Analytical modeling of the emergency draining tank for a molten salt reactor

The Molten Salt Fast Reactor is a reactor concept developed by the European Union based on a liquid fuel salt circulating through the reactor core. A peculiar emergency system, which takes advantage of the liquid fuel state, is represented by a tank located underneath the core, where the fuel can be passively drained and cooled; its geometry ensures that the fuel remains in subcritical conditions. In the framework of the SAMOFAR project, a design for the Emergency Draining Tank has been proposed: the tank shall be equipped with vertical cooling elements, arranged in a hexagonal grid; the liquid fuel salt, which heats up due to decay heat, will fill the gaps between the elements. In this work, analytical methods (Green's functions and orthogonal decomposition) are employed to study the transient heat transfer associated with the proposed design and to perform a preliminary dimensioning of the system, such that overheating is avoided in any moment of the transient and the fuel salt is kept in a liquid state and in safe conditions for a long time. The models are constituted by multilayer monodimensional slabs and cylinders, with a pure heat conduction model. The assessment of the available grace time and preliminary considerations about fuel salt freezing and its influence on the system effectiveness are also included.