The White House said Friday that President Bush has chosen a replacement for the man ousted as head of the government's nuclear weapons program in the wake of reports of embarrassing security breakdowns.

Bush selected Thomas P. D'Agostino, who currently serves as deputy administrator of defense programs at the National Nuclear Security Administration, to succeed Linton Brooks in the top job there on an acting basis.

Energy Secretary Samuel Bodman had said Thursday that Brooks, who also held the job on an acting basis, would resign within the month. The agency maintains the nuclear weapons stockpile and oversees the nation's weapons research laboratories.

"I have decided it is time for new leadership at the NNSA," Bodman said.

Brooks, a former ambassador and arms control negotiator, said he accepted the decision, one he understood was "based on the principle of accountability that should govern all public service. This is not a decision that I would have preferred."

Brooks was reprimanded in June for failing to report to Bodman the theft of computer files at an NNSA facility in Albuquerque, N.M., that contained Social Security numbers and other data for 1,500 workers.

Then in October hundreds of pages of classified weapons-related documents from the Los Alamos National Laboratory in New Mexico were found during a drug raid in the home of a woman who had worked at the lab.

That security breakdown was especially troubling, a department inspector general's report said, because it came after tens of millions of dollars had been spent to upgrade cyber-security at Los Alamos. A new management group also had been put in charge only a few months earlier — also a fallout over the repeated security problems.

The New Mexico laboratory is one of three major research labs that are part of the nuclear weapons complex under the NNSA. The agency was created after the security flap involving Los Alamos scientist Wen Ho Lee in the late 1990s in hopes that a single agency within DOE might provide more control over security.

Meanwhile, lab spokesman Steve Sandoval said Friday the installation in New Mexico plans to implement an expanded substance abuse policy that includes random drug tests of employees and pre-employment drug screening for lab workers and contractors.

All lab policies, including current substance abuse guidelines, have been under review since last year, before Los Alamos National Security LLC took over the lab's management in June from the University of California, which ran the lab for the DOE for decades, he said.

Michael Anastasio, the lab's director, notified employees about the new policy last month "to let people know this was coming and take it seriously," Sandoval said.

In announcing Brooks' resignation, Bodman said the NNSA had "done its best" to address the problems, but that progress had not been adequate.

"Therefore, and after careful consideration, I have decided that it is time for new leadership at the NNSA," Bodman said.

Some members of Congress questioned whether Brooks' departure is enough to make the changes that are needed.

"It will take more than a new boss to fix the problems, which are far more systemic and pervasive in nature," said Rep. Edward Markey, D-Mass., a member of the House Energy and Commerce Committee, which is considering hearings on DOE security.

Rep. Ellen Tauscher, D-Calif., said she also plans a hearing by her House Armed Services subcommittee on "the important policy and structural changes" planned to improve the nuclear agency. Her aides said she believes the issue is one that goes beyond Brooks, whom she praised.

Sen. Pete Domenici of New Mexico, the ranking Republican on both the Senate Energy Committee and the Appropriations subcommittee on NNSA spending, said Bodman "has sent a clear message" that improvements are needed at the agency.

A number of lawmakers as well as private watchdog groups have maintained that Brooks had not responded forcefully enough to the Los Alamos security breakdowns.

"His departure is long overdue," Rep. Joe Barton, R-Texas, said Thursday. He had called for Brooks' immediate firing last summer when the theft involving information on the 1,500 employees came to light.

In November, the Project on Government Oversight, a private watchdog group, urged that Brooks be fired, saying he had been slow in implementing a two-year-old policy to do away with removable storage devices in weapons-related computers.

In his message to employees, Brooks, who came to NNSA in July 2002, bemoaned the lack of progress in solving security problems at Los Alamos. "We have not yet done so in over five years," he said.

But the rash of security problems date back to the late 1990s, frustrating senior DOE officials.

They include the disappearance of two hard drives containing classified material that later were found behind a copying machine and the disappearance of two computer disks that forced a virtual shutdown of Los Alamos. It later was learned the two disks never existed.

Among other incidents were lost keys to classified areas containing highly enriched uranium, use of less secure e-mail systems to transmit classified material, scientists losing track of vials of plutonium and the alleged improper use of government credit cards.