontinuous measurement of the absorbed I-123 after defined accumulation times
(CF)
-
I2 concentration in air space by gas scrubber sampling
Session 3“Source Term Issues”, paper 3.6
3/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
Total iodine (sum of I2 and other aqueous iodine species, mainly I- resulting from
I2/steel reaction) concentration in water by sump water sampling
-
The entire system was operated at 1.5 bar pressure and a constant target temperature
level of 60 °C. Cooling and condensing the humidity in the air recirculation line downstream of the filter was necessary to protect the air pump. Before reentering the gas space
above the sump water, the steam-free air was reheated to 60 °C. The air injection to the
gas space was guided over a flow distributor covering about ¼ of the periphery, in order to
establish a smooth air flow field across the water surface. Temperatures, system pressure,
air and water flow rates were measured by conventional instrumentation. The air space
above the water pool and the air flow connection line to the cumulative filter were kept at
a slightly higher temperature level than the pool water in order to avoid any steam condensation on walls. The length of the connection line to the filter was minimized in order
to reduce iodine deposition on the pipe surface.
Test conditions were maintained over extended periods of time, where either stagnant or
moving water conditions were imposed. Sump water flow was driven by the sump recirculation loop, which injected water axially upward from the sump bottom center, and the
water was drained from a lateral position. The water flow pattern at the surface was
measured in a pretest and showed nearly symmetric radial outward flow; the particular
position of the lateral drain port did not significantly disturb the radial flow symmetry.
3
TEST RESULTS
An overview of the test procedure is given in Figure 3. At time t = 0, iodine is almost instantaneously injected into the sump pool and distributed homogeneously by the ongoing
water recirculation. After having established the intended initial state of well-mixed dissolved I2 concentration in water, intervals of 2 h duration with stagnant and recirculated
water motion follow. The final recirculation phase has a longer duration.
14
Iodine
injection
Water recirculation flow
12
10
8
Sump recirculation on
6
4
Sump recirculation off
2
0
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
Time (h)
Figure 3: Test intervals of stagnant and moving sump water flow
The most important iodine measurement results are shown in Figure 4. By differentiating
the measured iodine mass accumulation on the filter with respect to time, the I2 mass flow
from the water surface was determined. The data are based on the results from the online
Session 3“Source Term Issues”, paper 3.6
4/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
radiation detector (OD) and the offline determination of I2 mass on the cumulative filter
(CF) after refreshing the filter medium. Both measurements are in good comparison. The
theoretical curve “Model” is explained in the next chapter. The oscillations of the experimental data are caused by the error magnification effect associated with the time derivatives. The increasing oscillations at later times are related to a systematic decrease of
measurement accuracy because of (i) the depletion of volatile iodine available for
transport to the filter, and (ii) the radioactive decay of the I-123 tracer.
The figure clearly shows that high iodine mass flow takes place during times with recirculating sump water, while low mass transfer is associated with stagnant water conditions.
Furthermore, the data show a general decrease of I2 flow rates during the entire experiment. This general decrease is caused by the chemical reaction of dissolved I2 with submerged steel walls, generating non-volatile I- ions in the water and thus decreasing the
aqueous I2 concentration. Since the available instrumentation did not allow a separate
quantification of iodine species in water, the I2
I- conversion rate was estimated in the
data analysis.
Iodine mass flow (logarithmic scale).
0
0
OD
CF
Model
0
0
0
2
4
6
8
10
Time (h)
Figure 4: Iodine mass flow from water surface to air space
4
DATA ANALYSIS AND MODELING
According to the two-film model [5, 9], the iodine mass flux from the water surface to the
gas space can be written as
Vg I 2g
A
t
1
1
kw
kw G / w P
kw kg G / g
I 2w .
(1)
Here I2g and I2w are the iodine concentrations in gas and water, respectively; Vg is the gas
volume, A the water surface area, P the partition coefficient, kg and kw the gas- and water-side transfer coefficients, g and w the gas and water densities, and G the steam
Session 3“Source Term Issues”, paper 3.6
5/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
evaporation mass flux. Due to a low I2 concentration in the gas space, the small gaseous
volume above the sump and the small steel surface area exposed to this gas phase in the
present experiment, the deposition of gaseous I2 on the water surface is neglected. The
conversion of dissolved I2 to iodide ions I- by surface reaction with submerged steel is written as
Vw I 2 w
Aw t
(2)
kc I 2w
where Vw is the water volume, Aw the submerged steel surface area, and kc the transfer
coefficient associated with the chemical reaction I2
I- in water. Present data and previous THAI iodine tests lead to estimated values of kc = 1.9E-5 m/s for the recirculating pool
and kc = 1.6E-5 m/s for the stagnating pool. The different values of kc in two different
sump motion conditions indicate an influence of mass transfer in the water phase, which is
covered by the effective rate constants. These values can be compared to previous experience from I2/steel interaction data: analyses of THAI test Iod-9 [6] revealed 5E-6 m/s at
63 °C, whereas the rate constant from lab-scale tests [7] would only be 9.2E-8 m/s at the
Iod-9 temperature (always reflecting the same steel type no. 1.4571).
For the gas side mass transfer coefficient, the value kg = 1.4E-3 m/s is taken from the present standard value in the AIM-3 model [8]. The water-side mass transfer coefficient is
modeled on the basis of the unsteady molecular diffusion expression
kw
D
tc
(3)
where D is the molecular diffusivity of I2 in water and tc the contact time of the aqueous I2
solution with air. Equation (3) is the result of the surface film renewal theory. The formation of a diffusion boundary layer at the water surface begins when bulk water first gets
in contact with air at the water surface, and it may end after a contact time tc when surface water flows back into the bulk. In this case the surface film is renewed by bulk water
according to the flow velocity distribution in the water. For stagnant water conditions, the
contact time is the exposure time of the entire water surface to the air space, beginning
with the moment when the recirculation pump is switched off. For recirculating conditions, the contact times of individual water parcels from reaching the surface (at radius r)
until leaving it near the lateral wall (at Radius R) have to be averaged over the total surface. An approximate estimate of this average contact time can be obtained by assuming a
constant radial flow velocity v. The average contact time tc is given by
tc
1
R2
R
0
R
r
v
2 r dr
R
.
3v
(4)
The experimental data indicate an average radial water surface velocity v = 0.7 m/s. With
the pool surface radius R = 0.7 m the average contact time is tc = 0.67 s. Values of kw for
the present experiment are shown in Table I. For comparison, the default value from the
AIM-3 model is also shown.
Table I: Water-side mass transfer coefficient values
tc (s)
kw (m/s) s (m)
Stagnating sump < 7200 > 3.5E-7 <0.0025
Mixed sump
0.67
3.6E-5
2.4E-5
AIM-3 default
1E-5
-
Session 3“Source Term Issues”, paper 3.6
6/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
The last column in Table I gives an estimate of the boundary layer thickness s, calculated
according to
tcD
s
.
(5)
The “Model” curve in Figure 4 is the result of the numerical integration of equations (1)
and (2) by making use of the parameters and contact times as described above. The good
quantitative representation of the measured data, in particular of the differences between
the stagnant and recirculating phases, validates the detailed model of the water-side diffusive boundary layer.
According to the present data analysis, the water-side iodine mass transfer coefficient is
determined by molecular diffusion. This is not obvious because during recirculation the
water surface shows chaotic waves of substantial amplitude which would suggest the presence of turbulent water motions. An effective turbulent diffusivity of the water in the surface boundary layer would give rise to a much larger diffusion-like iodine transport than
the transfer based on molecular diffusion only. This apparent discrepancy is resolved by
considering the fact that the turbulent eddies are damped completely at the water surface. The wavy motion of the surface is formed by periodic gravitational and capillary
waves which both represent irrotational velocity fields, without any net material
transport. Therefore, molecular diffusion is the only possible iodine transport mechanism
in this boundary layer, the thickness of which is mostly below 1 mm. The present model is
applicable for laminar and turbulent conditions in the bulk water, as long as there is no
breaking of waves. It cannot be applied for boiling conditions because the rising steam
bubbles disturb the surface boundary layer. It could be applied to the SISYPHE data [3] because the formation of the surface boundary layer does not depend upon the mass flow
direction of the soluble gas.
5
CONCLUSIONS
The analysis of the THAI test Iod-23 provides a comprehensible model to predict I2 mass
transfer between sump water and atmosphere for non-boiling conditons in common severe
accident containment codes. The experimental demonstration of the significant influence
of water motion is reflected by the model.
The present investigation shows that the water-side iodine mass transfer coefficient is
governed by molecular diffusion, and it is dependent upon the contact time of bulk aqueous I2 solution with the air space, according to the surface film renewal theory. The present experimental setup allows to estimate the contact times for two different states of
water motion, the stagnant and the recirculating phase. In case of a reactor pool, the estimation of contact times should be based on a consideration of the water flow distribution. In particular, the contact times are governed by the horizontal velocity distribution
at the water surface. This distribution depends on the pool geometry and the water discharge and recharge operations. Spray droplets impinging on the water surface may destroy the boundary layer and enhance the mass transfer.
Uncertainties of the present analysis are related to the following aspects:
-
Experimental uncertainties (no explicit measurement of I2w; reduction of I2w by reaction with steel wall; determination of tc; influence of air flow upon surface exchange during stagnation conditions)
-
Model simplifications (e.g. uniform boundary layer thickness; uniform contact time)
However, parametric calculations indicate that the validity of the film renewal model is
well confirmed within the error margins of the present experimental data.
Session 3“Source Term Issues”, paper 3.6
7/8 pages
5th European Review meeting on Severe Accident Research (ERMSAR-2012)
Cologne (Germany), March 21-23, 2012
The dissolved iodine concentration in the water may not always be homogeneous; like the
water temperature it may show stratified conditions. In such case, only the iodine in the
uppermost layer can participate in the mass transfer. A mechanistic simulation of such
conditions requires the availability of a suitable 3-dimensional flow simulation model for
the water pool. While current CFD codes offer the needed modeling features, their computational effort generally does not allow to run such models in parallel with conventional
containment system codes. A special fast-running 3-dimensional pool simulation code is
currently under development for COCOSYS. Such a model should also be able to give the
needed information for estimating the contact time relevant for iodine mass transfer. Together with such information, the present model of the water-side iodine mass transfer
coefficient based on the film renewal theory can be applied to reactor conditions.
ACKNOWLEDGEMENT
The authors gratefully acknowledge funding of the THAI Iod-23 experiment by the German
Federal Ministry of Econonmics and Technology (BMWi) under project no. 1501361.
REFERENCES
[1]
Sources and effects of ionizing radiation. UNSCEAR 2008 report, Volume II, Annex D
Health effects due to radiation from the Chernobyl accident. United Nations Publication ISBN 978-92-1-142280-1, April 2011
[2]
B. Clément et al., State of the Art Report on Iodine Chemistry. Report
NEA/CSNI/R(2007)1 (2007)
[3]
L. Cantrel, P. March, Mass Transfer Modeling With and Without Evaporation for Iodine Chemistry in the Case of a Severe Accident. Nuclear Technology, 154, 170-185
(2006).
[4]
F. Funke et al., Multi-Compartment Iodine Tests in the ThAI Facility. EurosafeForum Berlin (2004),
http://www.eurosafe-forum.org/files/pe_91_24_1_2_08_thai_funke_271004.pdf
[5]
R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena. Wiley, New York,
ISBN 0-471-07392-X (1960)
[6]
G. Weber et al., ASTEC, COCOSYS, and LIRIC interpretation of the iodine behaviour
in the large-scale THAI test Iod-9. Proceedings of the 17th International Conference
on Nuclear Engineering, ICONE-17, July 12-16, 2009, Brüssel; contents being identical with additional publication in J. Eng. Gas Turbines Power 132, Article 112902,
November 2010
[7]
F. Funke et al., Iodine-steel reactions under severe accident conditions in lightwater reactors. Nucl. Eng. & Des. 166 (1996) 357-365
[8]
W. Klein-Heßling et al., COCOSYS – New Modelling of Safety Relevant Phenomena
and Components. Eurosafe Forum Cologne (2010), http://www.eurosafeforum.org/userfiles/1_05_Eurosafe-2010-COCOSYS-Klein-Hessling.pdf
[9]
L.E. Herranz, J. Fontanet , L. Cantrel, Modeling liquid–gas iodine mass transfer under evaporative conditions during severe accidents. Nuclear Engineering and Design
239 (2009) 728–734
Session 3“Source Term Issues”, paper 3.6
8/8 pages