Sen. Joseph R. Biden Jr. wondered aloud one day in 2002 whether someone could build an atomic weapon from parts available on the open market. His audience, the leaders of the government's nuclear laboratories, said it could be done.

Then do it, the Delaware Democrat, then chairman of the Senate Foreign Relations Committee, instructed the scientists in a confidential session. A few months later, they returned to the soundproof Senate meeting room with a workable nuclear weapon, missing only the fissile material.

"It was bigger than a breadbox and smaller than a dump truck, but they were able to get it in," Biden said in a recent speech. The scientists "explained how -- literally off the shelf, without doing anything illegal -- they actually constructed this device."

The relative ease with which U.S. scientists built an explosive nuclear weapon illustrates the need to secure plutonium and highly enriched uranium scattered in armories and research sites around the world, a pair of Harvard University researchers argue in a new study that contends the Bush administration is not doing enough.

Less fissile material was secured in the two years after Sept. 11, 2001, than in the two years just before, according to the Harvard report, which was obtained by The Washington Post. Half the equipment dispatched to Russia nearly four years ago as a fast, interim solution remains in warehouses, uninstalled because of bureaucratic disputes.

Calling it a "dangerous myth" that terrorists could acquire a nuclear weapon only with the help of a rogue state, authors Matthew Bunn and Anthony Wier use the Biden example to allege that a failure of U.S. commitment and leadership could lead to a nuclear calamity. They also warn that, in an unstable country, a nuclear weapon could be bought or stolen.

"What's missing is a sense of urgency," said former senator Sam Nunn (D-Ga.), who heads the nonprofit Nuclear Threat Initiative, which funded the 111-page study. Nunn believes President Bush must focus on removing bureaucratic hurdles and work more pointedly with Russian President Vladimir Putin.

"If one of the great cities of the world goes up in smoke, and you look back on these obstacles, it will make our retroactive rear-view mirror look at September 11th look like a waltz," Nunn said yesterday in an interview. "It would be so obvious that the obstacles should have been overcome by the presidents."

Bunn and Wier credit the Bush administration, particularly the leadership of the Department of Energy, for making strides. But they write that the U.S. commitment is no match for the danger. As they put it, U.S. authorities are not meeting Bush's own pledge to "do all we can."

In one case, plans were announced six years ago to destroy 68 metric tons of plutonium stripped from bombs and warheads in the United States and Russia, but the project remains stalled because of a dispute over who would pay if an accident or sabotage occurred in Russia. Sen. Pete V. Domenici (R-N.M.) has blamed "trivial negotiating issues."

In another example, the administration on average has requested less money to control nuclear materials and technology than was sought in the final Clinton administration budget, adjusted for inflation.

Although 16 percent more money has been spent than if the Clinton numbers had continued, "essentially all" of the increase was injected by congressional initiative, write Bunn and Wier, who reviewed federal spending on nonproliferation as an analyst at the Office of Management and Budget.

They report that the United States has taken more effective action than any other country, spending $9.2 billion from 1992 to 2004 to dismantle and secure weapons of mass destruction. Yet they note that the Defense Department is seeking $9.2 billion in the 2005 budget year alone to build a largely unproven defense system against a small number of missiles in a corner of the United States.

"It's very easy in the standard political debate for them to point to the successes and not put them in the context of how small they are, and not showing what they have not yet done," Bunn said in an interview. "The president has an opportunity to take action now that would drastically reduce the danger of nuclear terrorism in a few years."

The Bush administration is preparing to announce an expanded effort to secure nuclear stockpiles and supplies of bomb-grade material, officials have said. In a Feb. 11 speech, Bush promised a series of strong steps to curtail the production and spread of fissile material that could be used in a nuclear explosive or scattered in a radiological device called a "dirty bomb."

Basic security improvements have not been made at dozens of facilities in Russia, where more than 60 percent of the country's plutonium and weapons-grade uranium is kept, the General Accounting Office has warned. In a more recent report, the GAO said U.S. government facilities are also vulnerable to an increased risk of terrorism.

Despite improvements in Russia, Bunn and Wier report that visitors continue to see broken detectors, decaying fences, vulnerable seals and paper records never designed for careful monitoring. They also note that fissile material exists in "hundreds of buildings in more than 40 countries."

Evaluating an extreme case, they point to Pakistan, which has perfected nuclear weapons, as a potential target of terrorists with potent weapons and political connections.

Bunn and Wier, analysts at the Project on Managing the Atom, challenge the argument that the danger of terrorists assembling a bomb and acquiring fissile material is small, unless sponsored by a nuclear-capable government.

"We believe that this view is profoundly wrong," the authors write. They contend that the availability of nuclear designs and the work of U.S. weapons scientists in the Biden experiment prove their point.

Sen. Joseph R. Biden Jr. (D-Del.) challenged scientists in 2002 to build a nuclear weapon with parts from the open market, and they succeeded.