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.

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