New safety measures proposed for European Sodium Fast Reactor in Horizon-2020 ESFR-SMART project

Following up the previous European projects EFR and CP-ESFR, a new Horizon-2020 project, called ESFR-SMART, was launched in September 2017. This project, starting from the CP-ESFR design, will apply the new safety rules taking into account the lessons learned from the Fukushima accident, in order to increase the safety level of this European Sodium Fast Reactor (ESFR). In order to reach these new safety objectives, propositions are made to simplify as much as possible the design by using all the positive features of the Sodium Fast Reactors (SFR), i.e. low coolant pressure; high level of natural convection; possibility of decay heat removal by atmospheric air; high thermal inertia and long grace time before the human intervention.
These new safety objectives are presented in the paper from viewpoint of severe accidents prevention, defence in depth principles, extreme natural events to take into account, mitigation measures, etc. In all the cases, even in case of severe accident, early or significant radioactivity release requiring evacuation of the population will be avoided.
This paper gives a first list of propositions about ESFR, e.g.:

• Improved primary sodium confinement: The new design of the pit will be able to receive and confine the sodium in case of leak from the primary vessel. The level of sodium in the primary vessel in this case will remain high enough to assure natural convection through the core. A massive metallic roof above the pit assures the sodium containment even in the case of the worst severe accidents. Other measures are taken to avoid, even in this case of severe accident, primary sodium leaks in the above-roof area.
• Secondary loops design efficient in natural convection: Even in case of loss of feed water in the steam generators and loss of electricity supply for the secondary pumps, the measures taken on the secondary loops aim at ensuring an efficient decay heat removal by active or passive ways. These measures will include an optimized geometry of the secondary loops to promote the natural convection of the secondary sodium, the use of passive thermal pumps to increase the cooling flow rate, and the use of the steam generators modules to promote the cooling of their external surfaces by the natural convection of atmospheric air.
• Core design with improved safety parameters: special geometry and composition will significantly decrease a global void reactivity effect, and contributes to prevention of the severe accidents and mitigation of their consequences. Three types of control rods will be considered, including active and passive measures, i.e. activated by physical parameters, e.g. sodium temperature or flowrate.
• Three different systems will allow safe decay heat removal in all situations aimed to achieve the practical elimination of the loss of this function.

In conclusion, the paper gives a first review of the new propositions to enhance the ESFR safety. Some of these safety measures need additional R&D work for validation and some of them will be assessed in more details at the next phases of the ESFR-SMART project. The compliance of this new design with all safety rules has not yet been established at this stage of the project and will be studied later in dedicated tasks.