The EAGLE in-pile ID1 test was performed by Japan Atomic Energy Agency to demonstrate the effectiveness of fuel discharge from a fuel subassembly with an inner duct structure during a core disruptive accident in a sodium-cooled fast reactor. The experimental results suggested that early duct wall failure observed in the test was initiated by high heat flux from the molten pool of fuel and steel mixture, and the post-test numerical calculation and analyses showed that the high thermal load may be enhanced effectively by molten steel with a rather high thermal conductivity. In this study, to overcome weakness in conventional fluid-dynamic calculations, we adopted a fully 3D Lagrangian approach based on the finite volume particle method to analyze the mechanisms of heat transfer from the molten pool to the duct wall in the ID1 test. A series of behaviors representing pin disruption, molten pool formation, as well as the mixing and separation of molten steel and fuel in the pool was simulated to investigate their effects on molten pool-to-duct wall heat transfer. The present 3D particle-based simulation, which moderated some inherent defects in our previous 2D calculations, clarified that direct contact of the solid fuel with nuclear heat and liquid steel near the duct wall can expose the duct wall to a large thermal load, which led to the duct wall failure in the experiment.