Adsorption and revaporisation studies of thin iodine oxide and CsI aerosol deposits from containment surface materials in LWRs (AIAS)

Forskningsprojekt, 2011 – 2013

During a severe nuclear accident released fission and radiolysis products can react with each other to form new species which might contribute to the volatile source term. Iodine will be released from UO2 fuel mainly in form as CsI aerosol particles and elemental iodine. Elemental iodine can react in gaseous phase with ozone to form solid iodine oxide aerosol particles (IOx). Within the AIAS-2 (Adsorption of Iodine Aerosols on Surfaces) project the interactions of IOx and CsI aerosols with common containment surface materials was investigated. Common surface materials in Swedish and Finnish LWRs are Teknopox Aqua V A paint films and metal surfaces such as Cu, Zn, Al and SS. Non-radioactive and 131I labelled aerosols were produced from a KI solution and ozone with a new facility designed and built at VTT Technical Research Centre of Finland. CsI aerosols were produced from a CsI solution with the same facility. A monolayer of the aerosols was deposited on the surfaces. The deposits were analysed with microscopic and spectroscopic measurement techniques to identify the chemical form of the deposits on the surfaces to identify if a chemical conversion on the different surface materials had occured. The revaporisation behaviour of the deposited aerosol particles from the different surface materials was studied under the influence of heat, humidity and gamma irradiation at Chalmers University of Technology, Sweden. Studies on the effects of humidity were performed using the FOMICAG facility, while heat and irradiation experiments were performed in a thermostated heating block and with a gammacell 22 with a dose rate of 14 kGy/h. The revaporisation losses were measured using a HPGe detector. The decomposition effect of the radiolysis product carbon monoxide was tested on IOx aerosols deposited on a glass fibre filter. Iodine oxide particles were produced at 50 °C, 100 °C and 120 °C and deposited on filter samples in order to study the chemical speciation of the particles. The formation of HIO3 was verified with Raman analysis regardless of the reaction temperature. Furthermore, elemental iodine was also observed in the measured Raman spectra. Probably, iodine oxide particles had reacted with air humidity forming iodic acid and elemental iodine. IOx and CsI particles that were deposited on various sample surfaces were synthesized at 120°C. According to XPS analysis, it seemed that IOx particles were mainly in form of HIO3 on the metal and on the painted surfaces. The XPS spectrum of CsI was observed on all metal and painted samples on which CsI particles were deposited. However, the CsI particles seemed to have dissolved at least partially by air humidity. Iodine was observed at areas outside the caesium iodide deposits on metal and on painted surfaces. According to the XPS analyses, iodine was in oxidised form. The measurements indicated that iodine may have reacted with the oxidized metal surfaces to form metal iodates. Only trace amounts of oxidized iodine were detected on the painted surfaces. An interesting result in the XPS analysis was that a part of the acquired signal from CsI on the painted surfaces seemed to originate deeper from the structure of the paint when it was pre-treated either with heat or gamma irradiation. SEM analysis revealed that heat and gamma irradiation treatment increased the porosity of the paint. Therefore, dissolved CsI may have been transported into the matrix of the paint. Besides copper the studied metal surfaces underwent slow reactions with the iodine of the aerosol deposits which showed in the high revaporisation rates at room temperature and elevated temperatures. On the copper and paint samples it could be shown that these surfaces react more easily with the iodine from cesium iodide deposits. From the chemically converted metal iodides only copper iodide remained on the surfaces after exposure to hot humid air and as well after immersion in boiling water. Both, non aged and fresh paint films, showed to be very reactive towards the iodine in the aerosol deposits. At lower temperatures (< 50 °C) it showed that the solvent rich paint films showed reduced revaporisation. At high temperatures when the paint starts to significantly degrade the release of iodine from the solvent rich paints increased significantly. The containment conditions or conditions of a severe nuclear accident cause the revaporisation of paint solvents and thus an uneven distribution within the paint film. Near to the surface revaporisation starts and thus an higher paint solvent concentration is found in the center of the paint profile where the iodine species migrate to and possibly are chemically converted. Thus, the paint samples that had been for a long time aged at high temperatures showed the least ability to react and retain iodine from the deposited aerosols. Most paint solvents are less water soluble than the iodine species itself. Thus, elevated temperatures and hot water are required to wash out the iodine in the paint matrix. In all paint films iodine was still detected after 4 weeks immersion in hot water (50 °C).