Diagram of the WIPP Panel Closure (WPC) design , also known as the panel plug. This panel closure will consist of a 30' mortared concrete block explosion isolation wall and emplacement of 100' of run of mine salt as backfill. "Run of mine salt" refers to salt obtained from routine mining activities in the underground and used as backfill. This closure design is only about 1/3 as expensive as the original “Option D” Design. (USDOE)