Demolition work started Wednesday on the north end of the K-25 Building at the East Tennessee Technology Park in west Oak Ridge.

The building was constructed to enrich uranium during World War II as part of the top-secret Manhattan Project, and it was once the world’s largest building under one roof.

Previous plans had called for the north end of the mile-long, U-shaped building to be preserved for historic purposes. But an agreement signed this summer by federal, state, and local historic preservation groups allowed for the entire building to be demolished, including the north end, while still recognizing the historic significance of the site.

Demolition of the building’s east and west wings is complete, except for a small section of the east wing that has technetium-99, or Tc-99, a slow-decaying radioactive metal, according to a Wednesday press release from UCOR, the U.S. Department of Energy’s cleanup contractor in Oak Ridge. The north end forms the base of the “U” and is the smallest of the three sections.

“This is a tremendous day for employees,” said Leo Sain, UCOR president and project manager. “We have been working toward this since we arrived at ETTP, and it’s a testament to people here, at the Department of Energy, and at all the agencies who worked together to make this happen.”

DOE’s Oak Ridge Office of Environmental Management, or EM, expects the north end demolition to be complete in January. After that, workers will continue pre-demolition activities in the remaining Tc-99 area of K-25’s east wing, the release said.

“Completing demolition of the K-25 Building is our highest priority, and this is another significant step toward that goal,” said Mark Whitney, Oak Ridge’s EM manager.

The preservation agreement approved this summer calls for a replica equipment building and viewing tower, proposes a history center at a city-owned fire station at ETTP, and provides a $500,000 grant for the run-down Alexander Inn in central Oak Ridge.

During the decade-long discussion over preserving the north end, federal officials had expressed concerns about safety, the deteriorated condition of the building, and the cost of trying to keep that section.