A FTIR/chemometrics approach to characterize the gamma radiation effects on iodine/epoxy-paint interactions in Nuclear Power Plants☆
Graphical abstract
Introduction
In the event of a loss-of-coolant accident in a Nuclear Power Plant (NPP), leading to reactor core melt-down, fission products can be released from the nuclear fuel up to the reactor containment building. Among them, iodine is one of the most hazardous radionuclides because of its radiotoxicity and its ability to form volatile species. The amount and kinetics of iodine release to the environment due to containment leakages or containment venting procedure following the accident highly depends on its volatility in the containment. The evolution of the volatile iodine concentration in the containment is thus of main concern. It is determined by a balance between formation and deposition or destruction processes. The understanding of these processes considerably evolved after the performance of the series of Phébus Fission Products integral tests that were undertaken to investigate key phenomena involved in NPP severe accidents and particularly the iodine chemistry in the containment [1], [2], [3], [4]. Phébus test results showed that iodine may enter the containment in gaseous and particulate forms in various proportions depending on the accident conditions [5], [6], [7], [8]. Then, deposition processes occur in the containment (on inner surfaces, mostly epoxy-painted surfaces, in the sump) which reduces the overall airborne iodine concentration. After some couple of hours, the containment airborne iodine concentration reaches a steady state level, lower than concentration levels observed during the early stage of the accident.
Current assumptions to explain the rather fast disappearance of the gaseous iodine amount observed on the very short term of the accidental phase in Phébus tests are largely based on chemical interactions of gaseous molecular iodine (I2) with epoxy-painted surfaces of the containment [9], [10], [11], [12]. The iodine/Epoxy-paint interactions under radiation started to be studied in the sixties [13] and has gained and gained in importance in the 1990′ and later on [14], [15], [16], [17], [18]. Funke has developed an iodine-paint model in 1999 [19] with Marchand's data [20] to reproduce the iodine/Epoxy-paint behaviour under radiation. However, it does not fit well all the data in this area and large discrepancies still exist.
The aim of the present work is to study the Iodine/epoxy-paint interactions under radiation by Fourier Transform Mid-InfraRed Attenuation Total Reflectance (MIR-ATR). The experimental approach consists in characterizing the epoxy-paint before and after being exposed to radiation (gamma-ray), studying the radiation damages by identifying the chemical and functional modifications in the epoxy-paint, and checking their effect on the iodine-paint interactions. The influence of several parameters on the evolution of epoxy-paint MIR-ATR spectra was thus studied: the irradiation dose and the presence of iodine loaded on the epoxy-paint. Infrared spectra were used as responses of full factorial experimental design, built taking into account two factors: the gamma irradiation dose and iodine presence or not (the design is balanced).
The influence of the different experimental factors, as well as their interactions, were simultaneously analyzed using a multivariate analysis technique, the AComDim method [21]. This method for the detection of significant factors has been applied to spectral datasets from a variety of samples, e.g., wine, to study the influence of the vintage (year), maceration method and/or micro-oxygenation; apple, to study the influence of the cultivar and the maturity; starch–lignin mixtures, to study humidity, shape and lignin content [21]. This method has also been used to study the effects of experimental factors on the quality of spectral responses for commercial diesel [22]. In contrast to the ANOVA-PCA method [23], AComDim replaces successfully the separate PCAs with a single analysis to give an evaluation of significance of the effects [24.].
Section snippets
Epoxy-paint samples
The paint studied was obtained by curing the Diglycidyl ether of bisphenol A (DGEBA)-based epoxy prepolymer with a polyamidoamine (PAA) adduct. All the reactants were purchased from Freitag® and used without any further purification. To obtain the epoxy-paint, a mixture of 36 vol % of DGEBA and 64 vol % of PAA were blended at room temperature and deposited on a Teflon mould. After 24 h of drying under an extractor hood, the polymer obtained is a thin film of (85 ± 10) μm of thickness and it has
Results and discussion
The interpretation of AComDim infrared spectra requires as a first step the assignment of the characteristic vibrational bands initially present in the epoxy-paint. In the following discussion, the spectral data of each component of the epoxy-paint (DGEBA and PAA) will be first interpreted separately. Then in order to study the influence of the parameters: the irradiation dose and presence of iodine loaded on the epoxy-paint, all spectral data were analyzed using AComDim approach.
Conclusion
In case of a severe accident on a NPP, fission products can be released from the nuclear fuel to the reactor containment building. Among them, volatile iodine (I2) can be produced and can interact with the epoxy-paint (DGEBA/PAA paint, representative of the paint that covers the inner surfaces of the French nuclear reactor containment buildings). This work aimed to characterize the iodine-paint interactions, to identify the radiation damages on the epoxy-paint, and to check their effects on
Acknowledgments
The authors wish to acknowledge Loïc Bosland from IRSN for ab initio calculations and for scientific exchanges.
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Selected paper from XVI Chemometrics in Analytical Chemistry, 6–10 June 2016, Barcelona, Spain.