But recent federal investigations have found that the department may not be up to the task.
Just last month, Energy Secretary Spencer Abraham essentially shut down all nuclear weapons research, after two classified computer discs were reported missing at the Los Alamos National Laboratory in New Mexico.
It is the latest in a string of serious lapses that Correspondent Ed Bradley first told you about last February -- lapses that have led government investigators to conclude that security at nuclear weapons facilities may be inadequate.
As a senior Department of Energy nuclear security specialist, Richard Levernier's job was to test how well-prepared America's nuclear weapons sites were to defend against a terrorist attack. He says security is not only inadequate, but some facilities are at high risk.
"And when you're dealing with nuclear -- assets in terms of weapons and materials, operating at high risk is unacceptable," says Levernier, who ran annual performance tests in the years leading up to Sept. 11.
These were tests in which U.S. Special Forces, playing the role of terrorists, armed with simulated weapons, would try to penetrate the facilities, steal imitation nuclear material, and then escape. The security guards there were expected to stop the attackers.
"Overall, the test results that I was responsible for showed a 50 percent failure rate," says Levernier. "If you understand the consequences associated with the loss of that kind of material, it would make the World Trade Center event of Sept. 11 pale in comparison."
Linton Brooks, head of the National Nuclear Security Administration (NNSA), oversees the Department of Energy's nuclear weapons facilities -- where some 10,000 nuclear warheads and the tons of plutonium and highly enriched uranium used to manufacture them are stored.
He calls the state of security at those facilities "perfectly acceptable," and says that he's comfortable that these nuclear weapons facilities are safe.
Is there a problem defending against terrorists? "'Safe' and 'no problem' are not the same thing," says Brooks. "I am convinced that these facilities are secure and that nuclear material is not at risk. That's not the same thing as saying the there aren't a lot of things that we're working on, because this is a very difficult and demanding business."
But to Levernier, "difficult and demanding" is no excuse for the fact that the mock terrorists were able to penetrate nuclear weapons sites half the time -- even though the security guards knew exactly what day and virtually what time to expect the attacks.
When Levernier conducted an unannounced inspection of security guards one January weekend at a nuclear weapons plant in Colorado, he says he was stunned by what he found.
"We found that the patrols that should be patrolling and moving around the facility were not observed," says Levernier. "Upon further investigation, we found that the vast majority of the patrols were in a facility watching the Super Bowl game."
The Department of Energy has admitted that security guards at other nuclear facilities have recently left front gates wide open, and failed repeatedly to respond to emergency alarms in top-security areas. Some have actually been caught sleeping on the job.
"People should know that the Department of Energy facilities cannot withstand a full terrorist attack," says Levernier. "I mean, a realistic attack. Serious, state-sponsored, for business."
What does Brooks think of this? "These are training exercises, so we don't think that simplistic measures of won or lost are correct," he says. "I don't want to suggest that we're entirely happy with the results of all of these things. If you never do a test that shows a problem, you are not doing a rigorous enough test."
Since the Sept. 11 attacks, 60 Minutes has learned that terrorists have penetrated multiple layers of security on at least three occasions at the Y-12 nuclear complex in Oak Ridge, Tenn., the country's primary facility for processing weapons-grade uranium; and at the Los Alamos National Laboratory in New Mexico, where the first atomic bomb was developed.
The Department of Energy says it is now taking steps to bolster security, including more performance testing, installing more razor wire, better lighting, motion detection sensors and other new technologies, as well as the hiring of more guards.
But Matthew Zipoli, who's a member of a SWAT team of security guards at Lawrence Livermore National Laboratory just outside San Francisco, and vice president of the Guards Union, says that's not enough.
"It's all window dressing. There's really no substance to the security. It's what looks good from the outside," says Zipoli.
He adds that guards are required to participate in annual counterterrorism drills with neighboring police departments, but said it never happened on his watch.
"1996 was the last time local law enforcement agencies participated in exercises with Livermore Laboratories," says Zipoli, who adds that he doesn't think he's been adequately trained to perform his job.
"We haven't been trained on the proper skills to get past an enemy. We don't have the proper equipment, so no, we don't have the proper training. And that degrades the effectiveness of our force."
What's more, terrorists who might want to get into a nuclear facility may not even have to fight their way in. Hundreds of master keys and electronic key cards - some of which provide access to classified areas - have disappeared.
The Energy Department's inspector general found that officials at Lawrence Livermore lab, which holds top-secret information about the country's nuclear arsenal, failed to immediately report their missing keys.
And at Sandia National Laboratories, near Albuquerque, N.M., the locks have just been changed -- three years after keys there were reported missing. But this is something that Republican Sen. Charles Grassley, who's been leading the charge in Congress to improve nuclear security, finds hard to believe.
"If you were going to have your house keys stolen, you would change your locks right away, wouldn't you? It's unconscionable that after three years, locks had not been changed," says Grassley. "In that three-year period of time, how many times were those doors entered, and our classified information compromised?"
"I am concerned that bad guys could have had those keys. We don't know for sure if they did. But, the fact that they were lost, and there wasn't the proper concern about it, is a bigger problem," adds Grassley. "Because it -- once again, is evidence of people at these labs not taking their job at security seriously."
"I find it inexplicable and unacceptable that people don't take them seriously," says Brooks. "All I can tell you is they do now."
As the Department of Energy's senior safety official at Los Alamos, Chris Steele has seen his share of problems. He's responsible for making sure that the lab's operations do not put workers or the public at undue risk from an accident at a nuclear weapons plant.
What kind of grade would he give them?
"I'm giving -- in the process of giving them an F -- because they've had systematic and systemic nuclear safety violations," says Steele.
In 2003, Steele says he cited Los Alamos for an unprecedented 45 major nuclear safety violations: "Forty-five shows that their normal mode of operation is to have violations. That they view these as glitches, that there's no sense of urgency in fixing them. And they could be precursors to disaster."
For example, Steele says Los Alamos came up with a flawed set of safety guidelines that said that in the event of a large explosion at its radioactive liquid waste facility, the subsequent fire in thousands of gallons of nuclear waste would be extinguished by the sprinkler system. The sprinkler system there would extinguish the estimated hundreds of thousands of gallons of nuclear waste, which would catch fire.
"Under the tons of rubble, the sprinkler head would rise up somehow and put out the fire. Of course, this is impossible, for a sprinkler to work under tons of rubble," says Steele.
After pointing out this and other safety hazards to his bosses, Steele was suspended, allegedly for breaching security in an e-mail exchange with his co-workers at Los Alamos.
Matthew Zipoli, the Lawrence Livermore security guard, was fired after he allegedly organized a walkout of his fellow guards.
And as for Richard Levernier, who ran the mock terrorist drills, he was demoted after giving unclassified information about his security concerns to a newspaper.
All of them claim it was retaliation, which the Department of Energy denies. But it turns out many of the allegations they told us have been substantiated by various federal government agencies.
Zipoli has since been reinstated, and Steele was cleared and is back on the job. Over at the Department of Energy, Linton Brooks says they take all incidents and allegations of lax security seriously.
All of these concerns have been outlined in reports since 1997. And they continue to occur as recently as just in the last few months. Why? "Because this is a complex system. Because there are always going to be problems, and you have to continue to deal with those problems," says Brooks.
"And what we're trying to do is to make sure that when you're sitting here with my successor, that you don't have repetitions of these problems. Because we've got a long-term system to fix it."
Just last week, the Department of Energy's inspector general found that security guards at the Y-12 weapons plant have been cheating on mock terrorist drills for the past 20 years, claiming they were successful in defending their facility when in fact, in some cases, they were not.
In response to continuing security problems, Brooks and the Department of Energy are conducting special inspections of all nine nuclear weapons sites.
In the last several months, the Department of Energy says it has undertaken new steps to enhance security, including the creation of an elite paramilitary unit, and new technology to assist security officers.
As for the nuclear research that was shut down at the nation's nine nuclear weapons facilities, nearly 75 percent of that research was restarted recently -- after Energy Secretary Abraham said he was able to verify that adequate security measures were put into effect.