Sealant used in a nuclear weapons plant to prevent plutonium from leaking in case of an accidental blast is peeling, and a repair job could cost $20 million, a government report shows.

The Department of Defense's Pantex Plant (search) is the nation's only nuclear weapons assembly and disassembly plant and technicians work with radioactive and explosive materials at the complex around the clock.

According to a report by the Defense Nuclear Facilities Safety Board (search), sealant had been applied to faulty door welds on underground workshops at the plant after officials learned that a 6-year-old work order to repair them was never completed.

The government temporarily halted nuclear weapons (search) operations at the plant while it repaired the welds with the sealant. Afterward, operations resumed.

But in a July 21 report, an official with the nuclear facilities safety board found the sealant was peeling away in places. Now, safety board officials say sealing potential leak spots could cost between $15 and $20 million.

Engineers at the plant are studying the extent of the problem and will report its findings to the safety board.

The cells that contain the subterranean workshops are designed to prevent the spread of radioactivity in the unlikely event of an accidental blast. The cells are supposed to collapse inward and trap radioactive debris.