The breeding blanket is a critical component of a fusion reactor that involves the use of liquid metal (LM) as working fluid. One of the most challenging engineering issues in the blanket design is the necessity to account for the interaction between the electrically conductive LM and the magnetic field employed to confine the plasma. The LM is distributed to the blanket elementary cells composing the Breeding Zone (BZ) through a poloidal manifold. Breeder collectors are foreseen at blanket extremities to distribute it to and retrieve it from the BZ. From a geometrical point of view, the bottom outboard collector is the most complex, and it is where the 3D magnetohydrodynamic (MHD) effects are expected to contribute more to the pressure drops and flow distribution. The pressure drop increase, due to the MHD drag, is magnified when the channels are coupled with electroconductive walls. Moreover, there are considerable head losses located in correspondence with the connections between the manifold channels and the collector and near the collector feeding pipe, due to the 3D electric currents generated by velocity gradients in the stream-wise direction. In the framework of the development of the fusion reactor industrial demonstrator DEMO by the EUROfusion consortium, this project aims to investigate the 3D MHD flow numerically in complex geometrical elements, representing a simplified but realistic model of the bottom outboard collector and its connection with the spinal manifold. The main goals are the overall pressure drop estimate, the identification of the most relevant flow features, the evaluation of the 3D electric currents impact on the pressure drop and the study of electro-coupling phenomena between the channels including flow distribution. Parametric analyses on the position of the feeding orifice, the wall conductivity and the inlet velocity are performed to have a preliminary assessment of different breeding blankets concepts.
Nuclear fusion is considered the most promising energy source to meet the ever-increasing world demand for electricity consumption in the 21st century due to nearly carbon-free emissions, very limited production of only short-lived radioactive wastes and employing deuterium as nuclear fuel (cheap, uniformly distributed and virtually inexhaustible). However, the most promising fusion reaction involves the use of tritium that, as opposed to deuterium, has no significant natural source. Therefore, the design of a blanket able to breed tritium from lithium and able to guarantee the self-sufficiency of the reactor is a non-negotiable condition to improve the technology readiness level of the power generation from nuclear fusion and make it a viable commercial source.
In the framework of the EUROfusion programme, three breeding blanket concepts are considered, each with different configurations, in order to meet the DEMO design requirements whose feature radically different liquid metal hydraulic paths and manifold layouts. An accurate estimate of the pressure drops is necessary for the design of the PbLi loop and to estimate the performance of the blanket. Recent analyses have been conducted to evaluate the acceptable pressure loss in the PbLi loop, identifying the values of 2 MPa and 3 MPa, respectively for the Inboard and Outboard loops [1]. The main uncertainty is found for the collector for which, at the moment, there is not yet a complete understanding of the MHD effects, especially regarding the flow distribution. Therefore, this project aims to advance the state of the art in this direction.
The bulk of the MHD pressure drop in the blanket is localized in the inlet and outlet manifold where both high flow rates and complex geometrical elements are present and combine to form a 3D MHD flow that is not easily analysable using traditional analytical approaches derived for fully developed 2D flows. In this context, numerical simulations data from reliable CFD codes are an indispensable tool to provide a first estimate of the behavior of such flow, highlighting possible areas of concern, and to assess the pressure drop in the component. In the past, CFD codes have been used for the study of prototypical manifolds, mostly providing useful data for the flow distribution in basic rectangular geometries [2, 3 ,4, 5]. OpenFoam is an open source code that could meet the flexibility and adaptability requirements of MHD simulation, as commercial codes hardly allow the necessary implementation necessary for the simulation of these phenomena. Therefore, the development of an OpenFoam solver capable of analysing complex geometries would be a great step forward.
MHD flows through complex geometric elements represent some of the most advanced analyses regarding the flow of liquid metal through a magnetic field and a mandatory step in order to design a breeder blanket for a fusion reactor. This project can seriously enrich the state of art already in the first phase in which a simplified but realistic prototypical model of the OB bottom collector is analysed, including untreated complex geometrical elements such as a sudden symmetrical expansion. Moreover, the attention will focus not only on pressure drop, but also on the distribution of the flow between the manifold channels, a topic that has so far not found abundant attention in the literature compared to the estimate of pressure drop. In addition, in the hope of being able to manage as many possible analyses described previously, this project will allow for more and more realistic information compared to the actual collector configuration, which will provide help for the subsequent iteration of the design phase.
References
[1] A. Tassone, S. Siriano, G. Caruso, M. U., A. Del Nevo, "MHD pressure drop estimate for the WCLL in-vessel PbLi loop", Fusion Engineering and Design (in press).
[2] Smolentsev, S., Moreau, R., Bühler, L., & Mistrangelo, C. (2010). MHD thermofluid issues of liquid-metal blankets: phenomena and advances. Fusion Engineering and Design, 85(7), 1196-1205
[3] Morley, N. B., et al. "MHD simulations of liquid metal flow through a toroidally oriented manifold." Fusion Engineering and Design 83.7-9 (2008): 1335-1339.
[4] Rhodes, "Numerical Study of 3D Magnetohydrodynamic Flows Towards Liquid Metal Blankets, Including Complex Geometry and Buoyancy Effects" (2019) Doctoral Thesis.
[5] Mistrangelo, C., and L. Bühler. "Liquid metal magnetohydrodynamic flows in manifolds of dual coolant lead lithium blankets." Fusion Engineering and Design 89.7-8 (2014): 1319-1323.