Experts from national laboratories around the country have determined that an incompatible cocktail of nitrate salts and organic cat litter is to blame for a mishap that forced the closure of the nation's only underground nuclear waste repository.

The independent technical team released its report Thursday.

The team was charged by the U.S. Energy Department to investigate the chemical reactions that may have led to the release of radioactive material at the Waste Isolation Pilot Plant in February 2014.

The report comes more than a year after a single container of waste stored at the repository breached and contaminated 21 workers with low-level radioactivity.

The container came from Los Alamos National Laboratory.

While it couldn't determine the cause of the breach with absolute certainty, the team says it's clear a thermal reaction inside the container forced the lid to pop.