The U.S. Department of Energy says it has identified a key culprit behind a radiation leak last year at its underground nuclear dump in southeastern New Mexico: one of the drums contained "chemically incompatible contents," including the wrong kind of cat litter.

The finding is included in a 277-page report, the product of a year-long investigation into an accident at the federally-owned Waste Isolation Pilot Plant (WIPP) that released radiation into the air, exposing at least 22 workers, alarming residents of nearby Carlsbad, and leading to a suspension of operations at the site.

According to the investigators, Drum 68660 was the source of the "radioactive contamination" released. It had been packaged with incompatible contents at the Los Alamos National Laboratory in northern New Mexico and then sent to the WIPP facility for disposal.

These contents included nitrate salt residues, neutralization agent (triethanolamine), and organic sorbent—an organic brand of cat litter known as Swheat Scoop.

"Experiments showed that various combinations of nitrate salt, Swheat Scoop®, nitric acid, and oxalate self-heat at temperatures below 100°C," states a summary of the report. "Computer modeling of thermal runaway was consistent with the observed 70-day birth-to-breach of Drum 68660."

According to NPR, cat litter is widely used at nuclear laboratories because the "absorbent material is great at soaking up liquid nuclear waste." However, Los Alamos National Laboratory had apparently switched to using the organic brand of litter, which caused the problem.

And it was no small breach.

Last fall, the DOE estimated that the disaster cost at least $240 million dollars and that it could be years before the site is fully functional.

The incident raised concerns about the safety and sustainability of the dump, which forms the bedrock of the U.S. government's current approach to dispose of military-generated plutonium-contaminated transuranic waste from decades of nuclear bomb production and testing.

The facility, which stores nuclear waste deep beneath the earth's surface in salt formations, is the only underground repository for materials above the lowest level of radiation.

The Department of Energy claims on its website that "WIPP has set the standard for safe, permanent disposal of long-lived radioactive defense wastes."