One of the significant tasks facing defuelers in the aftermath of the Three Mile Island Unit 2 (TMI-2) accident has been the dismantlement and removal of the severely damaged reactor core. One of the most effective tools utilized to loosen and pulverize the debris bed and the once-molten, resolidified core was the core bore machine (CBM). This machine was very successful during the core stratification sampling program, which extracted core samples from the postaccident reactor core for data acquisition and analysis. The machine was later used to drill hundreds of holes in the hard, resolidified layer in the effort to advance the defueling process by pulverizing the core. Once again, the CBM proved effective. With all damaged fuel assemblies removed from the vessel, the majority of the fuel debris remaining in the reactor vessel is located in the lower core support assembly (LCSA) and the lower head of the reactor vessel. The only conceivable method of accessing the fuel remnants and debris is by severing and removing the massive stainless steel plates of the LCSA that inhibit deployment of defueling tools and equipment. A comprehensive program to remove the LCSA was initiated that incorporated the CBM and a plasma arc cutting system that could, in combination, be effective in removing the entire assembly. This paper describes the drilling equipment and the methods used to successfully remove the lower grid rib section utilizing the CBM as it has proved to be a viable technique for remotely cutting underwater stainless steel structures.