In fusion reactor breeding blankets (BB), liquid metals (LM) are employed as working fluid due to their attractiveness as coolant, tritium carrier, and breeder. Their interaction with the strong magnetic field causes the onset of magnetohydrodynamic (MHD) phenomena within the fluid piping network. Compared with an ordinary hydrodynamic case, a MHD flow shows altered features, such as a significant flow redistribution, enhanced pressure losses and a rearrangement of turbulent structures. Therefore, it is well known that heat transfer in a LM BB must be evaluated considering all these phenomena. In this project, a correlations-based model able to predict the MHD heat transfer in the forced convection regime will be developed and it will be implemented in RELAP5/MOD3.3 system thermal-hydraulic code.
The reduced model will accurately predict the heat transfer coefficient (HTC) for forced convection regime in square/rectangular ducts and circular pipes. The range of application is foreseen to be from the purely electromagnetic laminar flow to the turbulence regime. The correlations will be derived for this purpose by an extensive assessment of literature regarding MHD heat transfer for fusion applications. At last, the verification and validation procedure of the software will be carried out to verify the reliability of the code.
The MHD heat transfer model, together with the MHD pressure drop model already implemented by the Sapienza DIAEE research team, is going to extend the applicability of the code to nonisothermal MHD flows. Those newly capabilities will make the enhanced RELAP5/MOD3.3 version (referred as RELAP5-FUSION) a fast and reliable numerical tool for supporting fusion reactor operational and safety design. It will stand as the first prototype of best-estimate thermal-hydraulic system codes that, as for fission reactors practice, could be employed for fusion rectors licensing procedure, which is mandatory to build and operate a nuclear power plant.
Nowadays, a dedicated code able to simulate all MHD the phenomena involved in a liquid metal blanket is not yet available. However, the development of such a software is desirable since many fusion reactor elements are amenable to be treated in that a simplified way and, above all, that could be applied to safety demonstration for licensing process of fusion NPPs.
Generally, blanket-scale analyses of MHD effects are performed adopting a semi-analytical approach, in which empirical and semi-empirical correlations are supplemented with data extrapolated by direct numerical simulations (see e.g.[1]). This methodology, although effective to a certain degree, happens to be extremely time-consuming, not flexible in its scope, and with serious limitations in the achievable spatial and temporal resolution. Several computational fluid dynamic (CFD) tools have dedicated MHD modules, such as ANSYS-CFX, ANSYS-FLUENT and OpenFOAM. Unfortunately, reactor-scale analysis is not yet possible with CFD codes since it will be prohibitive in terms of calculation time and computational effort and it is going to be for the foreseeable future. Conversely, SYS-TH codes could enable the efficient and quick simulation of the blanket at a system level but, currently, they have limited or non-existent MHD capability. In this framework, Sapienza DIAEE researchers have already started a developing campaign for the system code RELAP5/MOD3.3 (extensively validated for a wide range of fission reactor applications), to include MHD features relevant for fusion design activities. Specifically, a module for evaluating MHD enhanced pressure drop has been implemented.
After these modifications, Sapienza RELAP5-FUSION is already competitive, in terms of capability with other system codes that include similar features. For instance, rudimentary and limited MHD pressure drop correlations are implemented in RELAP5-3D, MARS-FR and MELCOR. The code MHD-SYS, developed at the Imperial College of London [2], includes models for the simulation of flows in multiple electrical-coupled ducts and heat transfer for basic layouts. Unfortunately, MHD-SYS relies on coupling with CFD software, nullifying the main benefit of a stand-alone SYS-TH code, hampering its agile functioning.
A BE code valuable for supporting fusion licensing procedure should be able to manage the interplay of hydrodynamic, magnetohydrodynamic and heat transfer which is found in plasma-facing components and blankets adopting liquid metals as working fluids. The fundamental MHD modelling features that such a code should meet are:
- Electromagnetic drag losses evaluation (MHD 2D pressure drop)
- Electromagnetic losses evaluation in complex geometries (MHD 3D pressure drop)
- MHD forced convection heat transfer mechanism
- Electromagnetic coupling losses evaluation (Madarame effect [3])
The objective of the project proposed here is to further extend the level of innovation of RELAP5-FUSION that, as underlined above, owns already the capability of modelling 2D and 3D MHD losses of load. To overcome the current state of art it has been decided to develop and implement a reduced order model that can treat the forced convection heat transfer in MHD flows. Correctly assessing the heat transfer in a MHD environment is of paramount importance, for instance, to ensure the thermo-mechanical structural stability in blanket concepts with ancillary cooling system or an acceptable performance of a LM film divertor. Furthermore, this implementation could set the first step for the derivation of a module that can handle natural convection mechanism within the blanket piping network, which is a "second-order" phenomenon but could deeply impact the tritium transport efficiency and ,thus, the reactor safety.
When the project activity will be completed, RELAP5-FUSION will be the most advanced stand-alone BE SYS-TH code able to automatically evaluate MHD influence on blanket-scale operational or incidental transients and it will represent a solid prototype for prospective fusion licensing codes.
References
[1] A. Tassone, G. Caruso, and A. Del Nevo, Influence of PbLi hydraulic path and integration layout on MHD pressure losses, Fusion Engineering and Design, vol. 155, no. January, p. 111517, 2020. [Online]. Available: https://doi.org/10.1016/j.fusengdes.2020.111517
[2] M. J. Wolfendale and M. J. Bluck, A coupled systems code-CFD MHD solver for fusion blanket design,Fusion Engineering and Design, vol. 98-99, pp. 1902-1906, 2015.
[3] H. Madarame, K. Taghavi, and M. S. Tillack, Influence of Leakage Currents on Mhd Pressure Drop. Fusion Technology, vol. 8, no. 1 pt 2(A), pp. 264-269, 1985.