IODINE CHEMISTRY
S DICKINSON1, H E SIMS1, E BELVAL-HALTIER2, D JACQUEMAIN 2, C POLETIKO2, F
FUNKE3, Y DROSSINOS4, E KRAUSMANN4, B HERRERO5, T ROUTAMO 6
and B J HANDY7
1
AEA Technology plc, Winfrith, Dorchester, Dorset GB-DT2 8DH, UK
2
CEA/IPSN, CE Cadarache, FR-13108 Saint Paul lez Durance, France
3
Siemens AG KWU, Freyeslebenstrasse 1, PO Box 3220, DE-91050 Erlangen, Germany
4
CEC/JRC Institute for Systems, Informatics and Safety, IT-21020 Ispra, Italy
5
Instituto de Technologia Nuclear, Avenida Complutense 22, ES-28040 Madrid, Spain
6
IVO Power Engineering Ltd, Rajatorpantie 8, Vantaa, FI-01019, Finland
7
NNC Ltd, Booths Hall, Chelford Road, Knutsford, GB-WA16 8QZ, Cheshire, UK
SUMMARY
A shared-cost action on Iodine Chemistry has been completed as part of the CEC 4th
Framework programme on Nuclear Fission Safety. Organisations from six EC countries are involved
in an integrated programme of experiments and analysis focused on understanding and quantifying the
effects of silver on iodine behaviour. This work has provided new experimental data which will be
used to validate and improve the existing models and to stimulate code development.
In summary, this programme has provided extensive experimental kinetic data on the
reaction of aqueous iodine with silver surfaces. Experiments have shown that the volatility of iodine
from irradiated iodide solutions is greatly reduced in the presence of excess silver, and that this can
be understood in terms of the thermal reactions of I2 with the silver surface. Moreover, the AgI
product appears stable to irradiation at the dose rates studied.
The experimental data have been used to formulate kinetic models, which have been
incorporated into iodine chemistry codes and used in a source term evaluation. These calculations
have shown that reaction with silver has significant potential to reduce the formation of volatile iodine
under some severe reactor accident conditions. The effect of this reduction on the predicted release
of iodine to the environment depends strongly on the sequence being considered. The presence of
silver has the greatest potential impact under conditions of low pool pH and high Ag / I ratio.
A.
INTRODUCTION
Iodine is one of the most important fission products which would be released in the event of a
severe reactor accident. Plant assessments have shown that it contributes significantly to the source
term for a range of accident scenarios. However, the considerable differences between the iodine
behaviour observed in Phebus Test FPT0 and that predicted by containment chemistry calculations
indicate that the current models do not correctly treat all of the phenomena that could be important in
a reactor accident [1,2]. In particular, the presence of a large quantity of silver aerosol appears to
influence very strongly the volatility of iodine from solution.
It is assumed that iodine released from the fuel in a severe accident would be transported to the
containment primarily in the form of iodide. This would dissolve in containment water pools to give
involatile I-, which could then be oxidised under irradiation to volatile I2 . Reaction of either I- or I2
with silver to form insoluble AgI would substantially lower the amount of I2 formation, which could
have important consequences for active and passive safety measures and for accident management
strategies.
The shared-cost action F14S-CT95-0005 was a two-year programme which started in January
1996 and was completed on 31 December 1997. The project was focussed on understanding and
quantifying the effects of silver on iodine behaviour, and providing new experimental data that will be
used to validate and improve the existing models and to stimulate code development.
B.
WORK PROGRAMME
The work programme was divided into three activities: Activity 1, Experimental Studies; Activity
2, Assessment, Analysis and Model Development; and Activity 3, Source Term Evaluation.
Activity 1 Experimental Studies
This activity comprised three experimental programmes whose objective was to provide new
data on the reaction of aqueous iodine with silver under accident-relevant conditions. The two main
questions being addressed in this work were:
(i) How quickly are I- and I2 removed from solution by reaction with silver surfaces? and
(ii) Is the AgI formed in these reactions stable under irradiation?
This activity was divided into the four tasks described below.
Task 1.0: Definition of the experimental requirements, and in particular the range of conditions
and variables to be studied, based on the findings of the literature survey (Task 2.1) and results from
the Phebus-FP programme.
Task 1.1: Non-irradiated experiments to measure the rate of uptake of aqueous iodine (I2 or I-)
onto silver surfaces in the absence of radiation, in order to provide detailed kinetic data to
supplement and extend the existing database and to clarify anomalies in previous work. The main
focus of this work was initially to investigate the effects of impurities on the reaction rates. As the
work progressed the different behaviour of different types of surface became evident and additional
tests were performed to clarify these observations.
Task 1.2: Experimental studies on the possible effects of irradiation on the silver - iodine
reaction. The radiolytic formation of nitric acid in irradiated air-water mixtures could lead to the
production of Ag+, which could react directly with I- ions in solution. One of the objectives of this
task was therefore to provide experimental data on the rate of HNO3 production at elevated
temperature. The other objective was to quantify the effect of silver on the volatility of iodine from
irradiated solution, and in particular to assess whether the AgI formed is stable to irradiation.
Task 1.3: High-temperature radiolysis experiments. Results from Phebus test FPT0 indicated
that silver could be partially present in the sump as a colloidal suspension together with soluble silver.
In this task, therefore, the stability of colloidal AgI species is studied under conditions relevant to a
severe accident, to quantify the effects of temperature, radiation and Ag+ concentration on the
decomposition of silver iodide.
Activity 2: Assessment, Analysis and Model Development
The overall objective of this activity was to assess both new and existing experimental data on
iodine interactions with silver, and to develop a suitable model for incorporation into different iodine
chemistry codes. This activity was divided into the following three tasks:
Task 2.1: Literature review on the heterogeneous reactions of aqueous I- and I2 with silver
surfaces, with particular emphasis on kinetic data, in order to identify remaining uncertainties and
gaps where further research is required.
Task 2.2: Analysis of new data from the project, together with existing data from previous
programmes, with the objective of establishing the reaction mechanism and identifying the ratedetermining process and appropriate rate constants.
Task 2.3: Development of a kinetic model describing the reaction of iodine with silver under
conditions relevant to a PWR severe accident. This has been implemented in four different iodine
chemistry codes (INSPECT, IODE, IMPAIR and ACT-WATCH) in such a way that the models
used in the different codes are consistent and functionally equivalent.
Activity 3: Source Term Evaluation
The experimental work and model developments in the area of iodine chemistry have been
assessed in terms of those accident sequences which are important in the overall risk assessment of a
nuclear plant. This activity reviewed the implications of the work described above in terms of riskdominant sequences identified in probabilistic safety assessment studies for commercial reactor
plants. This involved generic plant calculations to assess the impact of the model developments on
reactor source term calculations.
C.
MAIN ACHIEVEMENTS
C.1
Experimental programmes
Several programmes of experiments were carried out to provide fundamental data for modelling
the iodine – silver reaction, and to study specific effects of irradiation on the reaction.
C.1.1I2 - Silver Reaction Kinetics
Experiments have been performed to study the kinetics of reaction of I2 in solution with silver
surfaces, and in particular to study the effects of representative impurities ( N O −3 , Cl-). The
measured first order rate constants are plotted against the silver surface / liquid volume ratio [Ag] in
Figure 1. This also shows the best-fit lines for the unstirred and fast-stirred experiments at 25 and
50°C.
The gradient of the regression line gives the pseudo-first order rate constant, or deposition
velocity, kd, which is defined by
−
d [I 2 ]
= k d [I 2 ][Ag ]
dt
The figure shows a strong effect of stirring on the reaction rate, implying that the reaction is
mass-transfer limited, at least in the unstirred or less rapidly-stirred experiments. The difference
between the rate constant at 25 and 50°C is quite small; the higher rate of iodine loss from the
solution at the higher temperature is mainly due to evaporation, as shown by the experiments with no
silver present. The closed diamonds represent tests with various additives; these do not differ
significantly from the other fast-stirred results.
A comparison of the Ag - I2 results with earlier data produced by Siemens [3] shows that the
measured reaction rate is somewhat higher in the more recent tests. Since the reaction is masstransfer limited, this difference can be attributed to differences between the two experimental
facilities. The apparent decrease in the reaction rate with time which was observed in the Siemens
tests probably arises from a combination of effects such as particle agglomeration and carry-over,
which were avoided in the new work by the use of a silver mesh in place of powder.
0.020
0.018
0.016
0.014
no stirring
kd = 1.8E-5 m/s
slow
medium
fast
kd = 1.3E-4 m/s
50°C, fast
kd = 1.7E-4 m/s
various
k s -1
0.012
0.010
0.008
0.006
0.004
0.002
0.000
0
50
100
150
200
250
300
350
-1
[Ag] m
Figure 1: I2 - Ag Reaction Kinetics
C.1.2 I- - Silver Reaction Kinetics
An experimental study of the kinetics of the Ag - I- reaction showed that the reaction of I- with
silver mesh is considerably slower than that of I2 , with pseudo-first order rate constants of 10-8 to 106
m s-1 being observed at 25°C. The reaction rate was slightly higher at 90°C and showed a fairly
weak dependence on pH. The rate was not affected by the presence of 0.01 M concentrations of
chloride or nitrate ions at 90°C and pH ~ 5. However, a significant effect of nitrate was observed at
low pH. This difference may be due to the oxidation of the silver by the nitric acid; however,
oxidation of I- to I2 would also increase the reaction rate.
The decrease in I- concentration with time measured in these tests could be fitted equally well by
a linear (zero-order rate law) or exponential (first-order rate law) correlation. However, the rate
constant was found to be dependent on the initial I- concentration indicating that a zero order rate
law is more appropriate. This could be explained by a two stage reaction involving, for example, an
initial, rate-limiting, surface oxidation step followed by rapid reaction of iodide with the product.
The tests using silver powders showed an initially rapid reaction when the silver was added to
the iodide solutions, which was not observed in the tests with silver mesh. This was attributed to
different degrees of surface oxidation of the silver powders. Further evidence for the role of surface
oxidation is provided by the tests in which silver mesh was immersed in a boric acid solution for 1 or
2 days before adding the iodide; the observed behaviour then resembled the powder tests with an
initially rapid removal of I- from the solution. Furthermore, in some high-temperature experiments in
which the silver was not fully submerged in the solution, unexpectedly fast reaction rates were
observed. This is thought to have been due to enhanced oxidation of the exposed silver in the steam
- air above the solution.
Earlier work by Siemens [3] has already established the importance of oxygen to the Ag - Ireaction, and the differences between the two sets of tests are probably due to the differing degrees
of surface oxidation on the different types of silver used. In the one case where the same type of
silver was used in the two programmes, the results were almost identical.
C.2
Task 1.2: Radiation Effects
C.2.1 Nitric Acid Formation
In this task, the work in the literature on radiolytic production of nitric acid has been reviewed
and an experimental programme on the effects of temperature and silver surfaces on nitric acid
production has been carried out. The temperature and humidity in the containment during a LOCA
would be somewhat higher than used in most studies of radiolytic formation of HNO3 and no
systematic measurement of the effect of temperature on nitric acid formation in air in contact with
liquid water has been reported.
The measured nitric acid yields are shown in Figure 2. The solid line shows the yield for a G
value of 2.2 molecules HNO3/100 eV. The total amount of nitrate formed in the solution was
proportional to the volume of gas in the vial and independent of the liquid volume, confirming that
HNO3 is formed mainly by a gas-phase process.. There was no observable effect of increasing
temperature from 25°C to 90°C on the yield of HNO3. In tests where silver metal was present,
silver was found to be dissolved at a similar concentration to the nitrate.
1.E-04
9.E-05
8.E-05
[NO3-] / mol dm-3
7.E-05
6.E-05
5.E-05
4.E-05
3.E-05
25°C
90°C
2.E-05
Ag1
Ag2
G = 2.2
1.E-05
0.E+00
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Dose / MRad
Figure 2: Nitric Acid Production in Irradiated Air-Water Mixtures
C.2.2 Iodine Volatility from Irradiated Solutions
In this task, the effect of silver on the radiation chemistry of aqueous CsI has been studied. The
experiments showed that the iodine volatility from irradiated solutions was significantly lower in the
presence of silver.
Simulation of these tests with the INSPECT model showed that the results were consistent with
the radiolytically-produced I2 reacting with the silver surface at a mass-transfer controlled rate. The
iodine volatility was therefore determined by the competition of this surface reaction with other
removal mechanisms. The effect of including the I- reaction was minimal, even at pH 7.
When a sample of silver with a surface coating of AgI was placed in a fresh solution of boric
acid, irradiated and sparged, less than 3% of the absorbed iodine was released in 64 hours of
irradiation (total dose 96 kGy). This is consistent the trace solubility of AgI and indicates that there
was no radiolytic decomposition of AgI under these conditions.
C.3
Task 1.3: High-Temperature Radiolysis
A series of bench-scale experiments was performed in order to study the stability of colloidal
AgI species in solution under conditions relevant to a severe reactor accident.
Initial tests showed that AgI colloids can be considered as stable species in solution at 80°C in
the absence of radiation. Irradiated tests showed that the amount of AgI decomposition increased
with temperature, from ~ 0.3% at 60°C to ~ 3% at 110°C. There appears to be no effect of the
initial form of the iodine (I- or I2 ) in solution or of the addition of Ag+, as shown in Figure 3.
3
% iodine deposited on the paint
I2, no Ag+
I2, 10ppm Ag+
2.5
I-, no Ag+
I-, 10ppm Ag+
2
1.5
1
0.5
0
40
50
60
70
80
90
100
110
120
Temperature °C
Figure 3: Stability of AgI Colloid under Irradiation
C.4
Data Analysis and Model Development
The kinetic data obtained in the experiments described above, together with data from
Funke [3], have been analysed with the objective of establishing an appropriate model and rate
constants for the reactions of I2 and I- with silver surfaces. The experimental results indicate that the
reaction of I2 with silver surfaces is limited by the rate of mass transfer of I2 in the liquid phase.
Slower rates observed in the earlier tests [3] can be attributed to different mass transfer conditions in
the early phase, and to agglomeration and/or colloidal AgI production in the later phase. This
reaction can thus be described by
d[I 2 ]
dt
= − k 1 [I 2 ]aq
S Ag
Vl
where
1  1
1 
=
+

k 1  k m k r [ Ag]
(7)
in which km is the mass transfer coefficient, kr = 2×10-4 m4 mol-1 s-1 and [Ag] is the silver
concentration in mol m-3. Since under almost all conditions the reaction with Ag will be very rapid,
kr[Ag] >> km, so (7) becomes:
S
d[I 2 ]
= −k m [I 2 ]aq Ag
dt
Vl
(3)
The importance of O2 on the reaction of I- with silver was demonstrated by the Siemens tests
[3], and differences between the results of the two programmes can be attributed to differing degrees
of oxidation of the starting materials. If an oxide layer is initially present on the surface, the uptake of
I- is limited by the I- mass transfer in the liquid phase. Once the oxide layer has been consumed, the
reaction proceeds much more slowly, probably via a rate-determining oxidation step. This can be
expressed simplistically as
Ag + O2 → Agox
(slow)
H+
Agox + I- → AgI + H2O
(fast)
The rate of I- uptake can thus be described by:
[ ] = − d[AgI ] = − k
d I−
dt
dt
S Ag
1
Vl
[I ]

1  1
1

=
+
k 1  k m k o [ Ag ox ] 
−
where
(8)
in which km is the mass transfer coefficient, and ko has a value of about 2×10-3 m4 mol- 1 s-1. The
oxide concentration [Agox] is given by
d[ Ag ox ]
dt
= k ox [ Ag] −
d[ AgI]
dt
where
kox = ks Sm
(9)
In the above equations, ks is the rate of oxidation per unit surface area (eg in mol dm-2 s-1), and Sm is
the specific surface of the silver (in dm2 mol-1). The value of ks in these tests was
(9 ± 4)×10-11 mol dm-2 s-1 at room temperature, with no clear pH dependence. At 90°C ks increased
to (2.4 ± 0.7)×10-10 mol dm-2 s-1 at pH 4.6 and (6 ± 1)×10-11 mol dm-2 s-1 at pH 7. This reaction can
be neglected at pH > 7 and under conditions where the dissolved O2 concentration is close to zero.
This model gives reasonable agreement with the experimental data for a range of I- and Ag
concentrations. However it should be noted that the rate of iodine uptake is very sensitive to the
initial degree of oxidation of the silver surface, and to the value of kox, both of which are largely
unknown under containment conditions.
C.5
Source Term Evaluation
The objective of this part of the work was to assess the importance of the Ag – I reactions in
terms of those accident sequences which are important in the overall risk assessment of a nuclear
plant. This involved generic plant calculations to determine the impact of the model developments on
the calculated reactor source term for risk-dominant sequences identified in probabilistic safety
assessment studies for commercial reactor plants, similar to those described previously by Dutton
[4]. Calculations were performed using the INSPECT, IMPAIR and ACT-WATCH codes.
Calculations were made for three severe accident sequences: (i) a large break LOCA into
the reactor building, (ii) a large break in the residual heat removal system (RHRS) at intermediate
shutdown, and (iii) a steam generator tube rupture (SGTR). The input data included design details for
the reactor plant and descriptions of the fault progression, thermal-hydraulic and fission product
behaviour as predicted by MAAP 3.0B.
The main parameters of the work were the Ag particle size, and the inclusion (or not) of the
reaction I- with Ag in addition to the I2 reaction. In cases where the I- reaction was important, the
degree of oxidation assumed for the silver particles was an important parameter.
The results showed that the impact of silver - iodine reaction modelling on the predicted
iodine releases to the environment is very sequence-dependent. In the LOCA cases, the predicted
releases are low and dominated by aerosol, so although the inclusion of a silver model can strongly
influence the chemical behaviour of the iodine, the resulting changes in volatile iodine production do
not substantially change the overall release. In contrast, the predicted releases from the auxiliary
building and SGTR faults are dominated by gaseous species due to the relatively low pHs and high
gas flow rates through the water pools. In these cases there is more sensitivity to the modelling used,
both in terms of differences between the models and of the assumptions made in the silver - iodine
modelling (particularly the mass transfer rates and particle sizes). However, silver was generally
found to have little effect on the iodine release in the SGTR case, largely because of the fairly low Ag
/ I ratio (~ 1, compared with ~40 in the auxiliary building sequence).
D.
INTERACTIONS WITH OTHER ACTIVITIES
The results of this work will be particularly important for the interpretation of the Phebus-FP
tests. The results of the first two tests have shown that the containment iodine chemistry is dominated
by the formation of a stable, insoluble compound, which is almost certainly AgI. The new data from
this programme has been applied to the modelling of test FPT1, for example via the PHEBEN
programme.
E.
CONCLUSIONS AND BENEFITS
The 2-year shared-cost action on Iodine Chemistry began on 1 January 1996 and was
completed on 31 December 1997. The main findings are as follows:
•
The rates of reaction of I2 and I- with silver surfaces have been measured in the absence of
irradiation. The I2 reaction is very fast and can be treated as mass-transfer limited. The Ireaction with silver mesh surfaces is much slower and follows pseudo-zero-order kinetics under
the conditions of these studies. Test with silver powders showed much higher initial reaction
rates and this is attributed to differences in the extent of surface oxidation of the starting
materials. The presence of chloride and nitrate impurities has little or no effect on the reaction
rates.
•
The G-value for nitric acid production in air-water mixtures has been measured over the
temperature range 25 to 90°C and confirmed to be in the order of 2, with no effect of
temperature or steam pressure. These tests have demonstrated that the formation occurs
principally by a gas phase process. The G value appears slightly higher when silver is present in
contact with the gas phase.
•
The volatility of iodine from irradiated CsI solutions is greatly reduced when a silver surface is
present. Good agreement is obtained between these test results and INSPECT simulations using
rate constants derived from the non-irradiated studies. Very little volatile iodine is produced
after reaction with the silver surface. There is no evidence for substantial radiolytic
decomposition of the AgI under the dose rates used in this work.
•
Colloidal AgI species in solution are also found to be largely stable under irradiation. The
amount of volatile iodine production increases slightly with temperature, but no effect of pH or
Ag+ are observed.
•
Models have been developed to describe the uptake of I2 and I- onto silver surfaces. The effect
of this modelling on the calculated source term for three representative accident sequences has
been investigated using the INSPECT, IMPAIR and ACT-WATCH codes. This evaluation has
shown that the reaction has significant potential to reduce the formation of volatile iodine under
some severe reactor accident conditions. The effect of this reduction on the predicted release of
iodine to the environment depends strongly on the sequence being considered. The presence of
silver has the greatest potential impact under conditions of low pool pH and high Ag / I ratio.
The results of the work will be used by all the partners to aid in the validation of severe accident
computer codes, and thus reduce the uncertainties associated with determining the consequences of
such accidents. Where the utilities and regulatory bodies within the European Union make use of
plant safety (PSA) codes in which the treatment of source term behaviour is simplified, a key aspect
of the exploitation of this work undertaken within each organisation will be the "benchmarking" of the
relevant PSA codes against the more detailed modelling methods developed through this work.
References
[1] Phebus-FPT0 Preliminary Report, NT IPSN/DRS/SEA/LERES 9/94 (1994).
[2] C A Chuaqui, C Hueber, D Jacquemain and C Poletiko, “The role of silver in containment
chemistry”, NT IPSN/DRS/SEMAR 95/31 (1995).
[3] F Funke et al, 4th CSNI Workshop on the Chemistry of Iodine in Reactor Safety,
Würenlingen, June 1996, NEA/CSNI/R(96)6 (1996).
[4] Dutton, L.M.C., Grindon, E., Handy, B.J., Sutherland, L., Burns, W.G., Dickinson, S., Sims,
H.E., Hueber, C., and Jacqumain, D., 1996. 4th CSNI Workshop on the Chemistry of Iodine
in Reactor Safety, Würenlingen, June 1996, NEA/CSNI/R(96)6, 615-634.