A magnetohydrodynamic Method of Characteristics for the study of pressure transients in liquid metal piping systems

Abstract

The Liquid Metal Breeding Blanket (LM-BB) represents a promising advancement in magnetic confinement fusion reactor technology. The lead-lithium eutectic alloy (PbLi) is employed both as a tritium breeder and, in certain designs, as the primary coolant. For the EU-DEMO reactor, alternative configurations such as the Helium-Cooled Lead Lithium and Water-Cooled Lead Lithium (WCLL) are considered. The blanket is required to endure high-energy neutron irradiation, significant thermal loads, and steep pressure gradients, which heighten the risk of coolant channel failures, known as In-box Loss of Coolant Accidents (In-box LOCA). These incidents involve the rapid release of high-pressure fluid into the PbLi channels, creating complex two-phase flow and substantial pressure shocks. LOCA scenarios are inherently three-dimensional and are significantly influenced by the fluid compressibility. The rapid pressurization of the liquid metal underscores the necessity for detailed knowledge of fluid thermodynamic states to accurately model shock transients. Additionally, magnetohydrodynamic (MHD) phenomena, arising from the interaction between the electrically conductive fluid and the strong magnetic field, need to be considered. The full impact of MHD effects on shock pressurization remains largely unexplored both experimentally and numerically.

This research aims to provide an in-depth assessment of MHD effects on simplified single-phase pressure transients. Liquid metal hammers are analysed using the method of characteristics, which has been modified to incorporate the electromagnetic force term, dependent on magnetic field intensity and the electrical conductivity of the fluid and pipe walls. The methodology is validated against hydrodynamic experimental data and compared with limited numerical results available in the literature. A parametric analysis has been conducted to simulate pressure transients under fusion-relevant conditions for the WCLL test blanket module. Although not fully representative of safety–critical in-box LOCA scenarios, these analyses provide valuable preliminary insights into MHD effects on pressure transients. Observations indicate that within a magnetic field range of $1\le B\le 4$ T, the magnetic field can significantly dampen pressure waves, reducing transient times by three orders of magnitude. Similar conclusions are drawn when varying the wall conductance ratio of the structures from $0\le c<20$.

The method described in this study omits several underlying phenomena known to affect accidental scenarios. Nonetheless, the results presented offer the most advanced guidelines necessary for the development of more sophisticated tools.