Chemistry of Molten Salt Reactor Fuel Salt Candidates

Abstract

Molten salt reactor (MSR) concepts are based on using a molten salt mixture as a primary nuclear reactor coolant, while the fuel can be either directly dissolved in the coolant or can be solid. MSRs can be operated with (epi)thermal or fast neutron spectra, as simple convertors or breeder reactors utilising, e.g., the 232Th-233U cycle, but also as accelerator-driven sub-critical reactors or transuranium elements (TRU) burners. The main advantages of the MSRs using a liquid molten salt fuel resides in the enhanced safety features such as a large negative temperature coefficient of reactivity due to a high thermal expansion, low atmospheric pressure in the core during the operation and no need of transport and fabrication of new fuel elements in the case of closed fuel cycle option. In addition, the liquid fuel brings possibility of continuous fission-product removal via physical and pyrochemical processes which brings flexibility in the fuel cycle. In the last years, development of MSR technology is on the increase in many countries and since 2002, the MSR concept is included to the Generation IV initiative. Since 2001, the European Commission (EC) carries on coordinated R&D activities on the MSR concept within the EC/EURATOM framework programmes. Several projects have been funded exploring the feasibility and safety of the Molten Salt Fast Reactor (MSFR) concept studied in Europe. MSFR is based on a non-moderated epi-thermal neutron spectrum with its fuel dissolved in a fluoride molten salt carrier and utilising a closed thorium fuel cycle. Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. The present work is summarising the recent research in this field carried out at Joint Research Centre (JRC) of the European Commission. It is particularly focused on the synthesis and characterisation of pure actinide fluorides UF4, ThF4 and PuF3 needed for the experimental assessment of the thermodynamic properties of the MSR fuel salt candidates. In addition, the recent achievements of these studies are presented, e.g., re-determination of the UF4 melting point and measurement of the novel thermodynamic data of PuF3-LiF system.JRC.G.I.3-Nuclear Fuel Safet