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The U.S. Energy Department expects shipments to the nation’s only underground nuclear waste repository to resume in April.

Officials said Tuesday they have plans for nearly 130 shipments from laboratories and other national defense sites over the next year. Those sites must demonstrate that they’re ready to load the radioactive waste and that it meets new safety requirements.

The Waste Isolation Pilot Plant was forced to close in February 2014 after an inappropriately packed drum of waste ruptured, contaminating part of the underground facility in southern New Mexico. Some operations resumed in December after an expensive recovery effort.

Over the last three years, tons of waste left over from decades of nuclear weapon research and development has stacked up at sites around the country, hampering the government’s multi-billion-dollar cleanup program.