Interactions of Fission Product Vapours With

233 CH9700208 INTERACTIONS OF FISSION PRODUCT VAPOURS WITH AEROSOLS C.G.Benson1 and M.S.Newland1 Abstract Reactions between structural and reactor materials aerosols and fission product vapours released during a severe accident in a light water reactor (LWR) will influence the magnitude of the radiological source term ultimately released to the environment. The interaction of cadmium aerosol with iodine vapour at different temperatures has been examined in a programme of experiments designed to characterise the kinetics of the system. Laser induced fluorescence (LIF) is a technique that is particularly amenable to the study of systems involving elemental iodine because of the high intensity of the fluorescence lines. Therefore this technique was used in the experiments to measure the decrease in the concentration of iodine vapour as the reaction with cadmium proceeded. Experiments were conducted over a range of temperatures (20 - 35O°C), using calibrated iodine vapour and cadmium aerosol generators that gave well-quantified sources. The LIF results provided information on the kinetics of the process, whilst examination of filter samples gave data on the composition and morphology of the aerosol particles that were formed. The results showed that the reaction of cadmium with iodine was relatively fast, giving reaction half-lives of approximately 0.3 s. This suggests that the assumption used by primary circuit codes such as VICTORIA that reaction rates are mass- transfer limited, is justified for the cadmium-iodine reaction. The reaction was first order with respect to both cadmium and iodine, and was assigned as pseudo second order overall. However, there appeared to be a dependence of aerosol surface area on the overall rate constant, making the precise order of the reaction difficult to assign. The relatively high volatility of the cadmium iodide formed in the reaction played an important role in determining the composition of the particles. Microscopic examination of the particles showed that they were homogeneous and contained non-stoichiometric quantities of cadmium and iodine. The vapour pressure of cadmium iodide is significant at the temperatures at which some of the experiments were conducted, and therefore, it was concluded that co-condensation occurred to form homogeneous particles containing mixtures of cadmium and cadmium iodide. The implications of vapour-aerosol interactions for severe reactor accidents have been examined, and recommendations for further studies have been made. The LIF technique has been shown to be particularly useful in the study of the kinetics of vapour-aerosol reactions where the fluorescence intensity of the species in the vapour phase is appropriate. It could therefore be used to study other relevant systems (e.g. reactions with surfaces). 1 Chemical Physics Department, Materials and Chemistry Group, AEA Technology pic, Winfrith, UK. 234 1. INTRODUCTION The interaction of fission products with structures and aerosols in the primary circuit during a severe accident in a light water reactor (LWR) will determine the magnitude of the source of radioactive material released to the reactor containment. This could have a significant effect on the source term to the environment, notably for accident sequences in which the containment building is by-passed. A recent state-of-the-art report on the source term [1] identified the interaction of fission product vapours with aerosols and surfaces as important areas of uncertainty in primary circuit modelling codes. Suspended aerosol particles readily serve as condensation or absorption sites for fission product vapours. The majority (> 90%) of the aerosol resulting from an accident could be associated with that generated from the predominantly non-radioactive components of the reactor core (e.g. control rods, soluble absorber, structural materials). The subsequent transport and distribution of the fission products in the environment will depend on the behaviour of the host aerosol. However, whilst the importance of this phenomenon has been established, few studies have been undertaken to characterise and quantify vapour-aerosol interactions in situ, with the majority conducted with fission product vapours and deposited material [2]. Models to describe all potential interactions are not included within the relevant codes. For example, VICTORIA [3] only restricts the extent of reaction by thermodynamic considerations or mass transfer; whilst MELCOR [4] uses kinetic models, it contains insufficient data on the rate laws and rate constants to be applied to each specific interaction. This report describes the results of the studies of experiments conducted to examine the interaction of cadmium aerosol and I2 The experiments have principally utilised laser induced fluorescence (LEF) to obtain data on the kinetics of the reaction between iodine vapour and cadmium aerosol, although post-test analysis techniques have also been used to characterise the resulting aerosols. LEF requires the species of interest (i.e. iodine) to absorb some of the incident light and subsequently relax back to its ground state, releasing energy in the form of light. The amount of light released (the fluorescence intensity) can be used to indicate the concentration of the species. Therefore, the depletion in vapour-phase iodine concentration resulting from the reaction with cadmium has been examined using the technique. 2. EXPERIMENTAL DETAILS Before the interaction experiments could be performed the iodine vapour and cadmium aerosol generation rigs were calibrated so that the vaporisation rate as a function of time was known for a given temperature. The iodine vapour and cadmium aerosol were generated in separate furnaces and introduced into the main reaction vessel. The matrix of vapour-aerosol experiments is listed in Table 1. Experiments were conducted at different temperatures, and the concentration of cadmium was varied by changing the temperature of the source, as listed in the table. 2.1 Description of the Facility The facility consisted of three sections: (i) The iodine generator (Figure 1) consisted of a specially designed TT-tube that enabled the carrier gas to by-pass the iodine source until the correct generation temperature had been reached. This 'IP-tube was contained in a temperature controlled water bath so that 235 any temperature within the range from 20 - 60°C could be achieved. For experiments above 20°C the pipework connecting the generator to the reaction vessel was heated using hot air blowers. (ii) The cadmium generator (Figure 2) consisted of a Nichrome wire wound silica tube, insulated with glass fibre blanket and Dalfratex insulating tape. An alumina crucible containing the cadmium was located inside the furnace and the temperature measured using a 'K'-type thermocouple. The furnace was connected to the furnace by a tube (length 150 mm> ID 15 mm) which was heated to 320°C using a heating tape connected to a variable resistance. The temperature of this pipework was also measured using a 'K'- type thermocouple. (iii) The main reaction vessel (Figure 3) consisted of a Pyrex glass tube (length 580 mm, OD 28 (±0.5) mm^ wall thickness 2 (±0.2) mm). The iodine generator was connected to the inlet and introduced into the aerosol gas flow through an inner glass tube (length 180 mm, OD 12 mm), which was blanked off at the end with four holes (diameter 3 mm) placed 6 mm from the end. The cadmium generator was connected at the same end at an angle of 45° to the tube. 100 mm capillaries (0.5 mm ID) were positioned at ~ 50 mm intervals along the length of the vessel, starting at 105 mm from the inlet. These capillaries contained 'K'-type thermocouples used to control the temperature of the reaction vessel (see Section 2.4). All thermocouple measurements throughout the system were recorded using a Schhimberger 'Orion' datalogger. The outlet of the reaction vessel was connected to a dilution chamber (length 100 mm, OD 65 mm). This had connections for dilution flow (if used), an optical window for the laser, an aerosol sampling port connected to a Nuclepore filter holder and pump, and a gas outlet. The gas outlet was connected to a 250 cm3 bubbler containing 100 cm3 carbon tetrachloride and 50 cm3 sulphuric acid (pH = 5.6). The bubbler was then connected to an absolute filter before venting to the local extract system. 2.2 Furnace Calibration 2.2.1 Iodine Helium was used as the carrier gas at a flow rate of ~1 1 min1. The water bath was set to the required temperature (20-50°C) and allowed to equilibrate. The 0°C point was obtained using an ice/water bath. Experiments performed above room temperature involved heating the tube between the generator and the bubbler to ensure that there was no condensation. When the required thermal-hydraulic conditions had been attained, the carrier gas flow was switched so that it passed through the iodine source. The vapour was collected in a 0.1M sodium hydroxide (NaOH) trap. After a set time period, the carrier gas was diverted through the by-pass system, and the apparatus switched off and allowed to cool. The mass of iodine vaporised from the generator was calculated by weighing the TT-tube before and after each experiment. The concentration of iodine in the bubbler was determined using inductively coupled plasma optical emission spectroscopy (ICPOES). The calibration graph is shown in Figure 4. 2.2.2 Cadmium Generator The carrier gas was helium (see Section 2.2.1 above) controlled by a similar flow controller to the iodine generator set to a flowrate of 10 1 min1. A pre-weighed alumina crucible containing 236 the cadmium (~ 5 g) was placed in the centre of the furnace. A 'K'-type thermocouple was positioned inside the furnace to monitor the temperature of the crucible. The delivery tube between the furnace and the bubbler was wrapped in heating tape and maintained at a temperature slightly higher than the melting point of cadmium (~321°C, monitored using a 'K'- type thermocouple) so that the aerosol generated reached the bubbler, or the reaction vessel (in a vapour-aerosol experiment), without condensing.

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