The demolition of the shuttered, mile-long K-25 Building where uranium was enriched for nuclear weapons throughout the Cold War has been pushed back more than two years to 2010, officials say.

Department of Energy contractor Bechtel Jacobs Co. was supposed to have K-25 — once the largest building under one roof in the world — torn down by Sept. 30, 2008, as part of a $1.6 billion accelerated cleanup contract signed in 2003.

But a lack of funding, safety concerns — including a worker falling through a floor in 2006 — and project revisions have delayed the undertaking, which is proving to be almost as much work as constructing K-25 in the first place during World War II’s Manhattan Project.

Still, Bechtel Jacobs spokesman Dennis Hill said “significant progress” is being made.

The K-25 Building, which covers 44 acres, is the heart of a site that is slowly being converted into an industrial park. The facility enriched uranium for weapons from 1945 until 1964.

About 800 people are working 50-hour weeks on the K-25 tear-down, which also includes demolishing a sister building named K-27.