A way to limit the corrosion in the Molten Salt Reactor concept: the salt redox potential control M. Gibilaro *, L. Massot, P. Chamelot Université de Toulouse, UPS, CNRS, Laboratoire de Génie Chimique, 118 Route de Narbonne, F-31062 Toulouse, France A R T I C L E I N F O Article history: Received 16 December 2014 Received in revised form 22 January 2015 Accepted 23 January 2015 Available online 30 January 2015 Keywords: redox potential control Molten Salt Fast Reactor U A B S T R A C T The possibility of controlling the salt redox potential thanks to a redox buffer in the Molten Salt Fast Reactor was investigated, the goal was to limit the oxidation of the reactor structural material. Tests were performed in LiF-CaF 2 at 850  C on two different redox couples to fix the salt potential, Eu(III)/Eu(II) and U (IV)/U(III), where the first one was used as inactive system to validate the methodology to be applied on the uranium system. A metallic reducing agent (Gd plate for Eu, and U plate for U system) was inserted in the salt, leading to a spontaneous reaction: Eu(III) and U(IV) were then reduced. Eu(III) was fully converted into Eu(II) with metallic Gd, validating the approach. On the U system, the U(IV)/U(III) ratio has to be set between 10 and 100 to limit the core material oxidation: addition of metallic U decreased the concentration ratio from the infinite to 1, showing the feasibility of the salt redox potential control with the U system. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction Generation IV International Forum (GIF) identified and selected six nuclear energy systems based on four broad areas: sustain- ability, economics, safety and reliability, proliferation resistance and physical protection [1]. Among them, the Molten Salt Fast Reactor (MSR) represents one of the most promising option as it can be used as transmuter, to burn plutonium and other transuranic elements. The MSFR originality is the use of a liquid fuel made of a molten fluoride circulating in both reactor core and heat exchanger: the fission material, 233 U, is dissolved in this carrier salt, and used as a heat-transferring agent. The MSR development started in the Oak Ridge National Laboratory (ORNL) in the US with the military Aircraft Nuclear Propulsion Program in the 1950s [2]. This novel technology was then turned to civilian applications with the construction of the Molten Salt Reactor Experiment (MSRE) in the 1960s, operated with LiF-BeF 2 -ZrF 4 -UF 4 and the Molten Salt Breeder Reactor (MSBR) in the 1970s, with LiF-BeF 2 -ThF 4 -UF 4 [3,4]. ORNL demonstrated few decades ago the feasibility of the molten salt nuclear reactor concept. During reactor operation time, plethora of information were recorded and experiments evidenced the excellent compatibility and the good corrosion resistance of Hastelloy-N with molten fluoride system at 650  C [5]. Moreover, post-operative examination of the reactor container material showed that corrosion was successfully minimised by the control of the redox potential of the salt [6]. To regulate the redox conditions of the salt, a buffer couple was used: U(IV)/U(III). The redox potential of the salt is thus fixed by the uranium redox couple thanks to the Nernst law: E salt ¼ E  UðIVÞ=UðIIIÞ þ RT F ln ½UðIV Þ ½UðIIIÞ   (1) Where E  is the standard potential of U(IV)/U(III) couple (V), R the ideal gas constant (J/K/mol), F the Faraday constant (C/mol) and [x] the concentration of the x species (mol/L). Two different redox conditions are expected [7]: - either the salt redox potential is too reducing; in this case, two options have to be examined:  carbon is present in the core and may react with uranium to form carbides as described below: 4UðIIIÞ þ xC ¼ 3UðIV Þ þ UC x (2)  carbon is absent from the core: the tritium, produced by fission as TF, is reduced into gaseous tritium (T 2 ) and might diffuse through the structural material [8]. * Corresponding author. Tel.: +33 5 6155 7219. E-mail address: gibilaro@chimie.ups-tlse.fr (M. Gibilaro). http://dx.doi.org/10.1016/j.electacta.2015.01.142 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved. Electrochimica Acta 160 (2015) 209–213 Contents lists available at ScienceDirect Electrochimica Acta journa l home page : www.e lsevier.com/loca te/ele cta cta