atw - International Journal for Nuclear Power | 05.2021

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atw Vol. 66 (2021) | Issue 5 ı September RESEARCH AND INNOVATION 54 Non-destructive Radioactive Tracer Technique in Evaluation of Photodegraded Polystyrene Based Nuclear Grade Ion Exchange Material Pravin U. Singare 1 Introduction Poly styrene-divinyl benzene type ion exchange materials having sulfonic acid and quaternary ammonium functional groups are widely used in various nuclear industries related applications where prolonged service in adverse environmental condition is required. In nuclear industrial applications organic resin materials are used for elimination of radioactive and other ionic impurities from water in moderator circuits [1]. In view of the extensive application of organic ion exchange materials in nuclear industries, their technological development started long back and are now made available at commercial scale to satisfy the needs of these industries. In spite of widespread applications of ion exchange materials, there are some problems related to their durability over a prolonged time period. The consistent performance of these materials depends upon the chemical nature of polymeric resin material, adverse environmental factors namely humidity, acid rain, temperature fluctuation and time of exposure to ultraviolet (UV) radiation, presence of traces of solvents, catalyst, metals and metal oxides from processing equipment and containers. In outdoor applications, these ion exchange materials very often deteriorate due to weathering process creating negative impact on their lifetime [2, 3]. The inability of polymeric resin materials to resist degradation conditions often becomes visible within a short span. In some situations, few hours of exposure to degradation conditions may result in extensive structural damage. The degradation process may lead to macromolecular chain bond breaking resulting in decrease in average molar mass or may lead to cross-linking thereby increasing the molar mass. Aging of polymeric materials will result alteration of polymer properties in long-term due to weathering conditions [4]. As a result, there is an increasing challenge in front of manufacturers to ensure about the life expectancy guarantee of their polymeric resin materials, particularly under the conditions which are difficult for inspection or failure catastrophic [5]. The wide spread utilization of polymeric resin materials has created the condition of emergence related to performance durability of these materials under stringent long-term exposure conditions. These problems related to durability of polymeric resin materials are associated with in-service environmental conditions and handling procedures during maintenance, repair and modifications. Since the repair or replacement of degraded polymeric resin materials is both labour and capital intensive, the durability of these materials is one of the critical issues from both safety and economic point of view. The photo-degradation brought about by solar UV radiations is the most serious problem associated with an organic based resin. The solar waves consist of UV radiations in the wavelengths range of 290 to 400 nm, corresponding to the energies in the range of 415 to 300 kJ/mol. These energies associated with the solar UV radiations are similar to the bond energies of many organic molecules. When specific functional groups of an organic compound absorb UV radiation the chemical reactions are initiated liberating free radicals which further speedup the photo degradation process. Among the UV radiations, most harmful are the UV-B radiations which are in the wavelength range of 280 to 315 nm having high energy in the order of 426-380 KJ mol -1 , while UV-A radiations in the wavelength range of 315 to 400 nm are less harmful having comparatively less energy in the order of 389 and 300 KJ mol -1 [6]. The deleterious effect of these radiations will depend on the chemical nature of the material, climatic conditions namely temperature, humidity, exposure time, presence of traces of solvents, catalyst, metals and metal oxides from processing equipment and containers [7]. Photo-degradation can take place via chain breaking or cross-linking in absence of oxygen and via photo-oxidative degradation in presence of oxygen. In most polymers, elevation in temperature condition and prolonged exposure to pollutants will raise the photooxidative sensitivity thereby triggering the photo-oxidative degradation process [8]. Exposure to UV radiations is usually observed superficially which is indicated in terms of embrittlement (surface cracking), discolouration and loss of transparency. Further exposure to UV radiations will bring about photolytic, photo-oxidative, and thermo-oxidative reactions in the resin materials resulting in the photo-degradation of polymeric resin materials which is usually superficially and slowly degrades the entire material by changing the chemical structure of the polymeric material [9]. Depending upon the nature of the polymeric resin material, the photo degradation may results in polymer chain scission, cross-linking leading to irreversible change in physicochemical conditions and also changes at the molecular level [10]. Subsequent to UV exposure, the polymeric resin material follows different degradation routes via formation of free radicals and breaking of the polymer chains thereby losing its mechanical properties and molecular weight making the materials useless after some time [11]. The photo degradation of industrial grade ion exchange resins operating under severe environ mental stress conditions is a serious problem with economic and environmental implications. Previous research focused mainly on the study of bulk mechanical properties, surface chemistry and surface morphology of the polymeric materials exposed to UV radiations for different exposure period [12, 13]. The study was also performed to understand the role of sensitizers in accelerating the efficiency of the photo-degradation Research and Innovation Non-destructive Radioactive Tracer Technique in Evaluation of Photo- degraded Polystyrene Based Nuclear Grade Ion Exchange Material ı Pravin U. Singare
atw Vol. 66 (2021) | Issue 5 ı September and the mechanism of UV light induced photo-oxidation and photolysis processes in polymeric materials [14, 15]. A detail review was published on photo-degradation of polystyrene polymers which emphasise mainly on the degradation mechanism of polymers and role of stabilisers in photo- degradation [16]. However, in spite of extensive application of polymeric resin materials in nuclear as well as in the chemical industry, not much work is reported in the literature related to performance of photodegraded polymeric resin materials [17]. Therefore, in the present investigation, a systematic study was performed on the ion uptake reaction kinetics and uptake behaviour of fresh and photo-degraded nuclear grade anion exchange resin Indion GS 300, using non-destructive radiotracer analytical technique. 2 Materials and Methods 2.1 Ion exchange resin The anion exchange resin Indion GS- 300 as supplied by the manufacturer (Ion exchange India Limited, Mumbai) was a nuclear grade resin in the OH-form with quaternary ammonium functional group having polystyrene matrix. The particle size was in the range of 0.3 to 1.2 mm, operating pH range was 0-14, maximum operating temperature of 60 °C having the exchange capacity of 1.40 meq./mL. The moisture content of the resin was 51.9 %. 2.2 Radio isotope used The 82 Br radioactive tracer isotope used here was supplied by Board of Radiation and Isotope Technology (BRIT), Mumbai, India. The isotope used is an aqueous solution of ammonium bromide in dilute ammonium hydroxide having half life of 36 d, radioactivity of 5mCi and γ-energy of 0.55 MeV [18]. 2.3 Conditioning of the ion exchange resin The resin grains of 30-40 mesh size were used for the present investigation. The soluble impurities of the resin were removed by repeated soxhlet extraction using water. Moreover, distilled methanol was used occasionally to remove non-polymerized organic impurities. The resin in hydroxide form was converted into bromide form with 10 % potassium bromide in a conditioning column. Then the resin was washed with distilled deionized water until the washings were bromide free. The resin in the bromide form was air dried over P 2 O 5 and used for further study (hereafter referred as fresh resin). 2.4 Photo-degradation of ion exchange resin The photo-degradation of the resin was carried in a UV chamber in an ambient atmosphere where the temperature was nominally 25 °C- 29 °C, with a humidity of 30-50 %. The resin was photo-degraded in a UV chamber by exposing them to radiation of wavelength 284 nm and 384 nm for 24 h. After 24 h the degraded resin was washed with distilled water and ethanol mixture to remove the degraded polymeric fractions. The resin was then air dried and used for further study (hereafter referred as λ 384 photo degraded resin and λ 284 photo degraded resin). 2.5 Study on bromide ion uptake and reaction kinetics The fresh and degraded resins in bromide form weighing 1.000 g (m) were equilibrated with 200 mL (V) labeled bromide ion reaction medium (0.200M) of known initial activity (Ai) at a constant temperature of 30.0 °C. The temperature of the reaction medium was maintained constant using an in-surf water bath. The bromide ion-isotopic exchange reaction can be represented as: R-Br + Br* - (aq.) ⇌ R-Br* + Br - (aq.) (1) Here R-Br represents ion exchange resin in bromide form; Br* - (aq.) represents aqueous bromide ion reaction medium labeled with 82 Br radiotracer isotope. The activity in counts per minute (cpm) of 1.0 mL of the reaction medium was measured at an interval of every 2 minutes for 3 h. The solution was transferred back to the same bottle containing labeled reaction medium after measuring the activity. The final activity (A f ) of the equili brated labeled bromide ion reaction medium was measured after 3h. The activity in counts per minute (cpm) was measured with γ-ray spectro meter equipped with NaI (Tl) scintillation detector. The activity measured at various time intervals was corrected for background counts. The procedure adopted for labeling the reaction medium was same as mentioned previously [19]. The percentage and amount of bromide ions exchanged on the resin in mmol were obtained from the A i , A f , values and the amount of exchangeable bromide ions in 200 mL of reaction medium. The study was extended further by equilibrating the fresh and degraded resins with 0.300M and 0.500M labeled bromide ion reaction medium at 30.0 °C. Similar set of experiments were repeated by equilibrating 1.000 g of fresh and degraded resins in bromide form with 0.200M labeled bromide ion reaction medium at higher temperatures up to 45.0 °C. 2.6 Fourier-transform infrared spectroscopy (FTIR) analysis FTIR analysis of fresh and photodegraded resins was performed using a Bruker Optik, ALPHA-T FTIR spectrometer having gold mirror interferometer with ZnSe beam splitter. The ATR probe consisted of a zinc selenide focusing element and a diamond internal reflectance element. The probe was brought into intimate contact with the sample surface using mechanical pressure. 32 scans were collected over the spectral range of 400 cm -1 to 4000 cm -1 . The probe was purged with dry air and background spectra were collected before each sample spectrum was taken. 2.7 Scanning Electron Microscopy (SEM) analysis The degradation studies of ion exchange resins was also studied by examining the surface morphology of fresh and photo degraded resin samples using JSM-6380LA Scanning Electron Microscope (Jeol Ltd., Japan). The powders were precisely fixed on an aluminum stub using double sided graphite tape and then were made electrically conductive by coating in a vaccum with a thin layer of carbon, for 30 seconds and at 30 W. The pictures were taken at an excitation voltage of 15 KV and a magnification of ×250 to ×500. 3 Results and Discussion 3.1 Effect of photo-degradation on isotopic ion uptake reaction kinetics In the present investigation it was observed that the initial activity of solution decreases rapidly due to rapid ion uptake reaction, then due to slow ionic uptake the activity of the solution decreases slowly due and finally remains nearly constant. Previous investigations have shown that the above ionic uptake reactions are of first order, as a result the logarithm of activity when plotted RESEARCH AND INNOVATION 55 Research and Innovation Non-destructive Radioactive Tracer Technique in Evaluation of Photo- degraded Polystyrene Based Nuclear Grade Ion Exchange Material ı Pravin U. Singare
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