Effect of Pebble Bed Electrical Resistivity on Electromagnetic Force in WCCB TBM

Abstract

Functional materials (multiplier and breeder) formed a pebble bed are considered in many breeding blanket (BB) concepts in both ITER test blanket module (TBM) program and DEMO fusion reactor. In a magnetic confinement fusion reactor, it is crucial to thoroughly assess the impact of pebble bed electrical resistivity on electromagnetic (EM) forces during a plasma disruption. This is essential for maintaining the structural integrity and normal operations of the reactor in an intense and gradient magnetic field environment. However, there is a lack of information and rare study related to pebble bed electrical resistivity investigations affecting EM force performance in a magnetic confinement fusion reactor, which might impact the existing design achievements. The study proposes and conducts parametric pebble bed electrical resistivity analyses to investigate Lorentz force performance in the current structural design of water-cooled ceramic breeder (WCCB) under ITER major plasma disruption event load. The proposed electrical resistivities ( $7.2\times 10^{-7}~\Omega $ m, $3.0\times 10^{-4}~\Omega $ m, and infinity) cover a wide design range. The transient EM numerical analyses compare the performance of WCCB TBM with different electrical resistivities. The results show that the maximum Lorentz force of WCCB TBM is almost 38% higher when the pebble bed electrical resistivity is $3.0\times 10^{-4}~\Omega $ m compared to $7.2\times 10^{-7}~\Omega $ m. No significant difference in maximum Lorentz force is observed when the pebble bed electrical resistivity is increased to infinity. Further investigations reveal that the higher magnitude of Lorentz force in cases with higher pebble bed electrical resistivity is induced by conduction current flows. As the pebble bed electrical resistivity increases, conduction current moves from the pebble bed to the container and the U-shaped cooling channels fabricated from reduced activation ferritic/martensitic steel named F82H, owing to its lower electrical resistivity. Based on these findings, the application of higher pebble bed electrical resistivity could be recognized as a conservative and robust approach in the preliminary design phase of BB development to avoid adverse impacts on the design achievements.