Disposal of Surplus Plutonium at the Waste Isolation Pilot Plant: Interim Report evaluates the general viability of the U.S. Department of Energy’s National Nuclear Security Administration’s (DOE-NNSA’s) conceptual plans for disposing of 34 metric tons (MT) of surplus plutonium in the Waste Isolation Pilot Plant (WIPP), a deep geologic repository near Carlsbad, New Mexico. This report evaluates DOE-NNSA’s plans to ship, receive, and emplace surplus plutonium in WIPP and its understanding of the impacts of these plans on WIPP and WIPP-bound waste streams. This report, the first of two to be issued during this study, provides a preliminary assessment of the general viability of DOE-NNSA’s conceptual plans, focusing on some of the barriers to their implementation.