Modelling Radiolysis of Methyl Iodide
in the Gas Phase
S. R. Bowskill, S. Dickinson, H. E. Sims and G. M. Baston
Organic iodides, exemplified by methyl iodide (CH3I), have been determined to be the dominant form of
gaseous iodine under reactor fault conditions. In order to predict the behaviour of CH3I during an
accident, it is necessary to understand its formation and destruction. Experiments were conducted to
build on knowledge of CH3I behaviour and the rates measured here can be applied to containment models
and used to predict the behaviour of organic iodide during reactor faults.
Background
Experimental Methods
The experimental work was carried out in a small
irradiation vessel (fig. 1). An aqueous solution of
methyl iodide was injected through the septum before
the vessel was subjected to Co-60 gamma irradiation.
Samples of the gas inside the vessel were taken at
regular intervals and CH3I levels were detected by gas
chromatography.
Iodine is one of the most radiotoxic fission products and if released during a
severe nuclear accident poses a threat to public health. Mechanisms exist in
reactor containments to limit this iodine release but a release in the form of
organic iodides, such as CH3I, is still a risk.
The mechanism of CH3I formation is uncertain, but it probably occurs on
containment surfaces and in the aqueous phase. In a PWR containment the
majority of CH3I is expected to be found in the gaseous phase due to its low
partition coefficient and the large gas to liquid volume ratio.
Septum
Thermocouple
pocket
Capillary
tube
A range of experimental conditions were tested.
Results were modelled using a kinetic model of iodine
species in moist air, known as IODAIR, which has been
developed using FACSIMILE [1].
Understanding the behaviour of CH3I decomposition in the gas phase is
therefore important in understanding and preventing possible iodine releases.
2 cm
Figure 1. A standard irradiation vessel
Results
A range of experimental conditions were tested to try and identify any sensitivities of CH3I
decomposition that may be important in a reactor containment under accident conditions. Table 1
summarises the effect of all tested parameters on the rate of CH3I decomposition.
Table 1. A summary of the experimental results and the effect of changing various conditions on the rate of methyl iodide decomposition
Effect of Increasing
Change in Decomposition Rate
Temperature
_
O2
_
Surface Area : Volume
_
Water Vapour
_
Dose Rate
_
Initial CH3I concentration
_
No change
Figure 2. Comparison of IODAIR model with data from CH3I
radiolysis tests at 80°C: effect of initial CH3I concentration. The
points are experimental data and the lines are IODAIR calculations.
↓
↓
↓
↓
No change
Decomposition of CH3I at the low concentrations used in these experiments is an exponential or first
order process (fig. 2) and the model gives a good prediction of this behaviour. This is consistent with a
single dominant decomposition reaction.
Dose-rate dependence appears to be complicated, changing between low (5.5 Gy hr-1) and higher dose
rates (fig. 3). The model is unable to predict this complex behaviour. However, this behaviour suggests
that there could be two main reactants whose concentrations change differently with dose rate.
Conclusions
Possible Mechanism
• Dose-rate dependence is the only
significant sensitivity observed.
Previous modelling work using the IODAIR model has indicated
the importance of the reaction of the electron with CH3I:
CH3I +
e-
→
CH3●
+
• Decomposition rates measured here can
be applied to containment situations.
I-
In air, this reaction will be in competition with the reaction of the
electron with O2:
O2 + e- → O2Experimental results show a strong effect of the presence of
oxygen on CH3I decomposition. Removal of oxygen increases the
decomposition of CH3I, indicating a mechanism that is in
competition with oxygen (fig. 5).
Figure 3. Comparison of IODAIR model with data from CH3I radiolysis
tests at 20°C: effect of dose rate. The points are experimental data
and the lines are IODAIR calculations. Error bars give one standard
deviation
• Experimental data suggest that the
reaction of the electron with CH3I is an
important decomposition mechanism.
Figure 5. The decomposition of CH3I during γ
irradiation in air at 80°C with an oxygen (), air ()
or 1% O2 in N2 () atmosphere. Trend lines are fitted
to each data set.
• The IODAIR model has been shown to
give reasonable agreements with
experimental measurements of CH3I
radiolysis.
References:
1. www.mcpa-software.com
2. S. Dickinson, S. Bowskill, H. E. Sims, “The IODAIR model for radiolysis of gaseous iodine species in air: data comparisons and predictions”, Proceedings of the International OECD-NEA/NUGENIA-SARNET Workshop on the Progress in Iodine Behaviour for NPP
Accident Analysis and Management Paper 3.4.
3. G. M. Baston, S. Bowskill, S. Dickinson, H. E. Sims, “The radiolysis of gaseous methyl iodide in air”, Proceedings of the International OECD-NEA/NUGENIA-SARNET Workshop on the Progress in Iodine Behaviour for NPP Accident Analysis and Management Paper
3.3.
Contact: Susannah Bowskill (e. [email protected] t. 01925 289 995) or Shirley Dickinson (e. [email protected] t. 07894 598 707)