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Comparison of Iodine Behavior in a Containment's Sump Water and in A Transactions of the Korean Nuclear Society Autumn Meeting Gyeongju, Korea, October 27-28, 2016 Comparison of iodine behavior in a containment’s sump water and in a FCVS’s pool Sojin Shima*, Sanggil Parkb, Jaeyoung Leea aHandong Global Univ., Handong-ro 558, Buk-gu, 37554 Pohang, Korea bACT Co., Ltd., IT Venture Town 406, Techno-9-ro 35, Yuseong-gu, 34027 Daejeon, Korea *Corresponding author: [email protected] 1. Introduction thermally oxdated and emitted to containment’s atmosphere with forming volatile molecular iodine. This Iodine is one of major source terms that would be molecular iodine emission rate from sump water can be released into the environment from containment under a represented as fractional rate, the proportion of oxidized very extreme accident conditions. This source term I-, which consider both the oxidation of the ions and the could severely affect the environment with a severe reduction of iodine molecules. The fractional rate of radioactive potentially, which has a high radioactivity oxidation varies on thermal-radiolytic environments and and a high volatility in a high temperature. Difficultly chemical conditions, dependent primarily on the pH and caught and considered to give sustained damage on less on the I- concentration and the temperature of sump thyroid, the iodine source term is protected from water. releasing to the environment. Especially in the over-high temperature and pressure out of the limit of containment 2.1 Effect of pH on iodine’s radiolytic oxidation design during a sever accident, the research and experiments to improve the retention efficiency of pH significantly affects the behavior of iodine in iodine species in a containment and a FCVS(Filtered aqueous phase in a containment. [1, 7] Firstly, the rate Containment Venting System) which vents containment of cesium iodine’s radiolytic oxidation considerably atmosphere in order to stabilize the temperature and the decreases in a high pH, which emits molecular iodine. pressure have been conducted. The volatile molecular iodine is converted from Iodine is produced in and released from fuel rod and I- through radiolytic oxidation, the iodine oxidation converted to other species in a sump water bottom of the reaction with the water radiolytic products, containment, which pass thorough a pool in a FCVS radicals(hydroxyl radical(OH), hydrogen radical(H) and - during venting. The sump water and the pool have electron(eaq )) and molecule(superoxide(H2O2) and dissimilar chemical and thermal conditions and cause molecular hydrogen(H2)) induced by the radiation from different volatile and retentional mechanism of iodine. fission product in the sump water. In this paper, studies of phase and species conversion of The oxidation’s reaction rate and the reacting and iodine have been reviewed in order to fine the efficient produced species changes heavily depending on pH retention conditions in a containment and a FCVS. change. In a high pH, iodine reductant reaction is dominant and hydrogen peroxide and molecular iodine 2. Iodine retention in a containment’s sump water converts into I- through radiolytic oxidation. Cesium iodine’s radiolytic oxidation is related to hydroxyl The majority of released iodine as the form of radical, hydroperoxy radical and oxide ion, and also the molecular iodine combines with cesium, produces reaction is induced by hydrogen peroxide reacting as an cesium iodide aerosol and then is released to oxidant. In the alkaline aqueous phase, molecular containment while a few proportion is emitted to radiolysis productions, hydrogen peroxide and iodine, containment atmosphere. are considerably dependent on pH whose reaction is I2(g)+2Cs(g) → 2CsI(s) (1) considered as the one between hydrogen peroxide and Cesium iodine is retained to a sump water via hypoiodite ion. - - mechanical mechanisms such as gravitational HOI(aq)+OH (aq) ↔ IO (aq)+H2O(l) (3) - - sedimentation and inertial retention. This species is H2O2(aq)+IO ↔ I (aq)+O2(g)+H2O(l) (4) highly soluble so readily retained in the sump water with I- oxidation can produce iodine radical, which is producing I- in the aqueous phase. expected to have a higher solubility than the atom + - CsI(aq) → Cs (aq)+I (aq) (2) having similar solubility, xenon. High pH with phosphoric acid that is from RCS leads to I-(aq)+OH(aq) ↔ HOI-(aq) (5) produce the species that comes from the fuel and HOI-(aq) ↔ OH-(aq)+I(aq) (6) structural materials, which promotes radiolysis and In the range of pH 4 to 9, iodine volatility according to thermal oxidation of cesium iodide and increases I- radiolytic oxidation decreases as pH rise. [7] There solubility and retention of the aerosol. are still uncertainties for the optimal pH, but iodine Most metal iodides are soluble and readily reduced into volatility and emission decreases as pH increase. High an I- in the sump water. However, because of the pH decreases fraction of radiolytic oxidation and nuclides, this retained iodine is radiolytically and Transactions of the Korean Nuclear Society Autumn Meeting Gyeongju, Korea, October 27-28, 2016 promotes to produce iodate that is a thermally stable 2.3 Effect of concentration of iodine ion on iodine’s form, so radiolytic oxidation is dependent to pH. radiolytic oxidation Secondly, hydrolysis of molecular iodine is induced to produce hypoiodous acid that is the less volatile species. The effect of I- concentration on fractional oxidation is Iodine is soluble in water even though it still has less considered independent. In the concentration of I-, some solubility than other halogen elements, and when it is expects the oxidation rate will rise, but the increase of dissolves in aqueous phase, it reacts. the reaction rate is compensated with the risen reaction - + I2(aq)+H2O(l) ↔ I (aq)+HOI(aq)+H (aq) (7) rate between water radiolysis product. - - + 3I2(aq)+3H2O(l) ↔ 5I (aq)+IO3 (aq)+6H (aq) (8) Furthermore, molecular iodine converted from - - - - I2(aq)+OH (aq) ↔ I (aq)+HOI(aq) (9) I species is more soluble into the I solution. - - - - - 3HOI(aq)+3OH (l) ↔ IO3 (aq)+2I (aq)+3H2O(l) (10) I2(aq)+I (aq) → I3 (aq) (11) The reaction occurs bilaterally according to the pH. In Even though the reaction violates the Octant rule, it accordance with the equilibrium calculation, non- follows Lewis theory of acid-base reaction, molecular volatile iodine species such as I- and iodate are iodine, a Lewis acid, and I-, a Lewis base, react and dominant among the all oxidation-reductant potential produce triiodide ion, a Lewis abduct. This reaction has with high pH, but molecular iodine and hypoiodous acid faster reaction rate than the reductive reaction of are dominant in an acidic and oxidizing conditions. That molecular iodine with water. is, the fixed quantity of volatile iodine species formed in chemical reactions until reach the thermal equilibrium 2.4. Organic material’s radiolysis and organic iodide’s (Preliminary kinetic release model) is dependent on pH. conversion in a sump water In severe accident, molecular iodine release rate is decreased by reducing the fraction of iodine transferred Organic materials affect iodine volatility with radiolysis with high pH. In the meantime, not only starting with into carboxylic acid and carbon dioxide that decrease alkaline aqueous phase, but it is also critical to maintain the sump water’s pH. The aqueous pH affects the sump water’s durational pH higher than the titrated significantly steady-state concentration of molecular pH. After the initial accident, the species that decreases iodine in aqueous phase, the iodine volatility has a sump water’s pH inflows into the sump water dependence on consequently radiolysis of organic continuously. Three species dominantly acidize the materials. sump water with dissolved in aqueous phase: (1) carbon The radiolysis of organic materials reacts with dioxide that makes up the containment atmosphere with molecular iodine in an aqueous phase and induces the the great proportion; (2) nitric acid formed by humid production of a volatile and non-volatile organic iodides. atmosphere’s oxidation; (3) carboxylic acid formed by These organic iodides anticipate the decrease of pH and organic materials’ radiolysis. The pH buffer solutions high volatile iodine concentration and that is, the are available to compensate these continuously released organic iodides are given a secondary effect dependent acidic species. on pH. [10] 2.2 Effect of temperature on iodine’s hydrolysis 3. Iodine retention in a pool in a FCVS When significant radiation released into water with The component ratio of iodine in the atmosphere amounts of the fission products in a severe accident, the emitted from containment to FCVS is known as 95% of release brings containment’s inner temperature to rise. aerosol and 5% of molecular iodine and organic iodide The iodine volatility shows the dependence on in the documents of WASH-1233, NUREG/CR-4327, temperature especially at the initial accident, even NUREG/CR-5950. Although only 5% of gaseous iodine though the pH dependence is dominant. In chemical compose the total emission of iodine, many researches reactions between iodine’s ion species, I- and OI-, have conducted about it because it is particular to retain molecular species, I , and water, iodine volatility 2 the released iodine gas in FCVS. Especially for organic decreases as the temperature increases according to the iodide, there are still uncertainties about attaining high transition of the thermal equilibrium. In a high decontamination factor(DF) in an accident condition, temperature, activating the reaction of iodine hydrolysis which is a big scale with radiation. that converts molecular iodine to non-volatile species, the iodine volatility significantly decreases. In an aqueous phase without chemical additives, I-’s oxidation 3.1 Effect of chemical additives on gaseous iodine showed clear decrease in several experiments. retention Nevertheless, it is required to assess the dependence of hydrolysis equilibrium reaction constant in a high In order to increase the DF, chemical additives are temperature refer to the numerous experimental data, inevitably added into FCVS’s pool.
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