Analysis of salt leakage due to corrosion-induced failure in molten salt thermal convection loop (MSTCL)

Abstract

A chloride-based molten salt reactors (MSRs) is preferred for fast neutron energy spectrum, which offers flexibility on choosing fissile materials, but an information of material corrosion is insufficient to develop MSR. The molten salt thermal convection loop (MSTCL) has been designed and constructed from stainless steel 316L to understand material-related issues relevant to MSR. However, during testing corrosion specimens, an event of the salt leakage and heater failure was detected in the weld zone of the loop. Microstructural observation with scanning electron microscopy showed wall thinning of the external surface near the perforated hole, intergranular attack in the internal surface, and unusual crevice and crack propagation in the intact weld zone. A root cause analysis has identified three molten chloride corrosion phenomena of impurity-driven intergranular attack, stress-assisted corrosion in crevice, and high temperature oxidation and chlorination in the atmosphere. It is confirmed that stress-assisted corrosion in crevice is responsible for the salt leakage. Residual stress due to welding combined with impurity-driven intergranular attack may lead to stress-assisted corrosion in crevice. Therefore, it can be inferred that crevice and residual stress in the internal surface are major causes of wall penetration of the loop and salt leak. Since both crevice and residual stress in the weld zone may originate from the manufacturing process involving welding, sufficient consideration should be given to the design of weld joint and the welding procedure to prevent these problem-causing factors from occurring.