For the past 15 years, the Waste Isolation Pilot Plant (WIPP) has stored transuranic waste from the US nuclear-defense programme. The facility, located 650 meters below ground in the bedded salt deposits of southeastern New Mexico, is run by the US Department of Energy (DOE) and will be permanently sealed in 2033. Yet an arms-control agreement made with Russia in 2000 requires the United States to dispose of 34 tonnes of weapons plutonium, which a recent DOE panel recommended should be stored at WIPP. Tripling the amount of plutonium held at WIPP could increase the risk of release of radioactive material to the biosphere. Safety assessments have so far not adequately considered chemical interactions of this material with that already stored in the repository. In 2014, for example, plutonium-contaminated nitrate salts reacted with a wheat-based kitty litter used to absorb liquid wastes, resulting in a small radioactivity leak to the surface. Reassessment of the risk of potential human ‘intrusion’ in the future is also necessary. Inadvertent drilling through the repository, in the search for oil and gas, could release brine into the tunnels, spreading radioactivity to groundwater. The addition of this weapons plutonium will require expansion of the repository, increasing the probability of intrusion, and will increase the amount and chemical complexity of the radioactive material that might interact with the brine. The DOE should reassess its confidence in WIPP’s performance over the millennia during which this material will remain a threat to environmental safety before adding an additional 34 tonnes of plutonium to its inventory.