The U.S. Department of Energy may have mistakenly shipped “reactive” nuclear material that was incorrectly labeled as low-level radioactive waste into Nevada in dozens of shipments over the past six years.

The shipments started in 2013 and stopped in December 2018. They were supposed to send only low-level radioactive waste from the department’s Y-12 facility in Oak Ridge, Tennessee, to the Nevada National Security Site, roughly 90 miles northwest of Las Vegas, for secure disposal.

But on July 3, the Energy Department informed Nevada officials, that reactive material — which might be unstable under normal conditions and may have violent reactions when combined with other substances — may have been included in those shipments.