Proceedings of ICAPP 2017 April 24-28, 2017 - Fukui and Kyoto (Japan) A STARTING PROCEDURE FOR THE MSFR: APPROACH TO CRITICALITY AND INCIDENT ANALYSIS D. Heuer, A. Laureau, E. Merle-Lucotte*, M. Allibert, D. Gerardin LPSC-IN2P3-CNRS / Université Grenoble Alpes Address: LPSC, 53 avenue des Martyrs, 38026 Grenoble Cedex – France laureau.axel@gmail.com, merle@lpsc.in2p3.fr, heuer@lpsc.in2p3.fr Molten salt reactors are liquid-fueled reactors so that they are flexible in terms of operation (load-following capabilities…) or design (core geometry, fuel composition, specific power level…) choices, but they are very different in terms of design and safety approach compared to solid- fueled reactors. Such reactors call for a new definition of their operating procedures. Dedicated studies are performed in the frame of the European SAMOFAR (Safety Assessment of Molten Salt Fast Reactors) project of Horizon2020. This paper focuses on the behavior of the MSFR fuel circuit in interaction with the intermediate circuit. It is devoted to the start-up procedure of the MSFR, including the description of the proposed procedure, a presentation of the approach to criticality and the reactivity measurement during the filling of the core, and finally studies of accident scenarios (overcooling, reactivity insertion) at low power during the divergence step of the MSFR, highlighting the very good behavior of the reactor to such abnormal transients. I. INTRODUCTION The MSFR 1,2 , as a liquid-fueled reactor where the fuel also acts as the coolant and is circulating, calls for a new definition of its operating procedures taking into account the characteristics of such a system. Regarding the neutronics characteristics, the negative feedback coefficient of the MSFR, around -8 pcm/K coming half from the density effect and half from the Doppler effect, provides intrinsic reactor stability. Unlike with solid-fueled reactors, the negative feedback coefficient acts very rapidly since the heat is produced directly in the coolant, the fuel salt itself being cooled in the heat exchangers. Also in such a circulating-fuel system, the fraction of delayed neutrons is reduced because the fuel motion drift the delayed neutron precursors in low importance areas. The calculation of this important reactor kinetics parameter thus requires special tools 3,4 . More globally the modeling of such reactors requires specific treatments to take into account all the phenomena associated to the liquid fuel circulation. Finally a MSFR design characteristics also impacts strongly the reactor operation: no control rod is foreseen in the core, the reactor being driven by the heat extraction. This definition of the operating conditions of the MSFR is one of the main tasks to be accomplished in the frame of the European SAMOFAR (Safety Assessment of the Molten Salt Fast Reactor) project of Horizon2020. This definition will rely on a system code under development and on physical studies presented in this paper for the start- up procedure. These preliminary operation procedures are based on discussions, advice from experts and calculations with a coupled simulation tool for precise transient studies as well as simplified neutronics point-kinetics calculations to identify procedures. After a section dedicated to the description of the MSFR concept and of the startup procedure, the simulation tool used for the present study are presented in section III. The approach to criticality is then detailed in section IV through the reactivity measurement and prediction during the filling step of the core. Finally, relying on coupled neutronics- thermalhydraulics calculations, the behavior of the reactor is studied for two abnormal transients (over-cooling and accidental reactivity insertion) that may occur during the divergence step, at low power (section V). II. DESCRIPTION OF THE CONCEPT AND THE STARTUP PROCEDURE II.A. Molten Salt Fast Reactor Concept Since 15 years, the National Centre for Scientific Research (CNRS, Grenoble-France) has focused R&D efforts on the development of a new molten salt reactor concept called the Molten Salt Fast Reactor (MSFR) selected by the Generation-IV International Forum (GIF) due to its promising design and safety features 1 . The reference MSFR design is a 3000 MWth reactor with a total fuel salt volume of 18 m 3 , operated at a mean fuel salt temperature of 700°C (Ref. 2). The fuel salt is composed of a molten lithium fluoride salt containing the heavy nuclei: the fertile matter being 232 Th and the fissile matter that can be used being 233 UF4 and/or enr UF4 and/or (Pu- MA)F3. The present studies have been done with a fuel salt composition corresponding to the beginning of life of a