Small scale experiments on organic iodide production from lodine -painted surface interaction.

C. Marchand, M. Petit, OECD workshop on iodine aspects of severe accident management, 18-20 mai 1999, Vantaa, Finland.

In case of a severe accident with acore melt occurring in a Pressurized Water Reactor (PWR), iodine is released to the containment. As iodine is a volatile fission product, it is assumed that most, if not all, of the initial inventory enters the containment. Complex physical and chemical phenomena take place in the containment, depending on conditions such as temperature, dose rate and sump pH. In particular, depending on the nature of paints used in the containment, several processes may lead to the production of organic iodides. In this paper, results for a specific way of organic iodides are presented. Separate effect experiments have been carried out to determine the formation of organic iodides from iodine adsorbed on painted surfaces. The studied parameters include temperature (60-140°C), dose rate (0.05-5kGy/h), species adsorbed (I2, I-), and pH (3, 9). These parameters were chosen to be as representative as possible. For measurements of the gas phase composition, the main analytical methods used were: gas phase chromatography with electron capture detector, gas phase chromatography coupled with mass spectrometry and mass spectrometry. In a first step, some experiments have been conducted at relatively high concentrations in order to maximize the organic iodide production. Then, concentrations have been lowered in order to make the experiments more representative of nuclear power plant accident. The studied phenomenon, paint - iodine interaction, refers to the gas phase. It is pointed out that the obtained results cannot be used for interactions in the aqueous phase because organic iodide production is connected to paint oxydation. Dose rate and temperature effects were first studied separatly and then in conjuction. To compare the different effects, an experiment plan method has been used. This method allowed to determine the important parameters in the production of organic iodides while minimizing the number of experiments to be carried out. The separate effects of temperature and dose rate appeared to be different from joint effects. For separate effects, at relatively high iodine surface concentrations, temperature conducted to high organic iodide production compared to dose rate. It is not the case for combined effects at the same surface iodine concentration. Effects were showed to be dependant on iodine surface concentration. It is the reason why a representative surface iodine concentration is needed to study organic iodide production.