Workers began on May 10 to fill a 20-foot by 20-foot hole created when a railroad tunnel collapsed at a decommissioned nuclear production site in Washington the day before.

The tunnel was located next to a chemical processing facility known as the Plutonium Uranium Extraction Plant, or PUREX. The site was part of the Manhattan Project, which produced the first nuclear weapons in the United States, when it was built in the 1940s. Most of its reactors were shut down between 1964 and 1971.

The rail car tunnel that collapsed on Tuesday was used to store contaminated materials, but officials said no contamination leaked from the site.