Contribution to the Understanding of Iodine Transport Under Primary

Nuclear Materials and Energy 17 (2018) 259–268

Contents lists available at ScienceDirect

Nuclear Materials and Energy

journal homepage: www.elsevier.com/locate/nme

Contribution to the understanding of iodine transport under primary circuit T conditions: Csi/Cd and Csi/Ag interactions in condensed phase ⁎ Mélany Gouëlloa, , Jarmo Kalilainenb, Teemu Kärkeläa, Ari Auvinena a VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland b PSI Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

1. Introduction the gas stream or on the surfaces of the tube walls. While investigations concerning the first hypothesis were investigated at IRSN in theCHIP In the context of severe accident management, assessment of the (Chemistry of Iodine in the Primary circuit) programme [12,13], the involved phenomena has been the main objective of several scientific last point is being studied at VTT [9,14]. research programs for more than thirty years. Important chemical re- The SIC alloy composition is ∼80% silver, 15% indium and ∼5% actions affecting directly or indirectly iodine chemistry, have been cadmium by weight [15]. In the case of LOCA scenario, the coolant flow identified and are being studied in detail, so that their effects couldbe is reduced and therefore leads to uncovering the fuel rods. With the incorporated into accident analyses. A classification based on safety increase of temperature, fuel cladding starts to deteriorate and above importance was established by EURSAFE [1,2]. Among the severe ac- 1000 °C, the Zircaloy in the fuel cladding is oxidised by steam produ- cident phenomena recommended for further study, the understanding cing significant amount of hydrogen. Rising temperature and fission gas of iodine chemistry has been defined as a high priority issue [3]. The formation within the fuel pellets leads to increase of the pressure inside chemistry and the transport of iodine in different parts of the reactor the fuel rods, which can lead to cladding rupture. The timing of silver, during the core degradation, although extensively studied, is still the indium and cadmium release depends on the accident sequence [15]. In subject of numerous investigations. The experimental results obtained high-pressure accident sequence (small break LOCA or station black from the Phébus-FP programme, performed under Loss of Coolant Ac- out), despite the release of silver, indium and cadmium vapours is ex- cident (LOCA) representative conditions, have highlighted major points pected to be smaller than during a low-pressure accident sequence [15], regarding the behaviour of iodine in severe accident conditions [4]. In the SIC materials can have more effect on the transport of fission pro- particular, the role of Silver-Indium-Cadmium (SIC) control rods in the ducts. In such scenario, the SIC rods would fail later in higher tem- limitation of the release of gaseous iodine has been suspected. In peratures than in large break LOCA scenario, concurrently with the Phébus-FP, the gas fraction at the cold break varied significantly from release of fission products. During the Phébus-FP tests, the (Ag-In-Cd) one test to another. In the presence of boron carbide (B4C) as control control rod clad rupture occurred at 1100–1400 °C and the first sig- rod material (i.e. the absence of SIC), approximately 85% of the iodine nificant control rod material release was detected [16]. Cadmium was released into the containment was present in gaseous phase. In tests the first SIC material released on failure of the control rod [16] in conducted with SIC control rods, this fraction did not exceed 2% [5,6]. sudden bursts [17]. Afterwards, the release of silver and indium from Section 4.5 of NUREG-1465 [7] stated that at least 95% of the io- the molten absorber was observed. Concurrently, volatile fission pro- dine exiting the reactor coolant system is in the form of caesium iodide ducts release such iodine and caesium was noticed. (CsI). However, it has been shown that interaction with other fission The experimental results from the integral Phébus-FP tests have products or structural materials in the primary circuit can modify its been used for the validation of severe accident integral codes such as composition and transport. Some elements, such as boron or mo- Accident Source Term Evaluation Code (ASTEC) developed by the IRSN lybdenum, can indirectly affect the transport of iodine by reacting with [18]. ASTEC is structured in several modules allowing the modelling of caesium and leading to the formation of volatile iodine, as it has been the entire phenomenology of severe accidents except steam explosions. shown by previous work conducted at VTT [8–10] and at the “Institut SOPHAEROS, module of the ASTEC [19], computes chemistry and de Radioprotection et de Sûreté Nucléaire” (IRSN) [11]. On the other transport of the fission products in the PWR primary circuit and inthe hand, other elements, such as the SIC control rod materials can at- containment during a severe accident. The calculated iodine speciation tenuate or delay the release of iodine, forming stable and less volatile in the RCS with SOPHAEROS/ASTEC v1.3, integrated in the degrada- iodine compounds, as assumed based on Phébus-FP tests [6]. In the tion phase, for the four Phébus-FP tests showed that 12% (FPT-2) to Reactor Coolant System (RCS), the chemical reactions could happen in 77% (FPT-1) of the iodine mass released in the RCS was cadmium

⁎ Corresponding author. E-mail address: [email protected] (M. Gouëllo). https://doi.org/10.1016/j.nme.2018.11.011 Received 30 March 2018; Received in revised form 12 November 2018; Accepted 14 November 2018 Available online 22 November 2018 2352-1791/ © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

iodide (CdI2; CdI) [5]. Cadmium iodide (CdI2) is very stable at low least 95% caesium iodide [7], CsI was the main subject of the study. As temperature and condensed before the RCS cold leg temperature for the control-rod materials, silver and cadmium may be present in the (∼200 °C) [20]. Tests conducted by Beard et al. [21] demonstrated the RCS as metal (Ag and Cd), oxide (Ag2O and CdO) or as hydroxide interaction between caesium iodide vapour and the control rod alloy (CdOH) [31,28]. Metallic forms of the precursors were selected for this aerosol in a Thermal Gradient Tube (TGT). The authors suggested that initial investigation for their stability and making the compounds safer vapour of I2 or CsI condensed onto the cadmium-based debris in gas to handle. The commercial powders used as reactants in this study do phase, with subsequent reaction at the interface to form CdI2. Falcon not mimic exactly the physical forms of the compounds deposited or experiments [22], carried out in a flowing system, have also demon- condensed in the primary circuit in severe accident conditions. How- strated the condensation/sorption of caesium and iodine onto the ever, the first purpose of the present study is to define if interactions in cadmium aerosols with an atmosphere containing 3% steam. The for- the condensed phase could occur. In the case of positive conclusions, mation of the ternary caesium-cadmium-iodine compound Cs2CdI4 has further studies will be performed and next step will be to perform tests been postulated by several authors in the primary circuit from a reac- from deposited/condensed caesium iodide and cadmium/silver. tion of CsI with Cd [23–25]. For reasons that will be explained further in the paper, two different Spence and Wright [26] noted, in their studies at temperatures tube furnaces were used for the cadmium experiments. Experiments below 950 °C, that solid silver and vapour caesium iodide reacted to- conducted at 400 °C were conducted with a 40 cm-length tube furnace gether, forming relatively volatile silver iodide by a simple exchange and those at 650 °C were performed with a 110 cm tube furnace. All the reaction (1). However, Sallach et al. [27] found that caesium iodide silver studies were performed with the shorter tube furnace. The out- vapour in argon was rather stable in the presence of silver at 770 °C and side diameter of the tube remained the same (2.8 cm), but the distance 950 °C. On the other hand, experiments pointed out a reactive beha- between the end of the crucible and the first flow diluter was changed viour concerning hydrogen iodide HI and molecular iodine I2 towards (2 cm and 16.5 cm, respectively). The carrier gas, a mixture of argon, solid silver forming silver iodide between 400 °C and 660 °C. It was steam and hydrogen, was passed through the system at a flow rate of reported that introduction of water vapour into the carrier gas had no 3800 cm3/min once the target temperature was reached. The resulting measurable effect on the reaction rate. It can also be noted thatthe aerosols and gases were cooled down in the furnace tube and in the reaction between silver oxide and volatile iodine was found to be more dilution system. efficient than that with metallic silver [28]. The first diluter placed after the tube had the function to desaturate The reactions between the aerosols formed by the control-rod ma- the flow and to prevent the condensation of vapours. Argon wasused terials (designed as M in the following balanced equation) and vapours, for this dilution and was previously heated up to the target temperature can be described [29] by: of the tube furnace. A second diluter was used to cool down the flow. The cooling flow passes through a porous wall structure preventing the xCsIg()()()()+ Ms xCsg+ MIgx (1) retention of particles and vapours during the process. Argon at room x temperature was used as dilution gas. Downstream the dilution system, xHI()() g+ M s H2 ()() g+ MIx g 2 (2) reaction products were transported through a sampling furnace (125 °C) along the main line with an approximate length of 50 cm. The sampling The present work focused on determining the impact of reactions furnace contains three main sampling lines, each equipped with a between simulant fission product (caesium iodide) and simulant control polytetrafluoroethylene (PTFE) membrane filter (hydrophobic, poral rod materials (cadmium or silver) taking place on the surface of pri- grade 5.0 μm, 47 mm, Mitex®) and two liquid scrubbers made of glass mary circuit on the source-term. First, interactions of caesium iodide assembled in series. and cadmium were studied notably because of the observations from the Phébus-FP tests mentioned earlier in the introduction. Then, the 2.2. Distinctive features of cadmium experiments interactions with silver were investigated, in response to previous studies indicating the major role that silver could play in the transport of For the experiments conducted with cadmium, a stainless steel plate fission products [30]. In addition, the work aimed at determining the (∼8 cm) was placed at the bottom of tube inside the furnace before the effect of H2/H2O ratio in the gas phase on the transport and chemistry first diluter, characterised by a temperature gradient of about 100°C of iodine in RCS. taking place inside the insulation of furnace outlet. This plate had not This article is organized as follows. Description of the experimental been pre-oxidized before the experiment. facility, presentation of the analytical methods and experimental matrix Following Beard et al. [32], the exit gas was passed through two and procedure are given in the second section. In the first experimental solutions, the first consisting of 2 M nitric acid to remove cadmium and section, the revaporisation of deposited/condensed precursor from the the second containing 0.1 M sodium hydroxide to trap iodine. In the experimental facility will be discussed. In the fourth part, results ob- experiments with CsI and Cd precursors, the filters were first leached tained with cadmium are presented and discussed, and a similar section with sodium hydroxide in order to avoid the iodide ion being oxidized is dedicated to silver. In the final chapter, the results from all experi- in acidic solutions by atmospheric oxygen to free iodine, which would ments are assembled for conclusions. then be lost from the solution before analysis. After removing a sample for iodine and caesium analyses, the solution was acidified with nitric 2. Material and methods acid for the analysis of cadmium.

2.1. Facility description 2.3. Analytical methods

All experiments were performed with the EXSI-PC facility, which is The scrubber solutions and leachants from filters were analysed by shown in Fig. 1 and was fully described in a previous paper [10]. An Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analyses alumina or stainless steel crucible containing between 2 g and 11 g of were performed with a Thermo Fisher Scientific HR-ICP-MS Element2 precursor material(s) was located inside a stainless steel tube, which apparatus. was heated using a horizontal tube furnace. The precursors chosen to Deposited compounds on the stainless steel plates and the aerosol simulate the fission product CsI and control-rod materials silver and filters were examined by X-ray diffraction, for the experiments per- cadmium were in the form of powder or grain (CsI 99.9 wt%, AgI 99 wt formed with cadmium. The X-ray diffractogram was run from the %, Cd with 3–6 mm grain size provided by SigmaAldrich®). samples using a Philips X'Pert MPD diffractometer and the powder Because iodine exiting the reactor coolant system is composed of at method. In this method, crystalline compounds can be detected from

260 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Fig. 1. Schematic figure of the experimental EXSI-PC facility.

Table 1 Experimental matrix and test conditions of the experiments studying cadmium influence on iodine chemistry and transport in the RCS reported in thispaper.

Exp. Precursor Precursor mass [g] Carrier gas volume percentages Crucible material Crucible temperature [°C] Experiment duration [min]

CsI-1 CsI 9.39 Ar/H20 86.7% Ar + 13.3% H2O Al2O3 400 120

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

CsI-2 CsI 11.03 Ar/H20 86.7% Ar + 13.3% H2O 650 145

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Cd-1 Cd 8.02 Ar/H20 86.7% Ar + 13.3% H2O 400 120

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Cd-2 Cd 8.11 Ar/H20 86.7% Ar + 13.3% H2O 650 180

Cd-3 Cd 8.74 Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2 650 180

Cd-4 CsI Cd 1.94 9.13 Ar/H20 86.7% Ar + 13.3% H2O 400 129

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Cd-5 CsI Cd 2.00 8.79 Ar/H20 86.7% Ar + 13.3% H2O 650 60

Cd-6 CsI Cd 2.00 8.73 Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2 650 165 the sample. The diffractograms were recorded using a Philips X'Pert JobinYvon® (LABRAM HR) spectrometer. The Raman backscattering MPD X-ray diffractometer and the powder method in the range5 was excited at 632 nm. The spectral resolution of this instrument is ∼4/ −70°2θ. The radiation used was graphite-monochromatised Cu-Kα cm and the spatial resolution was estimated to be ∼1 μm3. radiation (λ = 0.1541 nm). The working conditions were 40 kV and In the silver experiments, aerosol number size distributions were 50 mA tube power. Identification of species was performed by com- measured with a TSI Scanning Mobility Particle Sizer (SMPS), with parison between recorded diffractograms with the International Center series 3080 platform, series 3081 Differential Mobility Analyzer (DMA) of Diffraction Data's powder diffraction file. The reference database and series 3775 Condensation Particle Counter (CPC). Aerosol mass used was PDF-4 + 2013. Depending on the fineness of the crystals concentration was monitored with a Tapered Element Oscillating present in the deposit, the X-ray diffraction method can only detect and Microbalance Series 1400a Ambient Particulate (PM-10) Monitor identify compounds present in amounts exceeding about 5% of the (TEOM). deposit. Raman micro-spectrometry was also applied on the sample analysed 2.4. Experimental matrix and procedure by XRD as a fingerprint method for species identification. However, not being the main analytical technique used for this work, Raman spectra The absolute pressure in the facility was fixed at one bar. The gas were used mainly as a complement. The detailed interpretation of flow rate was set at 38003 cm /min under Normal Temperature and spectra was not straightforward and was not attempted in the present Pressure (NTP). Three different mixtures of argon, steam and hydrogen work. Raman micro-spectrometry was primarily used in order to dis- were applied (Table 1). The H2/H2O ratio was in the low range of that tinguish between cadmium iodide and caesium iodide in small amounts expected in the RCS during a severe accident (typical values range from of solid-state compounds. Raman spectra were obtained from a 0.1 to 10) [33]. Even though argon is not representative of the

261 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268 atmosphere during a severe accident, it was used as the main inert gas Table 3 during the experiments in order to increase the flow rate through the Calculated saturation pressure of analysed species furnace, keeping the steam mass flow rate low enough to avoid its according to Clausius-Clapeyron formula at 650 °C. condensation in the coldest parts of the facility. In order to free the Compound log (Psat/Patm) reaction rate from influence of the flow rate, the flow rate inthere- action furnace was kept constant during the experiments. Keeping a HI 3.2 I 2.4 constant flow rate also prevents from unwanted resuspension ofthe 2 CdI2 −0.6 precursor or deposited particles. Cd −0.64 3 Dilution gas flow was fixed to 7000 cm /min in the first diluter and AgI −3.9 to 28,000 cm3/min into the porous tube, creating a dilution ratio of CsI −4.5 10.3 for the sampling line at 150 °C. The volumetric flow was about Ag −10 CdO −10.33 1400 cm3/min in the sampling line and about 7500 cm3/min in the online measurement line. In the revaporisation studies by Bottomley et al. [34], the tem- has been shown [36] that at 580 °C the caesium iodide powder sintered perature was chosen on the basis of observations made from the Phébus slowly, and was almost non-volatile under argon. However, it was revaporisation testing. The residence time of the species in the transport shown that at temperatures close to the melting point (620 °C), the zone (i.e. from the crucible down to the filter in the sampling line) was whole amount of caesium iodide (0.5 g) vaporised in approximately 1.3 s. This is quite short residence time by comparison to the residence 30 min under argon. Consequently, only from 620 °C to 650 °C, and times, which can be calculated in RCS down to the cold leg break in during the stabilisation phase when carrier gas was passed over the LOCA scenario (gas residence time of 2 to 6 s for instance for FPT0 and crucible (20 min), some caesium iodide can be vaporised. This was FPT1 [35]). Consequently, it is expected that the deposition of aerosols probably also the case with silver iodide. The amount of material re- by settling will not be an important phenomena in the present case. leased during the heating phase was considered as negligible for CsI, This test showed that the revaporisation of caesium commenced at AgI and Ag. However, it was measurable during heating the cadmium. 550 °C and was rapid until 750 °C, and so the temperature was chosen to Once the target temperature was reached in the reaction furnace, be in this range. As in previous studies conducted with the EXSI-PC the mixture of carrier gas was introduced. In previous studies [9], it has facility, the reaction furnace temperature (650 °C) was chosen so that it been shown that the system was approaching a steady-state tempera- would be just above the melting point of caesium iodide (621 °C) and ture profile after 20 min. The first sampling was then carried out.Dif- close to the hot leg temperature (700 °C) used in the Phébus FP ex- ferent approaches were used for the sampling process. For the cadmium periments [4]. A lower temperature (400 °C) was chosen to compare the experiments conducted at 400 °C, and for all silver experiments, the solid/liquid reaction for the cadmium experiments. At both these different atmospheres were successively studied with the sampling lines temperatures, the metallic cadmium is molten (melting point = 321 °C) of the EXSI-PC facility following the procedure used in the previous but the silver is not (m.p. = 962 °C). Table 1 presents the test conditions studies with this facility [9]. However, because of the possible reduc- of the eight cadmium experiments and Table 2 the test conditions of the tion of cadmium iodide (CdI2) to metallic cadmium and the formation six silver experiments reported in this paper. The parameters which of hydrogen iodide according to reaction (3) when heated with hy- were studied were the initial vaporised species, the carrier gas volume drogen [37], it was decided to work differently during the cadmium percentages, the crucible material and crucible temperature. experiments at 650 °C. During the heating phase, a small flow of argon (2003 cm /min) was introduced into the furnace tube in order to maintain a neutral atmo- heat CdIsl2 (,)+ Hg2 () Cdsl (,)2()+ HIg (3) sphere above the crucible and thus limit oxidation of the metallic cadmium before the target temperature was reached. During this phase, Changing the atmosphere between samplings presented another the vaporisation of a small amount of cadmium cannot be excluded. The disadvantage. The deposited material (following CsI heating) when vaporisation of silver was less probable due to the low vapour pressure changing the atmosphere composition (from Ar/H2O to Ar/H2O/ of silver (Table 3). The vapour pressure of silver calculated according to H2(10%)) at 650 °C can lead to the revaporisation of species. An as- Clausius-Clapeyron formula at 650 °C is indeed about 15 times lower sessment of these possible processes involved in the EXSI-PC facility than the one calculated for cadmium. In the case of caesium iodide, it when changing the atmosphere composition has been conducted. The

Table 2 Experimental matrix and test conditions of the experiments studying silver influence on iodine chemistry and transport in the RCS reported in thispaper.

Exp. Precursor Precursor mass [g] Carrier gas volume percentages Crucible material Crucible temperature [ °C] Experiment duration [min]

Ag-1 CsI Ag 2.50 7.50 Ar/H2O 86.7% Ar + 13.3% H2O Al2O3 650 120

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Ag-2 CsI Ag 2.00 6.00 Ar/H2O 86.7% Ar + 13.3% H2O 400 120

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Ag-3 AgI 10.00 Ar/H2O 86.7% Ar + 13.3% H2O SS304 650 120

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

Ag-4 AgI 9.16 Ar/H2O 86.7% Ar + 13.3% H2O 400 128

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

CsI-3 CsI 9.581 Ar/H2O 86.7% Ar + 13.3% H2O Al2O3 650 120

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

CsI-4 CsI 9.39 Ar/H2O 86.7% Ar + 13.3% H2O 400 120

Ar/H2O/H2(3%) 83.9% Ar + 13.5% H2O + 2.6% H2

Ar/H2O/H2(10%) 76.1% Ar + 13.4% H2O + 10.5% H2

262 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Table 4 Characteristics of aerosols according to the two atmospheres from experiments conducted at 650 °C and mass concentration of aerosols in the EXSI-PC facility during sampling. Results are presented for the three different sampling lines.

Experiment number REVAP-1 REVAP-1b REVAP-2 REVAP-2b Blank-1 Blank-2

Carrier gas conditions Ar/H2O Ar/H2O/H2(10%) Ar/H2O Ar/H2O Ar/H2O Ar/H2O/H2(10%) Precursor CsI none CsI none none none Experiment durationa [min] 121 85 121 85 93 90 SMPS [1/cm3] 4.0•109 4.4•108 1.8•109 1.9•109 _ _ Count Mean Diameter CMD [nm] (average) 30.8 N. A.b 34.2 21.8 _ _ Geometric Standard Deviation GSD (average) 1.6 N. A. 1.5 1.4 _ _ Cs/I molar ratio from particles on the filter F1 1.4 2.2 0.9 0.9 _ _ F2 1.4 1.9 0.9 1.1 _ _ F3 1.3 1.6 0.9 0.9 _ _ TEOM [mg/m3] L1 334.2 26.0 362.8 87.5 2.0 1.2 L2 328.2 29.9 283.0 70.3 2.0 1.2 L3 319.6 35.3 314.8 38.7 2.0 1.2 Gaseous iodine concentration ICP-MS [mg/m3] L1 24.9 17.3 2.0 2.0 _ _ L2 6.6 0.8 2.0 2.4 _ _ L3 3.1 1.0 2.6 1.8 _ _

a The experiment duration is counted from the time when the furnace reached 650 °C. b N. A. (not available). In this case, only the Condensation Particle Counter (CPC) was used (and not the combination of the Differential Mobility Analyzer (DMA) and (CPC)). experiments were performed in order to underline possible effects of 26 to 36 mg/m3, respectively). Consequently, it is not possible to at- revaporisation in the EXSI-PC facility when the carrier gas composition tribute the observed difference in the 2Ar/H O experiment (87 to 38 mg/ was changed. The results are presented in the following section. m3 by TEOM compared to 4 to 2 mg/m3 by ICP-MS) due to the limit of detection of ICP-MS. Moreover, the results of previous experiments al- 3. Investigation on possible revaporisation in the experimental lowed us to estimate that facility effects and analysis uncertainty can facility explain a maximal difference of 20% between the “online measure- ments line” and the “sampling lines”. Consequently, it can be assumed

After heating caesium iodide precursor at 650 °C under an Ar/H2O that either the generated particles, in that condition, were not stable in gas flow during 121 min (experiments REVAP-1 and REVAP-2, see alkaline solution, or solubilisation of the filters in alkaline solution was Table 4), the flow was stopped and temperature decreased to room poorly carried out, or a mismeasurement occurred in ICP-MS (Table 4). temperature to remove the crucible from the stainless steel tube. The As a conclusion, it was showed that deposited material (following facility was not leached. Following the previous study [9], more oxi- caesium iodide vaporisation) when changing the atmosphere compo- dizing conditions were used first in the experiments. In addition, the sition (from Ar/H2O to Ar/H2O/H2) at 650 °C led to the revaporisation experiment follows the scenario of the accident sequence described in of about 7 to 9% of aerosols and up to 35% of the total gaseous iodine. the introduction, where a steam phase is followed by the release of It would be expected that lower revaporisation processes occur at hydrogen. 400 °C. However, this effect is not excluded and must be taken into The second part of the experiment consisted of heating the furnace account in the data processing. once again at 650 °C, this time under: Based on the time needed (15 min) to measure an increase of particle concentration with the online measurement devices after the flow

(1) Argon/steam/hydrogen gas flow (carrier gas conditions2 Ar/H O/ rate was switched on led to the conclusion that the measured particles H2(10%) - experiment REVAP-1b); and gas after these experiments were due to revaporisation processes (2) Argon/steam gas flow (carrier gas conditions2 Ar/H O - experiment and not resuspension processes. Additionally, the immediate increase in REVAP-2b). the particle concentration after the flow was turned on, as observed for example by Hontañón et al. [38], was not detected. The heating of the facility without any precursor at 650 °C produced a notable concentration of particles (4•4.10+8 to 1.9•10+9 1/cm3) 4. Results of the study on cadmium (Table 4). TEOM results showed the presence of particles during the second In order to make all the measured data comparable (from the three part of the experiment, meaning that there was a release of elements main sampling lines and online measurement line), all the mass con- from the walls. For two samplings, the mass concentration of particles centrations expressed in this paper were reported to the corresponding was higher when the second stage of the experiment was carried out values inside the reaction furnace. They were systematically corrected under the carrier gas conditions Ar/H2O(Table 4). Experiments named by the dilution factor. Consequently, it is important to keep in mind “blank” corresponded with the recorded data from the heating of the that these results represent the minimal values of the amounts of gases facility at 650 °C under the different carrier gases without any pre- and aerosols that were released from the crucible. The calculation did cursor. REVAP-1 and REVAP-2 were carried out in the same conditions. not take into account all the material condensed or deposited on the Differences in the values concerning aerosol concentrations didnot way from the crucible to the sampling lines. exceed 20% between the two experiments. However, the difference was Table 5 shows the mass concentrations of the reaction products, significant concerning gaseous iodine. Heating the facility with anAr/ calculated from the aerosol filter and bubbling bottle ICP-MS data.

H2O/H2 flow led to the revaporisation of iodine and caesium. However, The first experiments with caesium iodide (CsI-1 & CsI-2) wereused it was more difficult to reach conclusions regarding the results fromthe as a blank to determine the fraction of gaseous iodine generated by

Ar/H2O revaporisation test (Table 4). A first observation would be to heating caesium iodide powder under different atmosphere composi- say that revaporisation was less effective under Ar/H2O gas flow. The tion and temperature. results of ICP-MS were close to the TEOM results for a lower mass The influences of the temperature and composition of the carrier gas 3 concentration during the Ar/H2O/H2 experiment (17 to 21 mg/m and on the vaporisation of caesium iodide were studied in previous paper

263 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Table 5 Iodine, caesium and cadmium mass concentrations in experiments, calculated from aerosol filter and bubbling bottle ICP-MS data.

Exp. Cond. Crucible temperature [ °C] Gas [mg/m3]a Aerosol [mg/m3]a

I Cs Cd I Cs Cd Cs/I Cd/I % I(g)

CsI-1 Ar/H2O 400 1.75 0.01 N/A 0.31 0.01 N/A 0.03 N/A 84.95

Ar/H2O/H2(10%) 2.15 0.99 N/A 0.3 0.01 N/A 0.03 N/A 87.76

CsI-2 Ar/H2O 650 21.98 0 N/A 195.5 190.49 N/A 0.97 N/A 10.11

Ar/H2O/H2(10%) 1.25 0 N/A 290.74 250.64 N/A 0.86 N/A 0.43

Cd-1 Ar/H2O 400 N/A N. A. 0.55 N/A N/A 2.13 N. A. N. A. N/A

Ar/H2O/H2(10%) N. A. N/A 0.2 N/A N/A 2.45 N/A N. A. N/A

Cd-2 Ar/H2O 650 N/A N/A 0.03 N. A. N/A 192.4 N/A N/A N/A

Cd-3 Ar/H2O/H2(10%) 650 N/A N/A 0.14 N/A N/A 162.55 N/A N/A N/A

Cd-4 Ar/H2O 400 0.64 0.65 1.72 0.17 0.04 27.2 0.24 160.00 79.01

Ar/H2O/H2(10%) 1.73 2.62 0.24 0.17 0.12 0.41 0.71 2.41 91.05

Cd-5 Ar/H2O 650 27.62 9.64 8.66 125.32 152.56 2125 1.22 16.96 18.06

Cd-6 Ar/H2O/H2(10%) 650 34.34 0.36 0.36 227.75 305.67 10,128.8 1.34 44.47 13.10

N/A: not applicable. a Uncertainties in chemical analysis data: Cd ± 5%; Cs and I ± 10%

[10]. As the temperature increased, the fraction of gaseous iodine space group Pm3¯1, unit-cell parameters a = 3.496, c = 4.702 Å) and ¯ compared to the fraction of aerosol decreased (more than 80% for CsI-1 CdO (space group Fm3 m, unit-cell parameters a = 4.694 Å), respec- to less than 19% for CsI-2 at 650 °C). In addition, at 650 °C it was tively. Semi-quantitative amounts were 70%, 8% and 22%, respec- manifest that the total amount of gaseous iodine decreased as the tively. The sample from the porous tube after Cd-3 was identified as amount of hydrogen in the atmosphere increased (about 10% in Ar/ metallic cadmium.

H2O by comparison to 0.4% in presence of hydrogen). Concurrently, the Heating cadmium at 400 °C and 650 °C led mostly to the formation 3 mass concentration of particles increased (190 mg/m in Ar/H2O vs of particles (Table 5). The greatest amount of cadmium detected on 250 mg/m3 in presence of hydrogen). Nevertheless, only a small frac- sampling lines was localised on filters, gaseous cadmium representing a tion of gaseous iodine was measured in the liquid scrubber, indicating maximum of 20% of the overall sampling line. Although it was not limited decomposition of caesium iodide. possible to determine the effect of changing the atmosphere on the Second part of the experiment was dedicated to assess the behaviour release of cadmium, a slight decrease of the aerosol mass concentration of cadmium, heated in the crucible under different atmosphere com- was observed at 650 °C from condition Ar/H2O to condition Ar/H2O/ position and temperature (Cd-1, Cd-2 and Cd-3). After the experiment H2(10%). On the filters, particles were scattered. Raman analysis of conducted at 400 °C, the precursor was not molten and despite a red- particles on the filter indicated an intense and very broad band at −1 −1 −1 dish-brown deposit was observed strewn in the crucible at the com- 272 cm and two bands at 731 cm and 920 cm on both filters, pletion of the experiment, original grey pellets were predominant. It is which could be assigned to CdO [39]. suspected that the temperature above the crucible did not reach 321 °C, The observations in this study suggest that after vaporisation at both the melting temperature of cadmium, and that metallic cadmium was 400 °C and 650 °C and under Ar/H2O or Ar/H2O/H2(10%) atmosphere, partially oxidized to cadmium oxide (CdO) in its beta form [37]. Oxi- cadmium reached the sampling lines at 125 °C mainly as particles. dation of metallic cadmium in a steam-argon atmosphere (10% H2O) However, the crystalline form of the deposits observed through the above 200 °C has previously been reported by Beard et al. [32] and by experimental facility when precursor was heated to 650 °C indicated Danţuş et al. [36] at 377 °C. that the deposits resulted from a vapour condensation process rather At 650 °C, all the cadmium in the crucible was vaporised. Most of than aerosol formation and sedimentation as observed by Beard et al. the precursor condensed/deposited in the diluters and piping upstream [32]. This would suggest that cadmium was mainly transported as va- of the sampling lines, where the gas temperature was higher than pours at temperatures higher than 150 °C. Changing the atmosphere 150 °C. In addition, the deposits on the stainless steel plates were dif- composition over the crucible at 650 °C had an effect on the cadmium ferent according to the presence, or absence, of hydrogen. When the speciation. When hydrogen was present in the carrier gas, the propor- carrier gas was only composed of argon and steam, dry droplets were tion of released metallic cadmium was higher. observed on the coolest points of the plate. When hydrogen was added In the final part of the cadmium study, cadmium and caesium iodide to the carrier gas, deposits were comparable to metallic droplets, for were mixed in the crucible (Cd-4, Cd-5 and Cd-6). Because the ex- which the size range varies from 2 µm to 20 µm. On the several spots, periment time was not the same for Cd-5 and Cd-6, it was decided to which were analysed by Raman micro-spectrometry, these metallic compare the amount of vaporised element per minute. The values were droplets did not exhibit Raman scattering, but merely reflected the calculated from the difference between the initial amount in the cru- incident light. This would be consistent with the presence of pure me- cible and the amount determined by leaching the crucible after the tallic cadmium, which should not exhibit Raman scattering, except in experiment. Vaporisation rates for the three elements were lower the presence of impurities and/or absorbed molecules. during Cd-5 than when hydrogen was present in the carrier gas in Cd-6

Under Ar/H2O atmosphere, in the main line (from 150 °C to room (Table 6). temperature), fluffy grey deposits easily removed by nitric acid leaching The Cs/I molar ratios in the crucible after the experiment and in the were observed. In addition to these fluffy grey deposits, metallic de- sampling lines are shown in Table 6. The ratios determined from cru- posits were present on the wall of the porous tube cible leaching after Cd-4 were not used to support the experiment be-

(200 °C > Tfluid > 150 °C) under Ar/H2O/H2(10%) atmosphere. The cause two conditions were studied during this experiment. At 650 °C the diffraction pattern of deposits sampled in the main line from Cd-2was amount of caesium remaining in the crucible was higher than that of identified according to powder diffraction data contained in thePDF iodine (Cd-5; Cs/I = 8.87), and became even higher when hydrogen cards Nos. 01-071-3769, 01-073-0969 and 00-005-0640. These dif- was added to the carrier gas (Cd-6; Cs/I = 37.10). Part of the released fraction patterns correspond to Cd (space group P63/mmc, unit-cell iodine during condition Ar/H2O is clearly not linked to caesium, and parameters a = 2.9794, c = 5.6186 Å), Cd(OH)2 (trigonal symmetry, the Cd/I ratio is also affected by the condition2 Ar/H O. Deposits were

264 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Table 6 even though the reaction between the two compounds has already been Assessment of the vaporised element per minute and Cs/I and Cd/I molar ratios mentioned in the literature [21,23–25]. However, it is important to for experiments conducted at 650 °C. underline that the observation of cadmium iodide CdI2, has previously Initial [mol] Final [mol] Released from crucible been made after interaction between cadmium in aerosol and gaseous [mol/min] iodine [40] and in the current study, interaction is investigated from

−2 −3 −3 condensed phase. Cd-5 Ar/H2O Cd 7.82•10 4.26•10 1.23•10 −3 −4 −4 The observation of more caesium than iodine in the crucible after I 7.70•10 2.43•10 1.24•10 Cs 7.70•10−3 2.15•10−3 9.25•10−5 the experiment, which was not noticeable after the CsI experiment, Cs/I 1.00 8.87 0.74 would suggest a reaction between iodine and cadmium. The detection Cd/I 10.16 17.57 9.92 of a higher amount of gaseous iodine and the absence of Cd-I com- −2 −5 −4 Cd-6 Ar/H2O/ Cd 7.77•10 5.25•10 4.70•10 pounds could be explained by the decomposition of the unstable Cd-I H2(10%) I 7.70•10−3 3.03•10−5 4.65•10−5 compound during the experiment. The formation and decomposition of Cs 7.70•10−3 1.12•10−3 3.98•10−5 Cd-I compounds during the heating phase under low argon flow rate Cs/I 1.00 37.10 0.86 cannot be excluded when the carrier gas was introduced. Another hy- Cd/I 10.09 1.73 10.12 pothesis would be the formation of CdCs2I4 in the crucible. This would explain the fact that the fraction of caesium remaining in the crucible

was higher than that of iodine. The decomposition of CdCs2I4 on the observed after Cd-5 on the coolest points of the stainless steel plate and filter would also release the gaseous iodine found in the bubbling looked like golden metallic droplets that grew with decreasing tem- bottle. However, as the molar enthalpy of formation of Cs2CdI4(s) was perature (Fig. 2a). The diffraction patterns of sample from the first di- established to be −920.3 ± 1.4 kJ/mol by Ball et al. [41], this last luter was identified according to powder diffraction data contained in hypothesis is less probable. the PDF cards Nos. 04-003-5667, 04-013-0259 and 01-078-3527. These Transport of cadmium was significantly increased by the presence of diffraction patterns correspond to Cd, CsI (space groupI (T3)) and Cd 23 caesium iodide. 10 to 60 times more cadmium reached the sampling (OH) , respectively. Semi-quantitative amounts were 27%, 54% and 2 lines compared to the experiments performed with only cadmium. Most 19%, respectively. On the filter, the diffraction patterns were identified of the cadmium was condensed on the tube's walls before reaching the according to powder diffraction data contained in the PDF cards Nos. sampling lines during vaporisation of cadmium, and it was highly 01-071-3769, 01-073-0969, 00-005-6082 and 04-005-4225, corre- probable that the cadmium had condensed on caesium iodide particles, sponding to Cd, Cd(OH) , CdO and CsI (Fig. 3), respectively. The semi- 2 because of its lower vapour pressure. Beard et al. [42] concluded about quantitative amounts were respectively 71%, 5%, 19% and 5%. Some the condensation of gaseous caesium iodide onto cadmium particles reflections referred to caesium cadmium iodide (Cs CdI , No. 00-037- 2 4 and observed a reaction at the interface to form cadmium iodide. In the 1499) but, because the amount was very small, this identification was present study, it seems more probable that cadmium had condensed on uncertain. caesium iodide particles and there was no concrete proof of the reaction When hydrogen was added to the carrier gas (Cd-6), a significant at the interface in this case. amount of white crystals was observed all along the plate (Fig. 2b). With two strong fundamental Raman peaks at 110.93 cm−1 and 5. Results of the study on silver 91.449 cm−1, the spectrum presented the same fingerprint as the Raman spectrum of the commercial caesium iodide powder recorded First set of experiments focused on studying interactions between under similar conditions. Only caesium iodide was identified from the caesium iodide and silver (Ag-1 and Ag-2). When metallic silver was deposits in the diluter. The amount of aerosol collected on the filters added to caesium iodide at 650 °C, the behaviour of caesium and iodine was distinctly increased by comparison to experiments with pure resembled the pure caesium iodide test conducted in the same condi- compound, indicating an interaction between the simulant fission tions (CsI-3): the mass concentration of gaseous iodine decreased as the product caesium iodide and cadmium. The molar ratios determined on amount of hydrogen in the atmosphere was increased (Table 7). Most of the filters are presented in Table 5. The main point to notice was the the iodine in the sampling lines at 150 °C was in particulate form. The amount of caesium, which was higher than that of iodine on the filters. amount of silver (in gaseous phase and in aerosol phase) at 150 °C was The molar cadmium to iodine ratio was even more pronounced. From low, meaning either that the formation of silver-compound was not the filters, the same compounds as the ones identified from filters inthe significant in the conditions of the experiment, or that the compound experiment under Ar/H O (Cd-5) were measured with semi-quantita- 2 condensed before the sampling lines. At 400 °C, the experiment showed tive analysis giving 69%, 2%, 28% and 1%, respectively. the predominance of gaseous iodine over aerosol iodine. Unexpectedly, At 400 °C, the total amount of gaseous iodine trapped into bubblers mass concentrations of gaseous iodine were considerably higher than in was almost the same as the amount trapped during vaporisation of pure the same experiment conducted at 650 °C. As in Ag-1, a small amount of caesium iodide under the same conditions (more than 80% of gaseous silver was present in the sampling line. Calculation of Cs/I ratios on the iodine). By contrast, at 650 °C, the addition of cadmium to caesium filter confirmed that the presence of caesium iodide particles wasminor iodide tended to increase the amount of gaseous iodine (from less than in the study at 400 °C (Table 7; Cs/I lower than 1). 10% for caesium iodide experiment by comparison to more than 10% in Overall, the addition of metallic silver on caesium iodide precursor presence of cadmium) and to decrease the mass concentrations of the did not have a significant impact on the release of gaseous iodine. aerosols. The proportion of iodine in gaseous form1 was slightly in- However, the fraction of released gaseous iodine was higher than for creased by addition of hydrogen (Table 5). The observation of caesium the experiment performed with cadmium under the same conditions. and cadmium in the bubbling solution could be either due to a vapour Experiments at 400 °C, with and without silver precursor mixed with compound containing caesium and cadmium, as well as iodine, or be- caesium iodide, featured a higher fraction of gaseous iodine in the cause of a leak from the filter. overall release. The difference observed between 400 °C and 650 °Cwas The first unexpected result from the cadmium study is thatCd-I interpreted as due to the decomposition of caesium iodide rather than compound was not detected neither by XRD nor Raman spectrometry, to a more effective reaction between silver and caesium iodide at 400 °C. Second set of experiments investigated the behaviour of silver io- 1 i.e. the amount of gaseous iodine in comparison to the total amount of dide precursor (Ag-3 and Ag-4). In both experiments, the fraction of gaseous and aerosol iodine. gaseous iodine increased as the amount of hydrogen in the atmosphere

265 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Fig. 2. Optical microscope pictures of the deposits observed on the stainless steel plate (thermal gradient of about 100 °C along the coupon, from 650 °C to 550 °C) after vaporisation of Cd + CsI at 650 °C under a) Ar/H2O atmosphere and b) Ar/H2O/H2(10%) atmosphere.

Fig. 3. X-ray powder diffraction patterns of the filter from Exp.7.

Table 7 Iodine, caesium and silver mass concentrations in experiments, calculated from aerosol filter and bubbling bottle ICP-MS data.

Exp. Cond. Crucible temperature [°C] Gas [mg/m3]a Aerosol [mg/m3]a

I Cs Ag I Cs Ag Cs/I Ag/I % I(g)

Ag-1 Ar/H2O 650 61.87 0.10 0.07 169.16 125.80 1.52 0.74 0.01 26.78

Ar/H2O/H2(3%) 10.74 0.12 0.07 110.43 122.78 0.05 1.11 0.00 8.86

Ar/H2O/H2(10%) 0.69 0.15 0.07 131.71 136.17 0.03 1.03 0.00 0.52

Ag-2 Ar/H2O 400 20.66 0.06 0.31 0.02 0.01 0.30 0.76 16.39 99.91

Ar/H2O/H2(3%) 326.40 0.24 0.06 3.81 0.11 2.83 0.03 0.74 98.85

Ar/H2O/H2(10%) 153.24 0.11 0.12 0.07 0.05 2.23 0.77 31.06 99.95

Ag-3 Ar/H2O 650 134.16 N/A 0.22 62.55 N/A 227.05 N/A 3.63 68.20

Ar/H2O/H2(3%) 371.83 N/A 0.21 4.99 N/A 77.08 N/A 15.46 98.68

Ar/H2O/H2(10%) 659.29 N/A 0.41 87.54 N/A 77.70 N/A 0.89 88.28

Ag-4 Ar/H2O 400 8.04 N/A 0.00 397.54 N/A 10.64 N/A 0.03 1.98

Ar/H2O/H2(3%) 2.27 N/A 0.10 0.14 N/A 22.27 N/A 162.94 94.32

Ar/H2O/H2(10%) 103.77 N/A 0.07 0.35 N/A 67.51 N/A 191.11 99.66

CsI-3 Ar/H2O 650 116.47 0.00 N/A 88.47 116.89 N/A 1.32 N/A 56.83

Ar/H2O/H2(3%) 66.84 0.00 N/A 109.99 134.49 N/A 1.22 N/A 37.80

Ar/H2O/H2(10%) 1.66 0.00 N/A 147.95 149.95 N/A 1.01 N/A 1.11

CsI-4 Ar/H2O 400 1.75 0.01 N/A 0.01 0.31 N/A 51.37 N/A 99.66

Ar/H2O/H2(3%) 1.94 0.04 N/A 0.01 0.35 N/A 42.40 N/A 99.57

Ar/H2O/H2(10%) 2.15 0.99 N/A 0.01 0.30 N/A 42.30 N/A 99.68

N/A: not applicable. a Uncertainties in chemical analysis data: Cd ± 5%; Cs and I ± 10%

266 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268

Fig. 4. Measured average particle number size distribution according to the atmosphere. increased (Table 7). The percentage gaseous iodine was 1.3 to 50 times 6. Conclusions higher for the carrier gas containing the higher amount of hydrogen

(Ar/H2O/H2(10%)). However, although the experiment conducted at Comparing Phébus FPT2 and FPT3 test data, it was hypothesized 650 °C presents a decrease of aerosols containing silver with the amount that the control rod materials would, at least partly, explain the dif- of hydrogen, which is consistent with a decrease of gaseous iodine, the ference in the measured gaseous iodine fraction in the containment. The experiment at 400 °C showed an opposite behaviour. The substantial experiments presented in this paper aimed at determining and quanti- mass concentration of aerosol iodine in condition Ar/H2O at 400 °C may fying compounds released due to reactions between fission products be due to an error in the analysis, because no matching amount of silver and SIC materials on primary circuit surfaces. Possible interactions was detected on the filter. between caesium iodide and cadmium and caesium iodide and silver in In all the experiments at 400 °C, the number of particles monitored condensed phase at 400 °C and 650 °C under different atmospheres by SMPS was very low and could not be distinguished from the back- were studied. In addition, the behaviour of silver iodide at 400 °C and ground. On the other hand, at 650 °C, the average particle number size 650 °C was investigated. distributions can be drawn for the experiments (Fig. 4). The first point From this study, it appeared that none of these reactions led to an to notice is the difference between the two experiments. The average extensive decomposition of caesium iodide when present as a deposit at size distribution from the Ag + CsI experiment was similar to the CsI 650 °C. At 400 °C, the behaviour and transport of the elements was experiment for condition Ar/H2O. The second noteworthy point was the rather uncertain due to the low release rates. By comparing the influ- slight drop in the particle number concentration when adding hy- ences of cadmium and silver on the transport of iodine in the studied drogen. In the case of the Ag + CsI experiment, the average particle size facility, it seemed that the transport of gaseous iodine was slightly in- increased as the amount of hydrogen increased. The opposite behaviour creased in the presence of silver. Experimental results showed signs of a was observed for the AgI experiment and the CsI experiment. reaction between silver and caesium iodide, although the amounts of Silver iodide has a higher saturation pressure than caesium iodide. gaseous iodine released were tripled compared with the caesium iodide Consequently, it should exist in gaseous phase at lower temperature vaporisation. than caesium iodide. The proportion of gaseous silver was low, as was The transport of cadmium was clearly influenced by the presence of measured by ICP-MS in the liquid traps. This would explain why there caesium iodide. When vaporised alone, cadmium was mainly trans- was not a lot of silver in the gas phase during sampling. The presence of ported as vapour and mostly condensed on the facility's walls. gaseous iodine in the experiments with silver iodide precursor proved Therefore, it was not surprising that the caesium iodide aerosols, re- that the decomposition of silver iodide occurred and appeared to be leased in the experiments with both precursor, acted as condensation more efficient under 2Ar/H O/H2 atmosphere. Aerosols made of silver centres for the volatile cadmium vapours. By contrast, the transport of without iodine may be formed. This is in agreement with what has silver did not appear to be increased by the presence of caesium iodide. already been shown by Kovács and Konings [43], who observed the When examining the influence of atmosphere composition, it was persistence of Ag in condensed phase but also pointed out that the apparent that the transport of iodine, caesium and cadmium as aerosols decomposition was not total and that AgI, like (AgI)3, did exist. was increased in an Ar/H2O/H2(10%) atmosphere. The decrease of Contrary to the case of caesium iodide, the release of gaseous iodine gaseous iodine release with the addition of hydrogen in the carrier gas from silver iodide was enhanced by the presence of hydrogen. This when silver was added to caesium iodide at 650 °C also seemed to be difference could be related to the difference of solubility in water ofthe related to the lower formation of caesium hydroxide and silver iodide. two compounds. Silver iodide is not soluble in water and would pre- By comparing how cadmium and silver affect the chemistry and cipitate in aqueous aerosol droplets, whereas caesium iodide is highly transport of caesium iodide, the following observations can be made. soluble in water. First, the amount of gaseous iodine released at 125 °C was two times higher with silver (27 mg/m3 with cadmium vs 60 mg/m3 with silver

for vaporisation in Ar/H2O atmosphere at 650 °C) than with cadmium

267 M. Gouëllo et al. Nuclear Materials and Energy 17 (2018) 259–268 for an initial mass of caesium iodide similar (respectively 2.5 g and 2 g). 06.040. In addition, the two elements showed opposite behaviour with the [19] F. Cousin, K. Dieschbourg, F. Jacq, New capabilities of simulating fission product transport in circuits with ASTEC/SOPHAEROS v. 1.3, Nucl. Eng. Des. 238 (2008) addition of hydrogen to the carrier gas. While the amounts of gaseous 2430–2438 http://www.sciencedirect.com/science/article/pii/ and aerosol iodine increase in presence of cadmium, they decrease in S0029549308001866. presence of silver. [20] L. Cantrel, F. Louis, F. Cousin, Advances in mechanistic understanding of iodine behaviour in PHEBUS-FP tests with the help of Ab initio calculations, Ann. Nucl. Energy. 61 (2013) 170–178, https://doi.org/10.1016/j.anucene.2013.02.034. Acknowledgments [21] A.M. Beard, C.G. Benson, B.R. Bowsher, S. Dickinson, A.L. Nichols, I.R. Beattie, P.J. Jones, J.S. Ogden, N.A. Young, E.R. Buckle, Fission product and aerosol behavior This work was funded by the Finnish Research Programme on within the containment, 1990, Report EUR 12844 EN. [22] C.G. Benson, B.R. Bowsher, A.L. Nichols, The interaction of fission product vapors Nuclear Power Plant Safety, SAFIR2014. with aerosols in severe reactor accidents, Fission Prod. Transp. Process. React. Accid. Hemisphere Publishing, New York, NYUSA, 1989, pp. 433–446. References [23] R.G.J. Ball, B.R. Bowsher, E.H.P. Cordfunke, S. Dickinson, R.J.M. Konings, Thermochemistry of selected fission product compounds, J. Nucl. Mater 201 (1993) 81–91 https://doi.org/10.1016/0022-3115(93)90161-Q. [1] D. Magallon, A. Mailliat, J.-M. Seiler, K. Atkhen, H. Sjövall, S. Dickinson, J. Jakab, [24] R.J.M. Konings, A.S. Booij, E.H.P. Cordfunke, High-temperature infrared study of L. Meyer, M. Buerger, K. Trambauer, L. Fickert, B.R. Sehgal, Z. Hozer, J. Bagues, the vaporization of CsI, CdI2 and Cs2CdI4, Vib. Spectrosc. 2 (1991) 251–255 F. Martin-Fuentes, R. Zeyen, A. Annunziato, M. El-Shanawany, S. Guentay, https://doi.org/10.1016/0924-2031(91)85033-J. C. Tinkler, B. Turland, L.E.H. Puebla, European Expert Network for the Reduction of [25] R. Mishra, S.R. Bharadwaj, A.S. Kerkar, S.R. Dharwadkar, Standard Gibbs energy of Uncertainties in Severe Accident Safety Issues (EURSAFE), Nucl. Eng. Des. 235 formation of Cs2CdI4, J. Nucl. Mater. 240 (1997) 236–240 https://doi.org/10. (2005) 309–346, https://doi.org/10.1016/j.nucengdes.2004.08.042. 1016/S0022-3115(96)00674-5. [2] B. Schwinges, C. Journeau, T. Haste, L. Meyer, W. Tromm, K. Trambauer, Ranking [26] R.D. Spence, A.L. Wright, The importance of fission production/aerosol interactions of severe accident research priorities, Prog. Nucl. Energy. 52 (2010) 11–18, https:// in reactor accident calculations, Nucl. Technol. (1987) 77 https://doi.org/10. doi.org/10.1016/j.pnucene.2009.09.006. 13182/NT87-A33980. [3] W. Klein-Heßling, M. Sonnenkalb, D. Jacquemain, B. Clément, E. Raimond, [27] R.A. Sallach, R.M. Elrick, S.C. Douglas, A.L. Ouelette, Reactions between some H. Dimmelmeier, G. Azarian, G. Ducros, C. Journeau, L.E. Herranz Puebla, cesium–iodide compounds and the reactor materials 304 stainless steel, inconel 600 A. Schumm, A. Miassoedov, I. Kljenak, G. Pascal, S. Bechta, S. Güntay, M.K. Koch, and silver, vol. II. Cesium Iodide Reactions, US NRC report NUREG, 1986. I. Ivanov, A. Auvinen, I. Lindholm, Conclusions on severe accident research prio- [28] B.R. Sehgal, Nuclear Safety in Light Water Reactors: Severe Accident rities, Ann. Nucl. Energy. (2014), https://doi.org/10.1016/j.anucene.2014.07.015. Phenomenology, first ed., Academic Press, 2011, http://books.google.fr/books? [4] B. Clément, R. Zeyen, The objectives of the Phébus FP experimental programme and hl=fr&lr=&id=8Vjmn6nVfp8C&oi=fnd&pg=PP2&dq=Nuclear+Safety+in main findings, Ann. Nucl. Energy. 61 (2013) 4–10, https://doi.org/10.1016/j. +Light+Water+Reactors+-+Severe+Accident+Phenomenology&ots= anucene.2013.03.037. ergL221ExE&sig=Zn8KIlOqXniMaN3VK3tH9UDqEQ4. [5] N. Girault, C. Fiche, A. Bujan, J. Dienstbier, Towards a better understanding of [29] E.H.P. Cordfunke, R.J.M. Konings, The release of fission products from degraded iodine chemistry in RCS of nuclear reactors, Nucl. Eng. Des. 239 (2009) 1162–1170 UO2 fuel: thermochemical aspects, J. Nucl. Mater. 201 (1993) 57–69, https://doi. https://doi.org/10.1016/j.nucengdes.2009.02.008. org/10.1016/0022-3115(93)90159-V. [6] N. Girault, F. Payot, Insights into iodine behaviour and speciation in the Phébus [30] N. Girault, S. Dickinson, F. Funke, A. Auvinen, L. Herranz, E. Krausmann, Iodine primary circuit, Ann. Nucl. Energy. 61 (2013) 143–156, https://doi.org/10.1016/j. behaviour under LWR accident conditions: lessons learnt from analyses of the first anucene.2013.03.038. two Phébus FP tests, Nucl. Eng. Des. 236 (2006) 1293–1308, https://doi.org/10. [7] L. Soffer, S.B. Burson, C.M. Ferrell, R.Y. Lee, J.N. Ridgely, Accident source termsfor 1016/j.nucengdes.2005.12.002. light-water nuclear power plants, Final Report, 1995. http://inis.iaea.org/Search/ [31] N. Girault, L. Bosland, J. Dienstbier, R. Dubourg, C. Fiche, LWR severe accident search.aspx?orig_q=RN:26050072. simulation fission product behavior in FPT2 experiment, Nucl. Technol. 169 (2010) [8] J. Kalilainen, P. Rantanen, T. Kärkelä, M. Lipponen, A. Auvinen, J. Jokiniemi, 218–238 https://doi.org/10.13182/NT10-A9375. Effects of molybdenum and silver on iodine transport in primary circuit onsevere [32] A.M. Beard, B.R. Bowsher, A.L. Nichols, Interaction of molecular iodine vapour with nuclear accidents, Proceedings of the 2012 International Congress on Advances in silver-indium-cadmium control rod aerosol, Proc. an Int. Symp. Sev. Accid. Nucl, Nuclear Power Plants - ICAPP '12, Chicago, USA, 2012, pp. 1–14. Power Plants, SorrentoItaly, 1988, pp. 201–213. [9] J. Kalilainen, T. Kärkelä, R. Zilliacus, U. Tapper, A. Auvinen, J. Jokiniemi, Chemical [33] E.H.P. Cordfunke, R.J.M. Konings, Thermochemical data for reactor materials and reactions of fission product deposits and iodine transport in primary circuit con- fission products: the ECN database, J. Phase Equilibria. 14 (1993) 457–464 https:// ditions, Nucl. Eng. Des. 267 (2014) 140–147 https://doi.org/10.1016/j.nucengdes. doi.org/10.1007/BF02671964. 2013.11.078. [34] P.D. Bottomley, R.S. Dickson, T. Routamo, J. Dienstbier, A. Auvinen, N. Girault, [10] M. Gouëllo, J. Hokkinen, T. Kärkelä, A. Auvinen, A scoping study of chemical be- Revaporisation Issues: an overview, http://www.sar-net.org/upload/s4-6.pdf, haviour of caesium iodide in presence of boron in condensed phase (650 °c and (2007). 400 °c) under primary circuit conditions, Nucl. Technol. (2018) NT17–46 pa [35] L. Cantrel, E. Krausmann, Reaction kinetics of a fission-product mixture in a steam- https://doi.org/10.1080/00295450.2018.1429111. hydrogen carrier gas in the Phébus primary circuit, Nucl. Technol. 144 (2003) 1–15 [11] A.-C. Grégoire, J. Kalilainen, F. Cousin, H. Mutelle, L. Cantrel, A. Auvinen, T. Haste, http://cat.inist.fr/?aModele=afficheN&cpsidt=15161318. S. Sobanska, Studies on the role of molybdenum on iodine transport in the RCS in [36] I.R. Beattie, B.R. Bowsher, T.R. Gilson, P.J. Jones, The interaction of caesium iodide nuclear severe accident conditions, Ann. Nucl. Energy. 78 (2015) 117–129, https:// with boric acid, Proc. Spec. Work. Iodine Chem. React. Saf. (1985) 253–268. doi.org/10.1016/j.anucene.2014.11.026. [37] P. Patnaik, Handbook of Inorganic Chemicals, McGraw-Hill, New York, 2003ftp:// [12] M. Gouëllo, H. Mutelle, F. Cousin, S. Sobanska, E. Blanquet, Analysis of the iodine pvictor.homeftp.net/public/Sci_Library/Chem Library/Handbooks/Patnaik P. gas phase produced by interaction of CsI and MoO3 vapours in flowing steam, Nucl. Handbook of inorganic chemicals (MGH, 2003)(T)(1125s).pdf. Eng. Des. 263 (2013) 462–472, https://doi.org/10.1016/j.nucengdes.2013.06.016. [38] E. Hontañón, A. de los Reyes, J. . Capitão, The CAESAR code for aerosol re- [13] A.-C. Grégoire, H. Mutelle, Experimental study of the [Mo, Cs, I, O, H] and [B, Cs, I, suspension in turbulent pipe flows. assessment against the STORM experiments, J. O, H] systems in the primary circuit of a PWR in conditions representative of a Aerosol Sci. 31 (2000) 1061–1076, https://doi.org/10.1016/S0021-8502(00) severe accident, Int. Conf. Nucl. Energy New Eur. Ljubljana, Slovenia, 2012. 00028-8. [14] P.D. Bottomley, K. Knebel, S. van Winckel, T. Haste, S.M.O. Souvi, A. Auvinen, [39] Z.V Popović, G. Stanišić, D. Stojanović, R. Kostić, Infrared and Raman spectra of J. Kalilainen, T. Kärkelä, Revaporisation of fission product deposits in the primary CdO, Phys. Status Solidi. 165 (1991) K109–K112 https://doi.org/10.1002/pssb. circuit and its impact on accident source term, 6th Eur. Rev. Meet. Sev. Accid. Res. 2221650249. 2013. [40] C.G. Benson, M.S. Newland, Interactions of Fission Product Vapours with Aerosols, [15] D.A. Petti, Silver-indium-cadmium control rod behavior in severe reactor accidents, Winfrith, United Kingdom, 1996, pp. 233–281 Report PSI-97-02. Nucl. Technol. 84 (1989) 128–151 https://doi.org/10.13182/NT89-A34183. [41] R.G.J. Ball, S. Dickinson, E.H.P. Cordfunke, R.J.M. Konings, J. Drowart, S. Smoes, [16] A.-C. Grégoire, T. Haste, Material release from the bundle in Phébus FP, Ann. Nucl. Reactor safety programme 1988-91, Thermochemical Data Acquisition. Part 2, Energy. 61 (2013) 63–74 https://doi.org/10.1016/j.anucene.2013.02.037. 1992. http://cat.inist.fr/?aModele=afficheN&cpsidt=48404. [17] M. Steinbrück, U. Stegmaier, M. Grosse, Experiments on silver-indium-cadmium [42] A.M. Beard, C.G. Benson, B.R. Bowsher, The Interaction of Simulant Fission Product control rod failure during severe nuclear accidents, Ann. Nucl. Energy. 101 (2017) Vapours with Control Rod Aerosols in a Flowing System, United Kingdom Atomic 347–358, https://doi.org/10.1016/j.anucene.2016.11.039. Energy Authority, Winfrith, United Kingdom, 1988. [18] P. Chatelard, N. Reinke, S. Arndt, S. Belon, L. Cantrel, L. Carenini, K. Chevalier- [43] A. Kovács, R.J.M. Konings, Trimer formation of AgI. A gas-phase FT-IR and theo- Jabet, F. Cousin, J. Eckel, F. Jacq, C. Marchetto, C. Mun, L. Piar, ASTEC V2 severe retical study, J. Mol. Struct. 643 (2002) 155–160, https://doi.org/10.1016/S0022- accident integral code main features, current V2.0 modelling status, perspectives, 2860(02)00443-X. Nucl. Eng. Des. 272 (2014) 119–135, https://doi.org/10.1016/j.nucengdes.2013.

268