Study of the iodine kinetics in thermal conditions of a RCS in nuclear severe accident

Annals of Nuclear Energy 101 (2017) 69–82

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Study of the iodine kinetics in thermal conditions of a RCS in nuclear severe accident A.-C. Grégoire a,c,⇑, Y. Délicat a,b,c, C. Tornabene a,c, F. Cousin a,c, L. Gasnot b,c, N. Lamoureux b, L. Cantrel a,c a

Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES, Cadarache, Saint Paul lez Durance 13115, France Univ. Lille, CNRS, UMR 8522 – PC2A – Physicochimie des Processus de Combustionet de l’Atmosphère, F-59000 Lille, France c Laboratoire de recherche commun IRSN-CNRS-Lille 1, Cinétique Chimique Combustion et Réactivité (C3R), Saint Paul lez Durance 13115, France b

a r t i c l e

i n f o

Article history: Received 18 May 2016 Received in revised form 11 October 2016 Accepted 19 October 2016

Keywords: Source term Severe accident IODINE reactivity RCS Kinetics

a b s t r a c t During the PHEBUS-FP integral severe accidents simulation tests, gaseous iodine was detected in earlier stages of the simulated accident, coming from the experimental circuit modelling a reactor coolant system. One possible explanation is the existence of some kinetic limitations which promote the persistence of gaseous iodine at low temperature. This paper sums up some analytical and modelling works performed to check this assumption. Results show that the chemical speciation of iodine cannot be calculated by assuming chemical equilibrium, kinetics have to be considered, in particular for oxidising atmosphere with an excess of steam. A kinetic model for gaseous iodine is proposed and qualified by comparison with experimental works. Such modelling should be considered to calculate the transport of iodine through the reactor coolant system for a severe accident because it can significantly impact iodine source term evaluations. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Iodine behaviour in case of a severe accident (SA1) has been widely studied over the past years (Clément et al., 2007) and researches are still ongoing (Haste et al., 2012; NEA-STEM project, 2015) as this fission product is a major contributor to the radiological consequences at short term. Iodine is well known to be strongly volatile and almost entirely released from the degraded fuel (Pontillon and Ducros, 2010; Grégoire and Haste, 2013). The iodine

⇑ Corresponding author at: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES/SEREX, Bât 328, Cadarache, St Paul lez Durance 13115, France. E-mail addresses: [email protected] (A.-C. Grégoire), yathis.delicat@ gmail.com (Y. Délicat), [email protected] (C. Tornabene), frederic.cousin@ irsn.fr (F. Cousin), [email protected] (L. Gasnot), nathalie.lamoureux@ univ-lille1.fr (N. Lamoureux), [email protected] (L. Cantrel). 1 Abbreviations: ASTEC, Accident Source Term Evaluation Code; CHIP, Chemistry Iodine Primary Circuit; DCM, 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminos tyryl)-4H-pyran; EDF, Electricité de France; FP, Fission products; FTIR spectroscopy, Fourier Transform Infra Red spectroscopy; HT, High temperature; ICP MS, Inductively Coupled Plasma-Mass spectrometry; IR spectroscopy, Infrared Spectroscopy; IRSN, Institut de Radioprotection et de Sûreté Nucléaire; ISTP, International Source Term Program; LIF, Laser Induced Fluorescence; NIST, National Institute of Standards and Technology’s; NEA, Nuclear Energy Agency; PFA, perfluoroalkoxy alkanes; PWR, Pressurised Water Reactor; RCS, Reactor Coolant System; SA, Severe Accident; SARNET, Severe Accident Research network of excellence; SOPHAEROS, merging of the SOPHIE and AEROSOLS B2 codes; STEM project, Source Term Evaluation and Mitigation project. http://dx.doi.org/10.1016/j.anucene.2016.10.013 0306-4549/Ó 2016 Elsevier Ltd. All rights reserved.

chemistry occurring in the reactor coolant system (RCS) is complex, involving reaction with other released species (fission products, control rod elements), oxidising or reducing atmosphere combined to a high temperature gradient. During the integral PHEBUS-FP tests (experimental reproduction of a loss of coolant accident at 1/5000th scale (Clément and Zeyen, 2013), an aerosol/gas iodine partition is observed at the cold break of the RCS with some great variations depending on the boundary conditions of the test. The iodine gaseous fraction lied between few percent up to near 90 percent, the main difference probably comes from different reactions occurring in the RCS, partially due to variation in carrier gas composition but most probably due to some different chemical elements present (some tests were performed with control rods made of silverindium-cadmium and another with carbon carbide control rod). The gaseous fraction was not well predicted by the SA simulation software (Van Dorsseleare et al., 2009) and the discrepancies may be due to non-equilibrium gaseous chemical processes (Wren, 1983; Cantrel and Krausmann, 2003), not taken into account in the modelling. This gaseous fraction at the RCS break represents a safety issue because it may impact significantly the iodine releases in the environment (Chevalier-Jabet et al., 2014). Considerable efforts have been devoted over the past years at IRSN to improve the models; these efforts were held both on the validation/extension of the thermodynamic data base used by the ASTEC software (Chatelard et al., 2014) and on the development of thermokinetic models involving iodine in thermal conditions

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as close as possible of a RCS during a SA. One major issue is the lack of reliable data in the literature for iodine chemistry at high temperature (Cantrel et al., 2013). As a part of the International Source Term Program (Clément and Zeyen, 2005), the CHIP experimental program (Chemistry Iodine Primary Circuit) was launched at IRSN in 2008 (Haste et al., 2012). The main objectives of this program were to obtain information on iodine speciation under different circuit boundary conditions and to build up a data base for model validation. A first objective of this experimental program was to study elementary systems in order to provide step by step kinetic data for modelling the iodine chemistry in the RCS conditions, focusing on homogeneous gas phase reactions. This paper deals with the iodine-oxygen-hydrogen, {I, O, H} system. A kinetic scheme describing the reactivity of iodine in this system was developed by a combined approach associating literature data and theoretical studies (Cantrel et al., 2013; Canneaux et al., 2010; Hammaecher et al., 2011; Xerri et al., 2012). The main objective of the present work was to assess the capabilities of this model to predict the reactivity of iodine obtained from two independent experimental series by using various operating conditions and experimental techniques:

(Cousin et al., 2013). In both computations, the same kinetic network was implemented. This paper details in Section 2 the experiments performed by the flame technique and in Section 3 those in flow reactor. The capability of the {I, O, H} kinetic scheme to reproduce the experimental data is discussed in Section 4. 2. Flame study The flame study was performed on a set-up specifically developed for strongly corrosive reactants as iodinated species. The system consists of a flat flame burner which allows to stabilize H2/O2/ Ar low pressure premixed flames seeded with known amounts of HI2. Hydrogen iodide was used as the inorganic source of iodine, as it is under gaseous form at room temperature. The flames stabilized on the burner are one-dimensional (flat), laminar and stationary. All macroscopic variables of the flame (temperature, chemical species molar fraction) vary only with the gaseous flow axis and thus with the distance above the burner. 2.1. Experimental device and methodology


Iodine reactivity was firstly studied in several H2/O2/Ar low pressure one dimensional laminar premixed flames seeded with gaseous hydrogen iodide (HI at 2.85%/v). Detailed flame structure experiments are well suited for high temperature reactivity studies (up to 2000 K) and the determination of thermo kinetics mechanism (Hirschfelder et al., 1953). The temperature and some key chemical species (HI, H2O and OH radical) profiles in the flames were determined by using laser diagnostics. The study was conducted on a device derived from that developed for the studies of the combustion of halogenated species (Vandooren et al., 1992; Devynck et al., 1998; Richter et al., 1994). The high temperature gradient in the studied flames (1600–300 K) is close to the temperature gradient which may occur in a RCS of a PWR in SA.
The second experimental series consists in tests carried out in an open flow reactor in which molecular iodine was injected at high temperature (>1440 K) in a reducing or oxidising atmosphere. The aim of these more global tests was to determine the iodine gas speciation at 423 K after recombination in a strong temperature gradient but in conditions closer to those encountered in SA conditions. The influence of the atmosphere composition and the iodine concentration levels were the main experimental parameters. These tests were performed on the small scale test line initially developed by Lacoué-Nègre (2010) and Gouello et al. (2013) in the framework of the CHIP program. These two experimental sets allow covering a large range of conditions. The main interest of the flame technique, which is well suited for kinetic studies, is to get some concentration profiles which are needed for kinetic validation. However it presents also some limitations. High iodine concentrations have to be used (about 10 000 time higher than in the open flow reactor tests), not representative of SA conditions and high steam content conditions are excluded due to problems to stabilize the flame. Thus it was decided to complete the study by open flow reactor experiments. Even if the experimental data gained from the open flow reactor tests are less detailed than for the flame tests, this experimental device is easier to use and well suited for both low iodine and high steam concentrations. The experimental results obtained by the flame study were then simulated by the ChemkinII-Premix software (Kee et al., 1985, 1989). The data obtained in the open flow reactor device were interpreted with the severe accident ASTEC/SOPHAEROS software

2.1.1. Combustion chamber The measurements were performed in different H2/O2/Ar laminar premixed flames stabilized on a movable vertical flat flame burner. To ensure safe operating conditions, CH4 is firstly used to stabilize a CH4/O2/Ar flame; in a second step, CH4 is progressively substituted by H2 to finally stabilize the desired H2/O2/Ar premixed flames. The burner is placed in a low vacuum combustion chamber equipped with optical ports allowing laser diagnostics and with a quartz sampling microprobe for stable chemical species measurements (Fig. 1). Species concentration and flame temperature profiles were measured by moving the burner vertically. The pressure (8  103 Pa) and the global flow rate (4.18 ln min13) were adjusted to ensure visual stability of the flames as well as their flatness. A detailed description of the experimental device is reported in Délicat et al. (2011), Délicat (2012), and only the main features are recalled here (complementary information are provided in SD Table 1). Special attention was brought to the material of the experimental set up to limit iodine deposition and also subsequent corrosion of the system:
the gas supply lines were made of stainless steel (SS 316 L) or perfluoroalkoxy alkanes (PFA);
the combustion chamber and the gas supply lines were both heated at 100–150 °C to avoid steam condensation and iodine deposition;
the burner is a porous 7 cm diameter and 2 cm thickness made of SS 316 L;
the microprobe for gas sampling consisted of a 5 mm internal diameter quartz probe and ended by a small orifice of 210 lm inlet diameter. SuprasilÓ quartz free of aluminium was used to avoid chemical reactivity with iodine as observed with ‘‘standard” quartz;
the outlet of the combustion chamber was connected to several cold iodine traps filled with liquid nitrogen before the vacuum device. Working in corrosive conditions required a fairly heavy maintenance and a frequent cleaning of the whole system. 2 HI gaz purity 99.9+% (Custom Gaz). Nevertheless, only two gas suppliers – exclusively located in the US – were able to produce this gas and unexpected very long transport delays rendered this gas supply rather uncertain. 3 The gas flow rate are given for the reference conditions: 273 K, 101,325 Pa and are thus noted ln min1.

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Pressure gauge Heated Lines (100 °C)

Microprobe

Gas cell (100 °C)

FT-IR analyses H2 H2O

Steam generator

Studied flame

Quartz windows

Burner

In situ LIF analyses Combuson chamber heated at (100-150 °C) Vacuum

Ar O2 HI

Gas mixing lines heated at (100- 150 °C)

CH4 Mass flow controller

Gas supply lines

Fig. 1. Scheme of the experimental set-up including the gas supply lines, the combustion chamber and the microprobe system.

2.1.2. Measurements of chemical species and flame temperature profiles 2.1.2.1. HI and H2O concentration determination. The concentration profile of HI and H2O was determined by Fourier Transform Infrared spectroscopy (FTIR). The microprobe was connected to a gas cell (2 L, 50 cm length) equipped with gold coated mirrors of reflectivity higher than 90% in the IR spectral region allowing thus an optical path length of 10 m. The gas cell was coupled to a FTIR spectrometer (NEXUS-THERMO OPTEK). Scans were recorded with 0.1 cm1 resolution in the 400–4000 cm1 range. To prevent steam condensation, the gas cell was thermostated at 100 °C. Given the limitation of the FTIR spectrometer in the near IR (4000 cm1), only a narrow spectral zone free of H2O and CO2 interferences could be used for HI detection. HI quantitative determination was performed by integration of three individual peaks located in the 2264.5–2300.7 cm1 spectral range (Doizi et al., 2009)H2O concentration was determined by integration of the entire spectral zone between 1641.1 and 1754.9 cm1. The concentration of HI and H2O are related to the IR absorption by a linear relation (Beer-Lambert’s law). A calibration with known HI/Ar and H2O/Ar gas mixture was performed in order to determine the extinction coefficient of these gases in the spectral range of interest. The accuracy of the measurement in the flame is 8% for HI and 5% for H2O. 2.1.2.2. OH concentration determination. Laser Induced Fluorescence (LIF) with single photon excitation was used to determine profiles of relative concentration of OH (Desgroux et al., 1994; Gasnot et al., 1999). The laser system consisted of a frequency doubled Quantel Nd:YAG (TDL 50) at 532 nm laser pumping a dye laser. By using dyes mixture of rhodamine 640 and 4-(Dicyanome thylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), it was possible to obtain an excitation wavelength adapted to OH (307 nm). Optimal laser energy was selected to ensure linearity of the LIF signal with the laser power density. The laser beam

was focused by a 500 mm focal length lens in the chamber. Laser fluctuations were monitored by a post flame photodiode (Hamamatsu S1722). The fluorescence signal was collected at 90° by a two lens system (f1 200 mm and f2 300 mm of focal length) and focused on the entrance slit of a Jobin Yvon H25 spectrometer. The entrance slit was parallel to the laser axis and the output slit was modified to provide a band pass well suited to the fluorescence band under investigation. The fluorescence signal was amplified by A Photonis XP2020Q photomultiplier. Fluorescence signals and laser intensities were recorded simultaneously by a LECROY digitizing signal analyser at sampling rate of 1 GHz. LIF determinations cannot be performed at a distance from the burner less than 1 mm due to laser beam diffusion on the burner. The profiles of relative concentrations of OH were recorded by exciting the R2(7) line of the (0,0) band of the A-X system at 307 nm. The fluorescence signal was collected in the (0-0) OH vibronic band with a band pass of 20 nm (300–320 nm). In the case of OH (diatomic species), the LIF signal is related to the initial population or number density of the rotational level involved in the one photon absorption process. This population is converted into an absolute concentration using the temperature dependent Boltzmann factor and the quenching variations in the flames. The quenching corrections were experimentally determined from fluorescence time decay and the Boltzmann factor was determined given the temperature profile of the flame (see next paragraph). The spectrometric parameters of OH are well known and were taken from the literature (Lucht et al., 1978). For each position in the flame, the LIF intensity was averaged over 250 laser shots and corrected for the laser intensity fluctuations. The OH profile acquisition was repeated at least twice in each flame condition. Absolute calibration was performed using laser absorption spectroscopy (Lucht et al., 1978; Cattolica, 1979) to convert the fluorescence profiles into absolute concentrations. This technique was applied using the R2(7) line of the A-X system in the reference H2/O2/Ar flame in a zone where the OH concentration is stable

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Table 1 Initial conditions of the laminar premixed flames studied (gas composition is displayed in flow rate and fraction of the total gas flow). Flame

u

Pressure 3

1

0.5

8  10 Pa

2

0.5

8  103 Pa

H2

HI 1

0.63 ln min 15.0% 0.57 ln min1 13.58%

0 0 0.12 ln min1 2.85%

O2

Ar 1

0.63 ln min 15.0% 0.63 ln min1 15.0%

2.92 ln min1 Bal. 2.86 ln min1 Bal.

(Piller, 2003). The OH concentration profile is determined with a uncertainty of 5%. 2.1.2.3. Temperature profile determination. An accurate temperature measurement in the flames is required because of the high sensitivity of the flame structure and related kinetics to the temperature. This parameter is also essential to allow the conversion of the number density into mole fraction for species determination by the LIF technique. The temperature profile was determined by combining thermocouple measurements (<1 mm diameter chromel-alumel) close to the burner with an excitation LIF technique in both the flame front and the burnt gases. The temperature measurements by the LIF technique is based on the determination of the relative population of several rovibrational levels of a given species, which are directly linked to the temperature of the investigated medium – following the Boltzman Law (Rensberger et al., 1989). The LIF temperature measurements were performed by exciting several isolated rotational lines in the OH ground electronic state. The (0,0) vibration band of the A-X system was investigated using the R2(7), R1(3), R21(3) and R2(12) transitions recommended by (Piller, 2003) . The fluorescence signal was collected for these transitions lines and carefully analyzed according to a specific procedure (Piller, 2003; Desgroux et al., 1995). The temperature profile is determined with an uncertainty of ±25 K.

Fig. 2. Temperature profiles obtained in flame 1 (H2/O2/Ar) and flame 2 (HI/H2/O2/Ar).

2.2. Test matrix and main results Preliminary tests showed that flat and stabilized H2/O2/Ar flames seeded with HI and steam can be obtained in a wide range of conditions (HI input up to 3% of the total gas flow, steam injection with a steam/(steam + di-hydrogen) ratio up to 25–50%, total gas flow ranged between 4.2 and 5 ln min1, pressure between 6 and 10 kPa). Nevertheless, due to unexpected difficulties in the HI gasb supply and to relapsing corrosion of the experimental device4, only a reduced set of two flames has been studied. The test matrix consisted in a reference flame composed of H2/O2/Ar (flame 1) and in a second flame seeded with HI (flame 2), as displayed in Table 1. The H2/O2/Ar initial flame is seeded with an amount of HI allowing measurement of this key compound by FTIR technique. The equivalence ratio (defined as the ratio of the experimental fuel-to-oxidiser ratio to the same one in stoichiometric condition), the pressure and the total gas flowrate (4.18 ln min1) are the same for the two flames. The equivalence ratio has been calculated by taking into account the chemical equation corresponding to H2 combustion and to HI combustion:

2H2 þ O2 ¡2H2 O 4HI þ O2 ¡2I2 þ 2H2 O

u ¼ 1=2ðXH2 =XO2 Þ þ 1=4ðXHI =XO2 Þ where X is the molar fraction of the considered species. 4

Linked to the handling of large amounts of iodine up to 200/300 g of HI per day.

Fig.3. H2O molar fraction profile obtained in flame 1 (H2/O2/Ar) and flame 2 (HI/H2/O2/Ar).

The experimental temperature profiles are reported in Fig. 2. The mole fraction profiles are respectively reported in Fig. 3 for H2O, Fig. 4 for the OH radical and Fig. 5 for HI. The temperature profiles obtained in the two flames show a strong temperature gradient in the first cm above the burner and a temperature stabilization at 1500 K in the burnt gases zone. HI injection induces a slight shift of the temperature profile towards the burnt gases zone and above all a significant lowering of the temperature at the burner level (104 K compared to the reference flame 1). The temperature profiles obtained in these two flames will be used as input data in the modelling described in Section 4. The injection of HI induces also a significant lowering of the H2O molar fraction at the level of the burner and a shift of the overall H2O profile towards the burnt gases, as shown on Fig. 3. These observations may be owed to the possible inhibiting properties of HI on the oxidation kinetics, as already observed for halogenated species (Wilson et al., 1969). The final H2O concentration in the

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the burner and an overall shift towards the burnt gases. As for H2O, the final concentration level is the same in the two flames. The HI profile in flame 2 show a significant dispersion in the vicinity of the burner and a molar fraction slightly lower (2.6%) than the injection fraction (2.85%). The missing part (ca 9% of the injected HI) is attributed to losses of HI in the gas supply line and on the porous due to HI corrosion of the stainless steel. HI consumption is complete 4 mm above the burner. 3. Open flow reactor tests The open flow reactor tests were performed in a line developed for the study of aerosols and gaseous iodine formation in thermal conditions as close as possible of a RCS in severe accident conditions (Lacoué-Nègre, 2010; Gouello et al., 2013). In this open system, the reagents are injected in a continuous flow and heated close to 1440–1810 K. At this temperature, all the species are supposed to be under gaseous form at thermodynamic chemical equilibrium. Downstream the fluid is cooled in the reaction zone where chemical reactions take place. This reaction zone is characterised by a strong temperature gradient (from 1440 or 1810 K to 423 K) and a residence time of a few seconds. Chemical species are collected and characterised at the outlet of this transport zone. As low iodine concentrations were requested for this study (in the range 1  108 to 5  107 mol l1), the iodine source was solid molecular iodine. This species is indeed more suited for the generation of such concentrations. At high temperature, whatever its initial form, inorganic iodine is cracked into atomic iodine which is the most stable species in this temperature range.

Fig. 4. OH molar fraction profile obtained in flame 1 (H2/O2/Ar) and flame 2 (HI/H2/ O2/Ar).

3.1. Experimental setup Compared to the experimental set-up described in (Gouello et al., 2013), the line operated here was simplified with only an injection of gaseous iodine (Fig. 6). It consists of a high temperature tubular furnace (VECSTAR VTF7) equipped with an alumina tube (30 mm internal diameter, 1 m long). The reaction zone (from the high temperature zone down to the outlet) is 0.5 m long and is characterised by a strong temperature gradient. The carrier gas is fed up at the inlet of the test line and can be either a mixture of steam/argon or a mixture of H2 (3%vol max)/ argon. Gaseous iodine is obtained by sublimation of molecular iodine pellets and then transported in a low argon flow (0.1 ln min1 NPT) through a specific line (alumina injection tube)

Fig. 5. HI molar fraction profile obtained in flame 2 (HI/H2/O2/Ar).

burnt gases remains nevertheless very close in the two flames owing to the same initial H injected concentration. The OH molar fraction profiles (Fig. 4) are more dispersed than the H2O profiles, especially at the maximum concentration at 1 cm above the burner. The injection of HI induces similar changes in the OH profile as for H2O: a lowering of the OH concentration at the level of

Sampling lines (gas scrubbers and downstream filter)

Steam H2O / H2/ Ar supply line

Main line – alumina tube (1 m long) Upstream zone (423 K)

High temperature zone (> 1440 K)

Downstream zone (423 K)

I2 injecon line (alumina tube)

Outle lange PFA coated

Alkaline gas scrubbers (NaOH 0.1 M) Biphasic gas scrubbers (toluene/diluted HNO3) Aerosol filter (quartz 0.7 µm porosity + PTFE membrane filter) Fig. 6. Typical configuration of small scale test line for the study of the {I, O, H} system.

Common outlet gas scrubber

A.-C. Grégoire et al. / Annals of Nuclear Energy 101 (2017) 69–82

to be directly released in the high temperature zone (Fig. 6). For elemental iodine mass flow rates above 3  109 mol s1, iodine gas generation is based on the solid/vapour equilibrium described by (Sanemasa et al., 1983). A specific iodine generator (IG) was implemented, which can be operated in the 273–323 K temperature range and for carrier gas flow up to 1 ln min1 and pressure up to 0.2 MPa. Lower iodine flow rates were obtained by permeation tubes (PT, DynacalTM) filled with iodine pellets and sealed at both ends. The actual I2gas mass flow rate is controlled by the porosity of the tube which is only temperature dependent in the 30–100 °C temperature range (for a carrier gas flow rate below 1 ln min1 and a working pressure below 0.1 MPa). In the steady state, these generators produce a stable mass flow rate of gaseous iodine. The main line is terminated by 4 independent sampling lines equipped with two gas scrubbers assembled in series and terminated by an aerosol filter. During the test, only one sampling line is operated at the same time, As only gaseous species are expected to be transported in the flow, no inlet filter was implemented on the sampling lines to avoid parasitic iodine retention on the filtering device (Grégoire et al., 2015). These lines are made of PFA for its low affinity towards iodinated species and heated at 423 K up to the first gas scrubber to avoid steam condensation. Two types of lines can be distinguished:
one line is devoted to assess the total iodine mass flow rate in the steady state. The gas scrubbers were filled with an alkali solution (NaOH at 0.1 mol l1) to trap the iodine species readily soluble in aqueous media (as HI) and I2. Samplings of the solutions were performed at different time to determine the kinetic of iodine accumulation. This line is also operated during the heating up and cooling down phases;
the other lines (L1 to L3) are devoted to the determination of the gaseous iodine species released in the 423 K zone. The gas scrubbers are filled with an organic phase (toluene, maintained at 4 °C) and an aqueous phase (Gouello et al., 2013) (Grégoire et al., 2015): The molecular iodine is trapped in the organic phase whereas the other iodinated gaseous species are collected in the aqueous phase. The aqueous solution is diluted HNO3 (pH  1.8) to avoid I2 stripping from the organic phase. Qualification tests have confirmed the complete trapping of I2 in the organic phase in this condition.

iodine distribution from the high temperature zone down to the outlet gas scrubbers. Iodine species are quantitatively recovered in alkaline media. The solutions are analyzed by Inductively Coupled Plasma – Mass Spectrometry (ICP MS) for elemental quantification with an uncertainty of 8% (Bruker 810Ò spectrometer). UV–visible quantification of I2 in toluene was performed with an Agilent 8453Ò spectrometer at 309 and 498 nm with an uncertainty of 6%. 3.1.2. Test line thermal hydraulic boundary conditions The study of the {I, O, H} system featured several thermal hydraulic conditions as reported in Table 2: high temperature zone at 1440 or 1810 K, injection of a mixture of steam/argon or H2(3%)/ argon at 1 or 1.4 ln min1. The fluid temperature in the reaction zone (from the high temperature zone down to the outlet of the alumina tube) was determined for three conditions (noted (a) to (c) in Fig. 7) by chromel/alumel and Pt-Rh(30%)/ Pt-Rh (6 %) thermocouple measurements, with an uncertainty of ±30 K. The thermal profile seems to be slightly affected by the carrier gas composition in the 1100–400 K temperature region with a sharper temperature decrease in steam conditions (up to 85 K cm1, configuration a) compared to the pure argon case (up to 60 K cm1, b). The residence time in the reaction zone ranges from 7 s (c) to 7.5 s (a, b). 3.2. Tests conditions and main results Eight tests were performed with molecular iodine injection in a high temperature gradient. The aim of these tests was to determine 2000 (a) 1800

(b) (c)

1600

Temperature (K)

74

1400 1200 1000 800 600

3.1.1. Test conduct and post test operations The whole test line is slowly heated for 4–5 h in order to match the required thermal hydraulic boundary conditions. Iodine injection in the high temperature zone lasts ca 3 h. The sampling lines are successively operated in the meantime. After the test, the facility is dismantled and each part leached to establish the

400 0

10

20

30

40

50

Distance from HT zone (cm) Fig. 7. Temperature profile obtained in the main line (fluid temperature) – atmospheric pressure – condition (a) H2O/Ar 80/20 – Carrier gas mass flow rate 1 ln min1 (Gouello et al., 2013) (b) Ar, 1 ln min1 (c) Ar, 1.4 ln min1.

Table 2 IOH tests – conditions. Test

IOH-1

IOH-2

IOH-3

IOH-4

IOH-5

Pressure (MPa) Carrier gas flow (ln min1) and composition (v/v%)

Residence time (s)

0.106 1.0 Ar/H2 97.3/2.7 1820 (b) 7.5

0.105 1.0 Ar/H2 99/1 1820 (b) 7.5

0.120 1.4 Ar/H2 98/2 1440 (c) 6.9

0.106 1.0 H2O/Ar 50/50 1810 (a) 7.5

0.106 1.0 H2O/Ar 50/50 1810 (a) 7.5

I mean mass flow rate and I mean concentration in the HT zone Mass flow rate (mol s1) concentration (mol l1) Iodine injection device

4.1  109 3.8  108 IG

4.2  109 3.9  108 IG

1.7  109 1.7  108 PT

4.6  109 4.4  108 IG

2.4  108 2.3  107 IG

HT Zone max.temp (K) and temp. profile (reaction zone)

IG: Iodine Generator, PT: Permeation Tube device, Flow rates given at 273 K and 101,325 Pa.

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the iodine gas speciation at 423 K after high temperature (>1440 K) decomposition of the injected reagents (molecular iodine, steam and di-hydrogen) and their recombination in a strong temperature gradient down to 423 K. The influence of the atmosphere composition (oxidative with injection of steam or reducing with injection of di-hydrogen) and the iodine concentration levels were the main parameters taken into consideration. Significant iodine deposition was observed in the main line (including the outlet flange and the distribution ramp of the sampling lines), both in steam or di-hydrogen atmosphere. Such deposition is supposed to occur at relatively low temperature (close to 423 K as strong deposition are often observed on the outlet flange or PFA distribution ramp), but no trends could be inferred. The nature of the deposited iodine and the deposition process (adsorption or absorption) could not be determined. For some tests, the amount of deposited iodine represents more than 40% the initial injected mass. In this case, the iodine speciation determined in the sampling lines may not be fully representative of the actual one’s at 423 K. For this reason, only the five tests featuring iodine deposits below this limit will be presented and discussed. The test conditions are displayed in Table 2 and the main results in Table 3. The injected iodine mean mass flow rate or mean concentration in the high temperature zone is determined from the total injected element mass (based on the final element mass distributions in the line) and the injection duration assuming a steady injection rate. Given the uncertainty on ICP-MS analyses of each leaching solution, an average uncertainty of 5 % could be estimated.
Three tests (IOH-1, IOH-2, IOH-3) were performed in a reducing atmosphere composed of argon and a low fraction of di-hydrogen (below 2.7 v/v%). The main parameters are the elemental iodine and hydrogen concentrations in the injection zone, so that a reducer over iodine ratio (H/I) between 3200 and 26,000 could be obtained for those tests.
Two tests (IOH-4 and IOH-5) were performed in an oxidising atmosphere composed of a mixture of steam and argon (50/50). The thermal profile determined in conditions (a) was used. Two iodine concentration levels were considered with an iodine concentration about 5 times lower for the IOH-4 test compared to the IOH-5 test so that an oxidiser over iodine ratio (H2O/I) up to 79 000 could be obtained.

3.2.1. Tests in di-hydrogen/reducing atmosphere For all tests, significant iodine deposition was observed in the main line, though remaining below 40% of initially injected iodine. In reducing conditions (absence of O), the iodine detected in the aqueous phase of the gas scrubbers is attributed to HI (as I2 is trapped in the organic phase). Detectable amounts of molecular iodine (in the organic phase) were still observed at 423 K, showing that the {I, O, H} system undergoes some kinetic limitations in reducing conditions and for residence time of several seconds. The maximum temperature seems to have no significant influence

on the kinetic limitations (IOH-3 test) indicating that a temperature of 1440 K seems to be sufficient to obtain a total decomposition of molecular iodine. The H/I molar ratio seems to be the main parameter that governs the kinetic limitations – as the other thermal hydraulic parameters were very close from one test to another (temperature profile, pressure and residence time as reported in Table 2). As this ratio increases, the I2 fraction at 423 K decreases so that the system tends to reach thermodynamic equilibrium.

3.2.2. Tests in steam/oxidising atmosphere In oxidising conditions, the iodine species which could be present in the aqueous phase are HI and HOI. Large amount of HOI are nevertheless excluded because it is an intermediate species and in acidic conditions HOI will react quickly and quantitatively with HI (HOI + I + H+ ? I2 + H2O) and the I2 formed will be then transferred in the organic phase. Therefore we consider that the amount of iodine detected in the aqueous phase is attributed to HI. For the IOH-4 and -5 tests, up to 33% of initially injected iodine is found deposited in the main line. For both tests, the gaseous iodine flow at 423 K is mainly composed of molecular iodine (between 90 and 96 %). The remaining iodine fraction detected in the aqueous phase (9.1% for the IOH-5 test and 3.8% for IOH-4) is attributed to HI. The presence of measurable amounts of HI species indicates that the {I, O, H} system is also kinetically limited as in reducing conditions. The amount of HI species decreases with increasing H2O/I ratio as reported in Table 3.

4. Modelling and discussion A first kinetic scheme has been proposed for the {I, O, H} system (Cantrel et al., 2013; Xerri et al., 2012) but not validated. The elementary reactions used here are detailed in Tables 4 and 5. The data are derived from this first kinetic scheme with a total of 35 reactions. The rate constants come mainly from literature review (NIST data base, 2015), or theoretical chemistry calculations performed previously except for O2 + M ¡ O + O + M (experimental data (Payne et al., 1998)), for OH + I ¡ IO + H (chlorine analogy (Garret and Truhlar, 1979) and for I + OH + M ¡ HOI + M (estimation based on O-I bounding energy). The collision efficiencies for M are taken from (Nishioka et al., 1993) – H2O/18.6/H2/2.85/ O2/1.14), except for Ar for which the 0.7 default value has been applied. The direct rate constants are reported in Tables 4 and 5 for each reaction. The reverse ones are deduced from these direct rate constants and the Gibbs free energy of the reaction which is based on thermodynamic data of the reactants and products. It has been checked that the thermodynamic data used in the input files of the Chemkin software (see Section 4.1) and ASTEC software (Section 4.2) are either the same or very close to each other, with no impact on simulations.

Table 3 IOH tests – main results. Test Carrier Gas composition (v/v%) 3

Reducer H/I ratio  10 Oxidiser (H2O)/I ratio  103 Measured deposits in the main line) Iodine released at 423 K I as I2 I as HI (also possibly I or HOI)

IOH-1

IOH-2

IOH-3

IOH-4

IOH-5

Ar/H2 97.3/2.7 9.9 ± 0.7

Ar/H2 99/1 3.2 ± 0.2

Ar/H2 98/2 26 ± 1.8

H2O/Ar 50/50 79 ± 4

H2O/Ar 50/50 15 ± 0.7

(35.6 ± 2.8)% (64.4 ± 3.5)% (14.1 ± 2)% (85.9 ± 2)%

(26 ± 1.8)% (74 ± 5.2)% (20.5 ± 1) % (79.5 ± 1)%

(42 ± 3.3)% (58 ± 5.5)% (3.7 ± 0.4)% (96.3 ± 13)%

(17.5 ± 1.4) % (82.5 ± 4.8)% (96.2 ± 1)% (3.8 ± 1)%

(33 ± 2.9)% (67 ± 4)% (90.9 ± 6)% (9.1 ± 6)%

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A.-C. Grégoire et al. / Annals of Nuclear Energy 101 (2017) 69–82 Table 4 Elementary reactions involving oxygen, hydrogen and M (molecules for collision). Reaction OH + OH ¡ H2O + O OH + H2 ¡ H2O + H O + H2 ¡ OH + H O + OH ¡ O2 + H H2O + M ¡ H + OH + M OH + M ¡ O + H + M H2 + M ¡ H + H + M O2 + M ¡ O + O + M HO2 + H ¡ H2 + O2 HO2 + H ¡ H2O + O HO2 + H ¡ OH + OH HO2 + OH ¡ H2O + O2 HO2 + O ¡ OH + O2 O2 + H + M ¡ HO2 + M

kdirect(T) (m3 mol1 s1 or m6 mol2 s1) 3

Origin

1.14

1.5  10 T exp(420/RT) 1.8  103 T1.21 exp(19,710/RT) 2 2.67 5.1  10 T exp(26,270/RT) 2.8  105 T0.40 exp(3090/RT) 9 3.5  10 exp(440,000/RT) 2.4  109 exp(416,000/RT) 4.6  1013 T1.40 exp(437,000/RT) 1.6  012 T1.0 exp(494,000/RT) 6.7  101 T1.77 exp(2380/RT) 9.1  102 T1.47 exp(58,100/RT) 2.2  105 T0.88 exp(270/RT) 2.9  107 exp(2080/RT) 2.0  107 5.7  1014 T1.80

Literature review Theoretical chemistry Literature review Theoretical chemistry Literature review Literature review Literature review Experimental Theoretical chemistry Theoretical chemistry Theoretical chemistry Literature review Literature review Literature review

Table 5 Elementary reactions involving iodine, oxygen, hydrogen and M (molecules for collision). Reaction

kdirect(T) (m3 mol1 s1 or m6 mol2 s1)

Origin

I + H2 ¡ HI + H I + H2O ¡ HI + OH I + HI ¡ H + I2 I + OH ¡ HI + O I2 + OH ¡ HOI + I H2 + IO ¡ HOI + H OH + I ¡ IO + H HO2 + I ¡ HI + O2 HO2 + I ¡ OH + IO I + HOI ¡ HI + IO O + IO ¡ I + O2 HO2 + IO ¡ HOI + O2 OH + IO ¡ HOI + O OH + HI ¡ HOI + H OH + HOI ¡ IO + H2O O + I2 ¡ I + IO I2 + H2 ¡ HI + HI HOI + H ¡ I + H2O I + I + M ¡ I2 + M I + H + M ¡ HI + M I + OH + M ¡ HOI + M

241 T1.93 exp(137,300/RT) 30.7 T2.26 exp(184,700/RT) 577.9 T1.72 exp(164,200/RT) 282.9 T1.70 exp(127,700/RT) 12.0 T1.90 exp(12,000/RT) 7.2  107 T3.98 exp(44,400/RT) 1.5  108 exp(267,500/RT) 9.0  106 exp(9060/RT) 2.5  107 exp(3700/RT) 2.2 T2.29 exp(119,400/RT) 8.4  107 8.4  106 exp(4490/RT) 7.2  1012 T5.18 exp(11,900/RT) 9.0  101 T2.28 exp(103,600/RT) 2.2  109 T4.41 exp(19,900/RT) 8.4  107 1.9  108 exp(171,000/RT) 6.0  102 T1.55 exp(13,100/RT) 200 exp(4780/RT) 2  109 T1.87 103

Theoretical chemistry Theoretical chemistry Theoretical chemistry Theoretical chemistry Theoretical chemistry Theoretical chemistry Chlorine analogy Literature review Literature review Theoretical chemistry Literature review Literature review Theoretical chemistry Theoretical chemistry Theoretical chemistry Literature review Literature review Theoretical chemistry Literature review Literature review Estimation

4.1. Simulation of the interpretation of premixed flame tests The Chemkin software is widely used in combustion simulations (Wang et al., 2009) and well suited to model steady laminar one-dimensional premixed flames with the premix workpackage. The kinetics was applied to model the flame 1 without iodine and flame 2 with iodine. The thermal profile reported in Fig. 2 has been used as input data. The Chemkin thermo file format is based very closely on the NASA file format. For IO and HOI, the a1–a7 values were calculated following the formalism of (Kee et al., 1989). They fit Cp(T)/R, H°(T)/R and S°(T)/R coming from the ASTEC database and are displayed in Table 6. For I, I2 and HI the values are based on (Chase, 1998). Fig. 8 shows the H2O and OH profiles simulated by the kinetic network described in Tables 4 and 5. The trends are good with some discrepancies for the H2O levels which are slightly overestimated mainly due to a gap at the burner outlet pointing out a reactivity more pronounced with the kinetic scheme. The comparison between the HI experimental profile with the modelled one is reported in Fig. 9, curve a. It appears that even if the trend is quite good, the kinetic scheme do not reproduce the quantitative evolution of HI: an under-predicting factor of about 2.5 is observed all along the flame. This result can partially be explained by a too low HI molar fraction just at the outlet of the burner (x  0 cm): at this key position the missing iodine is under

Table 6 Thermodynamic fitting coefficients in the chemkin formalism. Species

Fitting coefficients (m3 mol1 s1) 273 < T < 1000 K

1000 6 T < 2000 K

IO

a1 = 3.150918 a2 = 3.52416  103 a3 = 3.22008  106 a4 = 1.97838  109 a5 = 5.91795  1013 a6 = 1.45598  10+4 a7 = 9.95699

a1 = 1.69342 a2 = 6.93689  103 a3 = 5.41679  106 a4 = 1.86749  109 a5 = 2.40545  1013 a6 = 1.45747  10+4 a7 = 17.65954

HOI

a1 = 3.07014 a2 = 3.52416  103 a3 = 3.22008  106 a4 = 1.97838  109 a5 = 5.91795  1013 a6 = 1.45598  10+4 a7 = 9.95699

a1 = 4.48627 a2 = 3.52416  103 a3 = 3.22008  106 a4 = 1.97838  109 a5 = 5.91795  1013 a6 = 1.45598  10+4 a7 = 9.95699

radical iodine form, I. After a deep analysis with some sensitivity calculations, the most consistent explanation for this modelling result is that H radical is overestimated and that leads to a rapid conversion of HI into I by the reaction HI + H ? I + H2. So, in order to reduce H concentration, the rate constants of the two following reactions: (H + H + M ? H2 + M) and (O2 + H + M ? HO2 + M) have been increased by a factor 10 which is the order of magnitude of

A.-C. Grégoire et al. / Annals of Nuclear Energy 101 (2017) 69–82

77

Fig. 8. Comparison between simulated and experimental profiles of H2O and OH.

influence is not very relevant and cannot explain the gap between simulated and experimental HI profile. To see the impact of these rate constant changes, profiles of OH and H2O were simulated again with no great influence on steam but with a slight higher consumption of OH (Fig. 10). The OH profiles are still in good agreement. This comparison experiment-modelling allows to validate the adjusted kinetic scheme on key chemical species induced in iodine chemistry, and in flame conditions which are close of the SA ones, on a thermal point of view. 4.2. Simulation of the open flow reactor tests

Fig. 9. HI profiles for (a) standard simulation (b) with rate constant increases for (H + H + M ? H2 + M) and (O2 + H + M ? HO2 + M) by 10. (c) Temperature profile shift of +25 K.

the uncertainties usually considered. This adjustment induces that the H molar fraction goes down 109–1013. HI profile is simulated again and compared to the experimental results with these changes (Fig. 9, curve b). As expected, the agreement is much better. The curve c in Fig. 9 corresponds to the maximum temperature profile uncertainties that had been checked (T ± 25 K), the

The ASTEC/SOPHAEROS software (Cousin et al., 2013), which computes the fission product and structural materials transport through the reactor coolant system, was used to simulate the experimental results in flow reactor conditions. In the vapour phase, SOPHAEROS computes chemical speciation, vapour condensation and vapour/surface reaction. It considers aerosol nucleation, coagulation processes, deposition mechanisms and aerosol resuspension. A sectional approach is adopted to model the aerosol distribution. The same species composition is considered in all the aerosol size classes. The thermal profile reported in Fig. 7 has been used as input data. The fission product (FP) speciation in the vapour phase is computed assuming a thermodynamic equilibrium. It depends on the element inventory, on the species involved in equilibrium and on

Fig. 10. Comparison between simulated and experimental profiles of H2O and OH with the revised rate constants.

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the associated free energy functions at the given temperature. In ASTEC/SOPHAEROS V2, kinetic reactions can also be used to compute FP speciation. FP species involved only in kinetic reaction are not implied obviously in thermodynamic equilibrium. The kinetic scheme concerning the {I, O, H} system (Cantrel et al., 2013; Xerri et al. 2012) has been implemented in SOPHAEROS and applied on the IOH GAEC tests. In these tests, a priori species are only in vapour form i.e. no species can form aerosols particles. In the simulation no iodine deposition on the line was modelled due to the lack of data describing the iodine loss with temperature and concentration. As observed in Table 3, for tests with a large excess of I2 (IOH-5) or HI (IOH-1) at the outlet, the deposited iodine amount is very close. We can think that the deposition process is similar for I2 and HI and thus has a weak impact on iodine speciation at 423 K. Therefore we consider that the simulated fractions of I2 and HI can be compared to the measured one’s. 4.2.1. Open flow reactor test series – tests performed in reducing condition 4.2.1.1. IOH-1 test. IOH-1 test has been conducted under hydrogen (H2) conditions with argon. It is especially dedicated to study I-H system. The experimental conditions are reminded in Table 2 and the main results in Table 3. Theoretically, only gaseous species can be formed taking account of chemical system involved. Around 64% of injected iodine was found in the sampling lines, 10% was retained in main line HT and 23% was found in the outlet flange. 2% was retained in the PFA tubing. Iodine retention is not clearly explained and iodine speciation (I2 or HI) is not determined. The SOPHAEROS calculation assuming a thermodynamic equilibrium provides only HI at the outlet. No retention is modelled because all species are only in gaseous phase and cannot condense or be sorbed on wall. HI computed at equilibrium is closer to experimental value if it is considered average value coming from sampling line or iodine retention in HI form. If kinetic system is activated, taking into account the adjustment on the rates constant of: (H + H + M ? H2 + M) and (O2 + H + M ? HO2 + M) to better fit the flame data, around 87% of iodine at the outlet is under I2 which is totally in disagreement with experimental value. An analysis of chemical reaction path has been done to understand iodine behaviour under hydrogen (H2). At high temperature, I2 injected is instantaneously decomposed in atomic iodine. Above 1000 K, HI is mainly produced by I + H + M ? HI + M whereas I2 is produced by I + I + M ? I2 + M. In stationary regime, I2 and HI are really produced when the temperature is lower than 1000 K. I radical is mainly consumed to produce I2. At the outlet, a global analysis of each reaction rate is done to know what are the main reactions producing or consuming HI and I2 (criteria 1%) (Table 7). HI is mainly produced by reaction between I2 and H and I and H2. There is no global loss rate of HI during the transport. I2 is only produced by I + I and consumed by reaction with H radical to form HI. Because of a disagreement between experimental values and modelled speciation at the outlet, several kinetic reactions listed

Table 7 HI and I2 global production/loss in stationary regime at the GAEC main line outlet (IOH-1).

HI

I2

Production

Production Loss

Reaction

%

I + H + M ? HI + M I + H2 ? HI + H H + I2 ? I + HI

2.6 46.7 50.0

I + I + M ? I2 + M H + I2 ? I + HI

100 99.6

in Table 4 are modified to increase HI production. Consequently, to promote HI production, kinetic reaction constant of H + I + M ? HI + M, H + I2 ? I + HI and I + H2 ? HI + H have been multiplied by 5. These values correspond to the uncertainty range of kinetic constant of each reaction. In this case, HI fraction at the outlet is increased and reaches 27%. Even if this percentage significantly increases, it is not enough to explain the experimental values. Consistently with the calculations performed in flame conditions, a second hypothesis is tested assuming that H radical amount is underestimated with the kinetic reactions. It is assumed therefore that H2 is in equilibrium with H radical. In this context only reactions including iodine are activated. In this case, 89% of HI is computed at the end of the facility. Results are summarized in Table 8. The computation performed with the carrier gas assumed to be in thermodynamic equilibrium has a higher impact on the nature of the iodine released at the outlet of the line than the modification of the kinetic reaction rate directly involved in I2 and HI production or loss. Indeed, when H2 and H are supposed to be in thermodynamic equilibrium, the global balance between production and loss is modified as follow (Table 9): Finally, the loss of I2 and HI production are enhanced. Because H2 and H are in equilibrium, if H radical is consumed to form HI, it is necessary to also consume H2. The main reaction which produces HI is in this case I + H2 ? HI + H. It allows respecting H2 and H equilibrium, producing H radical with H2. HI production is enhanced. HI production occurs at higher temperature (Fig. 11). This is more realistic because, even if HI is the more stable species at lower temperature, its production is very low at these temperatures. Like the previous calculation, one can see that there is no loss rate of HI under these conditions. The computation assuming H2 and H at equilibrium or using kinetic reaction show that the H2 concentrations are similar (ratio = 1, Fig. 12). It means that H2, using kinetic reaction is approximately at equilibrium and is not influenced by iodine species. Nevertheless, the H radical concentration, which is lower than the H2 concentration, is not at equilibrium with kinetic reaction. It appears several orders of magnitude between the two computations for this radical. It can explain the difference in the HI production rate. The NIST database shows that there is a very large uncertainty on constant rate for H + H + M ? H2 + M reaction at 1000 K and 1500 K, up to a factor 15. In our kinetic scheme, the value used is in the lower range of data provided by the (NIST database, 2015). Table 8 HI, I2 and I fraction at the GAEC main line outlet, experimental and modelling results (IOH-1).

Measurement Thermodynamic equilibrium Kinetic reaction Modified reactions H2 and H computed at equilibrium

HI (%)

I2 (%)

I (%)

85.9 100 5.2 27.7 90.5

14.1 0 87.2 66.5 5.7

0 7.60 5.8 3.8

Table 9 HI and I2 global production/loss in stationary regime at the GAEC main line outlet, H2 and H at equilibrium (IOH-1).

HI I2

Production Production Loss

Reaction

%

I + H2 ? HI + H I + I + M ? I2 + M I2 + H2 ? HI + HI H + I2 ? I + HI

99.9 100 60.7 39.2

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Table 11 HI, I2 and I fraction at the GAEC main line outlet, experimental and modelling results (IOH-3).

Measurement Thermodynamic equilibrium Kinetic reaction H2 and H computed at equilibrium

HI (%)

I2 (%)

I (%)

96.3 99.99 9.3 87.6

3.7 0.0 74.5 5.3

16.2 7.1

Fig. 11. HI, I2 and I production/loss rate (in mol s1), H2 and H at equilibrium (IOH-1 under H2).

Fig. 13. HI, I2, I and HOI production/loss rate (in mol s1), in kinetic mode (without any equilibrium) (IOH-4 under H2O).

molecular iodine assuming thermodynamic equilibrium, too much molecular iodine assuming kinetic reaction for all species and closer results to the experimental values assuming kinetic reaction only for iodine species. Fig. 12. H and H2 concentration ratio between kinetic and equilibrium calculations (IOH-1 under H2).

4.2.1.2. IOH-2 and IOH-3 test. IOH-2 test is also under hydrogen (H2). The hydrogen fraction has been reduced to limit HI production. The experimental conditions are reminded in Table 2 and the main results in Table 3: around 74% of injected iodine was found in sampling lines, 26% was retained. The gaseous iodine repartition at the outlet (I2 and HI) is recalled in Table 10. Modelling results are also given assuming either thermodynamic equilibrium, or kinetic reaction for all species or kinetic reaction only for iodine species, assuming that the carrier gas is in equilibrium. Due to a lower level of H2, the measurement shows that the HI production is lower than in the previous test. IOH-2 test is also well modelled assuming carrier gas equilibrium. IOH-3 test is under hydrogen (H2) but at a lower temperature and with a lower iodine injection. Around 58% of the injected iodine was found in the sampling lines, 42% was retained. Less molecular iodine is found in sampling compared to the previous test but this result has to be analyzed regarding more iodine retention. Modelling results show the same trend (Table 11): no

Table 10 HI, I2 and I fraction at the GAEC main line outlet, experimental and modelling results (IOH-2).

Measurement Thermodynamic equilibrium Kinetic reaction H2 and H computed at equilibrium

HI (%)

I2 (%)

I (%)

79.5 99.99 3.2 78.4

20.5 0.0 89.4 16.4

7.4 5.2

4.3.1.3. IOH tests in reducing conditions: first conclusions. Regarding all GAEC tests performed under H2, it appears that thermodynamic equilibrium computation is closer to experimental results but cannot explain all the speciation found in the sampling lines. If all {I, O, H} species, including carrier gas, are computed with the kinetic systems, computation trends are not satisfying for test IOH-1, IOH-2 and IOH-3. According to NIST database analysis on H2 production with H radical, it appears that in the kinetic scheme, the rate constant seems to be too slow. Assuming a thermodynamic equilibrium only for the carrier gas allows catching general experimental trends. 4.2.3. Open flow reactor test series – tests performed in oxidising condition 4.2.3.1. IOH-4 test. IOH-4 has been conducted under steam conditions with argon. In this case, the global {I, O, H} systems can be studied. Experimental conditions are reminded in Tables 2 and 3. Around 82% of injected iodine was found in sampling lines, the rest was found deposited in the main line. As for the previous tests (Section 4.2.1), no retention is computed. Assuming a thermodynamic equilibrium for all species, SOPHAEROS predicts only molecular iodine at the main line outlet which is close to the experimental results. With all the kinetic systems, around 70% of iodine is released as molecular iodine, 27% as HI, 2% as HOI and 0.3% as I. At high temperature, molecular iodine is decomposed into I radical. Firstly, HI is formed and then HOI. At lower temperature, I radical is consumed to produce molecular iodine (Fig. 13). A global analysis of the main iodine production/loss under steam conditions is given in Table 12. With these hypotheses, the HOI and HI fractions are too high compared to the experimental results. Molecular iodine is formed

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Table 12 HI, I2 and HOI global production/loss in stationary regime at the GAEC main line outlet (IOH-4). Reaction HI

Production

Loss

% (if all kinetic)

% (if Carrier gas at equilibrium)

HO2 + I ? HI + O2 I + H2 ? HI + H I + OH ? HI + O HI + IO ? I + HOI HI + OH ? I + H2O

99.0 0.9 0.0 32.7 67.2

1.5 91.4 5.8 0.0 99.9

I2

Production Loss

I + I + M ? I2 + M H + I2 ? I + HI I2 + H2 ? HI + HI I2 + OH ? HOI + I

99.9 91.3 8.6 0.0

100.0 0.0 0.0 99.9

HOI

Production

IO + H2O ? OH + HOI HI + IO ? I + HOI I + OH + M ? HOI + M HOI + I ? I2 + OH OH + HOI ? IO + H2O HOI + H ? OH + HI HOI + H ? I + H2O

5.5 94.1 0.0 100.0 0.0 0.0 0.0

0.0 2.5 97.4 79.2 10.6 5.4 4.7

Loss

Table 13 HI, I2, I and HOI fraction at the GAEC main line outlet, experimental and modelling results (IOH-4).

Measurement Thermodynamic equilibrium Kinetic reaction Carrier gas at thermodynamic equilibrium

HI (%)

I2 (%)

I (%)

HOI (%)

3.8 0 18.9 3.3

96.2 99.99 65.0 92.4

0.01 4.0 4.2

0.0 12.1 0.1

only with I radical. To enhance this production which occurs at lower temperature, it is necessary to decrease the HOI and HI production. A second computation is done assuming carrier gas at equilibrium, as already performed in reducing conditions. In this case, I2 represents around 92% of total iodine at the outlet. Only 3.3% of HI are formed and 4.2% are in I radical. The production/loss rate profile for I2, HI and I shows there is no HI or HOI production. Only I2 is produced below 1000 K. Because I radical concentration is higher, there is more molecular iodine in this case (Table 13). In Fig. 14 are represented the concentration ratio of several species from the computation with all the kinetic reactions and the computation with only carrier gas at thermodynamic equilibrium. The steam concentration is similar (ratio = 1) between the two computations and the main {O, H} species. Nevertheless there is a large difference for the radicals. HO2 and IO are enhanced with

Fig.14. H2O, OH, IO and HO2 concentration ratio between kinetic and equilibrium calculations (IOH-4 under H2O).

Table 14 HI, I2, I and HOI fraction at the GAEC main line outlet, experimental and modelling results (IOH-5).

Measurement Thermodynamic equilibrium Kinetic reaction Carrier gas at thermodynamic equilibrium

HI (%)

I2 (%)

I (%)

HOI (%)

9.1 0.0 18.1 2.7

90.9 99.99 80.1 96.2

0.0 0.9 0.9

0.0 0.9 0.2

kinetic system. These radicals are involved in the HI and HOI production (Table 13). Consequently, in the second computation, reducing these species concentrations also reduces the HI and HOI production. OH radical concentration is enhanced at high temperature with equilibrium assumption. But at these temperatures, only HI production can occur (Fig. 13) and because OH radical is involved in HI loss (Table 13), it explains also the reason why HI is not promoted with carrier gas at equilibrium. At lower temperature, with the decreasing HI and IO radical concentrations (Fig. 14), there is less HOI production. It is interesting to see that, assuming the same hypothesis on the carrier gas speciation, it is possible to model the experimental trends with only the iodine kinetic system. It appears that this system seems to be sufficient to provide the right iodine speciation. 4.2.3.2. IOH-5 test. Another IOH test in presence of steam has been performed. It is a similar test to the previous one but with a higher iodine concentration. The experimental conditions are reminded in Table 2. Around 67% of injected iodine was found in sampling lines, the rest being deposited in the main line (see Table 3). The SOPHAEROS computation at thermodynamic equilibrium gives only molecular iodine. With higher iodine concentration, using kinetic reactions, I2 fraction increases but the modelling results are closer to the experimental results assuming the carrier gas at equilibrium. In Table 14, gaseous iodine repartition at the GAEC main line outlet is recalled taking into account each hypothesis as regards I2 or HI retention. The modelling results are also given. 5. Conclusion In order to validate the kinetic scheme developed by Cantrel et al. (2013) and Xerri et al. (2012) and describing the reactivity of iodine with steam and di-hydrogen in PWR SA conditions, two independent experimental approaches were developed to gain confidence in the validation process. Iodine reactivity was firstly studied in several H2/O2/Ar low pressures one-dimensional laminar premixed flames seeded with gaseous hydrogen iodide. The temperature and some key chemical species (HI, H2O and OH radical) profiles were measured. The high temperature gradient in the studied flames (1500–300 K) is close to the one which may occur in a RCS of a PWR during a SA. This experimental approach allows the determination of an experimental database describing the chemical behaviour of iodine in such operating conditions and well suited to develop and/or optimize detailed kinetic schemes because we have access to concentration profiles as a function of temperature. The second experimental measurements were performed in an open flow reactor in which molecular iodine was injected at high temperature (>1440 K). The nature of the iodine released at 423 K after recombination in a strong temperature gradient was determined; oxidising conditions (H2O) as well as reducing conditions (H2) were tested. As expected from thermodynamic equilibrium, the presence of steam enhances the release of molecular iodine at the reactor outlet, whereas the presence of di-hydrogen

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induces the formation of large amounts of hydrogen iodide. Nevertheless, 10% of HI are still observed in oxidising conditions and up to 20% of I2 in reducing conditions indicating that the system undergoes kinetic limitations. The capabilities of the thermo-kinetic scheme of {I, O, H} to reproduce the experimental data from these two experimental series was tested. The kinetic scheme was implemented both in the Chemkin software to model the premixed flame and in the ASTEC/SOPHAEROS software to simulate the tests performed in the open flow reactor. The consistency of the thermodynamic data used in the Chemkin and ASTEC input files was previously carefully checked. In the two experimental lines, the measured temperature profiles were used as input data for simulations. The profile of the key chemical species (HI, H2O and OH radical) in the flames can be quite satisfactorily simulated (Chemkin modelling). It indicates the completeness of the kinetic scheme used and a good agreement for iodine is obtained by the adjustment of the kinetic rate constant of only two reactions involving H radicals (H + H + M ? H2 + M) and (O2 + H + M ? HO2 + M) within the uncertainty range of such constant. The HI and I2 amounts measured in the open flow reactor tests are also quite well predicted (ASTEC/SOPHAEROS modelling). This comparison experiment-modelling allows to validate the kinetic scheme of {I, O, H} in thermal conditions of a RCS in SA. The main conclusion deduced from this work is that equilibrium approach assumption is too simplistic and can lead to underestimate the gaseous iodine amount at the break as well as to give wrong iodine chemical speciation. Particularly in steam conditions, kinetics are not enough rapid to reach equilibrium in strong thermal gradient as present in the RCS for SA conditions. Before making computations for SA scenarios, some modelling efforts are still needed to better understand impact of control rod materials (Cadmium, Silver and Boron) on iodine species. This work is ongoing. The kinetic scheme is a first step of a more global modelling which has to be implemented in SA simulation software to better predict iodine source term. Acknowledgments The authors gratefully acknowledge the partners of the ISTP program and Electricité de France (www.edf.com) for their funding contribution to this research.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.anucene.2016.10. 013. References Canneaux, S., Xerri, B., Louis, F., Cantrel, L., 2010. Theoretical study of the gas-phase reactions of iodine atoms (2P3/2) with H2, H2O, HI, and OH. J. Phys. Chem. A 114 (34), 9270. Cantrel, L., Krausmann, E., 2003. Reaction kinetics of a fission product mixture in a carrier gas in the Phébus Primary Circuit. Nucl. Technol. 144 (1), 1–15. Cantrel, L., Louis, F., Cousin, F., 2013. Advances in mechanistic understanding of iodine behaviour in Phebus-FP tests with the help of ab-initio calculations. Ann. Nucl. Energy 61, 170–178. Cattolica, R.J., 1979. Laser Absorption Measurements of OH in an Atmospheric Pressure Methane-Air Flat Flame. Sandia Laboratories Energy Report SAND798717. Chase, M.W., Jr., 1998. NIST-JANAF Themochemical Tables, Fourth Edition, J. Phys. Chem. Ref. Data, Monograph 9, 1–1951. Chatelard, P., Reinke, N., Arndt, S., Belon, S., Cantrel, L., Carenini, L., Chevalier-Jabet, K., Cousin, F., Eckel, J., Jacq, F., Marchetto, C., Mun, C., Piar, L., 2014. ASTEC V2 severe accident integral code main features, current V2.0 modelling status, perspectives. Nucl. Eng. Des. 272, 119.

81

Chevalier-Jabet, K., Cousin, F., Cantrel, L., Séropian, C., 2014. Source term assessment with ASTEC and associated uncertainty analysis using SUNSET tool. Nucl. Eng. Des. 272, 207–218. Clément, B., Zeyen, R., 2005. The Phebus fission product and source term international programs. In: Proceedings of International Conference on Nuclear Energy for New Europe, Bled, Slovenia, 5–8 September. Clément, B., Zeyen, R., 2013. The objectives of the Phébus experimental program and the main lessons learned. Ann. Nucl. Energy 61, 4–10. Clément, B., Cantrel, L., Ducros G., Funke., F., Herranz, L., Rydl, A., Weber, G., Wren, C., 2007. State of the art report on iodine chemistry, OECD report, Nuclear Energy Agency, report NEA/CSNI/R(2007) 1. Cousin, F., Kissane, M.P., Girault, N., 2013. Modelling of fission-product transport in the reactor coolant system. Ann. Nucl. Energy 61, 135–142. Délicat, Y., 2012. Etude de la réactivité de l’iode transporté dans un mélange H2/O2/ Ar en conditions de combustion dans des flammes basse pression prémélangées (Ph.D. thesis of Lille-1) University – Science and Technology (in French). Délicat, Y., Gasnot, L., Pauwels, J.F., Grégoire A.C., Cantrel, L., 2011. Experimental and kinetic study of the iodine reactivity in low pressure H2/H2O/O2/Ar premixed flames. In: proceedings of ICAPP 2011, France Nice, May 2–5, paper 11 354. Desgroux, P., Gasnot, L., Pauwels, J.-F., Sochet, L.-R., 1994. A comparison of ESR and LIF hydroxyl radical measurements in flame. Combust. Sci. Technol. 100, 379. Desgroux, P., Gasnot, L., Pauwels, J.-F., Sochet, L.R., 1995. Correction of LIF temperature measurements for laser absorption and fluorescence trapping in a flame. Appl. Phys. B 61, 401. Devynck, P., Desgroux, P., Gasnot, L., Thersen, E., Pauwels, J.F., 1998. CCl, CH and NO LIF measurements in methane-air flames seeded with chlorinated species: influence of CH3Cl, CH2Cl2 on CCl and NO formation. Combust. Inst., 461–468 Doizi, D., Dauvois, V., Roujou, J.L., Lorin, V., Fauvet, P., Larousse, B., Carles, P., Hartmann, J.M., 2009. Experimental study of the vapour–liquid equilibria of HI– I2–H2O ternary mixtures, Part 1: experimental results around the atmospheric pressure. Int. J. Hydrogen Energy 34, 4275–4282. Garret, B.C., Truhlar, D.G., 1979. Generalized transition state theory. Canonical variational calculations using the bond energy-bond order method for bimolecular reactions of combustion products. J. Am. Chem. Soc. 101 (18), 5207–5217. Gasnot, L., Desgroux, P., Pauwels, J.F., Sochet, L.R., 1999. Detailed analysis of lowpressure premixed flames of CH4+O2+N2: a study of Prompt-NO. , 117. Combust. Flame 117 (1–2), 291–306. Gouello, M., Mutelle, H., Cousin, F., Sobanska, S., Blanquet, E., 2013. Analysis of the iodine gas phase produced by interaction of CsI and MoO3 vapours in flowing steam. Nucl. Eng. Des. 263, 462–472. Grégoire, A.C., Haste, T., 2013. Material release from the bundle in Phébus FP. Ann. Nucl. Energy 61, 63–74. Grégoire, A.C., Kalilainen, J., Cousin, F., Mutelle, H., Cantrel, L., Auvinen, A., Haste, T., Sobanska, S., 2015. Studies on the role of molybdenum on iodine transport in the RCS in the RCS in nuclear severe accident conditions. Ann. Nucl. Energy 78, 117–129. Hammaecher, C., Canneaux, S., Louis, F., Cantrel, L., 2011. A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms (2P3/2). J. Phys. Chem. A 115 (24), 6664–6674. Haste, T., Auvinen, A., Cantrel, L., Kalilainen, J., Kärkela, T., Simondi Teisseire, B., 2012. Progress with iodine chemistry in SARNET 2. In: Proceedings of International Conference on Nuclear Energy for New Europe, 5–7, ISBN 978961-6207-35-5, Ljubljana, Slovenia September. Hirschfelder, J.O., Curtiss, C.F., Campbell, D.E., 1953, The theory of flames and combustion. In: Symposium (International) on Combustion, vol. 4, tissue 190– 211. Kee R.J., Grcar J.F., Smooke M.D., Miller J.A., 1985. A FORTRAN program for modeling Steady State Laminar One-dimensional Premixed Flames. SANDIA National Laboratories Report N°SAND85-8240. Kee R.J., Rupley F.M., Miller J.A., 1989. SANDIA National Laboratories Report, SAND89-8009B, UC-706, Reprinted January 1993: ‘‘Chemkin II: A Fortran Chemical Kinetics Package for the Analysis of Gas Phase Chemical Kinetics”. Lacoué-Nègre, M., 2010, Chimie de l’iode dans le circuit primaire d’un réacteur nucléaire en situation d’accident grave – Etude de mélanges CsI/MoO3 sous vapeur d’eau (Ph.D. thesis of Lille-1). University – Science and Technology (in French). Lucht, R.P., Peterson, R.C., Laurendeau, N.M., 1978. Fundamentals of Absorption Spectroscopy for Selected Diatomic Flame Radicals. National Science Foundation, Report N° PURDU CL 78 06. NEA Source Term Evaluation and Mitigation (STEM) project, 2015. . Nishioka M., Takeno T., Nagakawa S., Ishigawa Y., 1993. NOx emission characteristics of rich methane-air flame. In: A.L Kuhl (Ed.), Dynamics of gaseous Combustion, vol. 151, Progress in Astronautics and Aeronautics, page 141. NIST Computational Chemistry Comparison and Benchmark Database, R.D. Johnson, III, NIST Standard Reference Database Number 17, Release 1.6.8, 2015, http:// cccbdb.nist.gov/ nuclear severe accident conditions, Annals of Nuclear Energy, 78 117–12. Payne, W.A., Thorn, R.P., Nesbitt, F.L., Stief, L.J., 1998. Rate constant for the reaction of O(P3) with IO at T = 298 K. J. Phys. Chem. A 102 (31), 6247. Piller, L., 2003. Formation de monoxyde d’azote dans les flammes prémélangées CH4/C2H6/C3H8/O2/N2: Etude expérimentale par diagnostics laser et modélisation (Ph.D. thesis of Lille-1). University – Science and Technology (in French).

82

A.-C. Grégoire et al. / Annals of Nuclear Energy 101 (2017) 69–82

Pontillon, Y., Ducros, G., 2010. Behaviour of Fission Products under severe PWR accident conditions; the VERCORS experimental programme – part 2: release and transport of fission gases and volatile fission products. Nucl. Eng. Des. 240 (7), 1867–1881. Rensberger, K.J., Jeffries, J.B., Copeland, R.A., Kohse-Höinghaus, K., Wise, M.L., Crosley, D.R., 1989. Laser-induced fluorescence determination of temperatures in low pressure flames. Appl. Opt. 28, 3556–3566. Richter, H., Vandooren, J., Van Tiggelen, P.J., 1994. Decay mechanism of CF3H or CF2CHCl in H2/O2/Ar flames. In: 5th International Symposium on Combustion. The combustion institute, pp. 825–831. Sanemasa, I., Kobayshi, T., Pao, C.Y., Digushi, T., 1983. Equilibrium solubilities of iodine vapour in water. Bull. Chem. Soc. Jpn. 57, 1352–1357. Van Dorsseleare, J.P., Seropian, C., Chatelard, P., Jacq, F., Fleurot, J., Giordano, P., Reinke, N., Schwinges, B., Allelein, H.J., Luther, W., 2009. The ASTEC integral code for severe accident simulation. Nucl. Technol. 165, 293–307.

Vandooren, J., Fristrom, R.M., Van Tiggelen, P.J., 1992. Experimental study of the kinetics of a hydrogen-chlorine-argon flame. Bull. Soc. Chim. Belg. 101, 10. Wang, J., Huang, Z., Tang, C., Miao, H., Wang, X., 2009. Numerical study of the effect of hydrogen addition on methane-air mixtures combustion. Int. J. Hydrogen Energy 34 (2), 1084–1096. Wilson, W.E., O’Donovan, J.T., Fristom, R.M., 1969. Flame inhibition by halogen compounds. Int. Symp. Combust. 12 (1), 929–942. Wren, D.J., 1983. Kinetics of Iodine and Cesium Reactions in the CANDU Reactor Primary Heat Transport System Under Accident Conditions, AECL-7781. Xerri, B., Canneaux, S., Louis, F., Trincal, J., Cousin, F., Badawi, M., Cantrel, L., 2012. Ab initio calculations and iodine kinetic modeling in the reactor coolant system of a pressurized water reactor in case of severe nuclear accident. Comput. Theor. Chem. 990, 194–208.