The effective removal of nuclear waste from fission has attracted significant attention, with numerous porous sorbents reported in recent decades. The practical application of current sorbents is often hindered by limited removal efficiency and low production scalability. Here, we developed activated carbon fibers (ACFs) as an ultrafast and effective iodine capture material using a scalable method. The engineered ACFs possess and extraordinary micro/mesoporous structure with a surface area exceeding 2900 m2 g−1 while maintaining mechanical and thermal stability. The resulting fibers demonstrate a superior iodine capture capacity of 3.10 g g−1 and a capture rate of 2.76 g g−1 h−1. To further augment these properties, a novel oxygen-doping strategy was implemented. This approach dramatically improves performance, achieving 51% higher capacity (4.68 g g−1) and 76% faster rate (4.86 g g−1 h−1). Notably, exfoliation reactions of iodine within carbon layers that induced structural changes were discovered. Our work underlines the promise of ACFs for nuclear waste management.