Classified nuclear threat reports warn that rogue countries and terrorists have learned it is possible to make atomic bombs using low-enriched uranium, a common fuel for nuclear reactors used to conduct research and generate power. The reports, described to USA TODAY by top federal officials, also conclude that it would be easier than previously believed for enemies of the United States to make such weapons using spent nuclear fuel, the waste generated by reactors.

Neither of those substances is listed as ''weapons usable'' under U.S. or international security protocols. As a result, they get little protection from theft at civilian nuclear reactors worldwide. That includes reactors in former Soviet states and nations such as Indonesia, where public sympathy runs high for Iraq and al-Qaeda.

And the threats are real.

Five years ago, U.S. scientists at Los Alamos National Laboratory secretly designed an atomic bomb with low-enriched uranium, USA TODAY has learned. The bomb, which could have fit easily in a small pickup, was weak in nuclear terms but strong enough to destroy a square mile of a city.

U.S. scientists also have proved in experiments that it is possible to create nuclear weapons using several elements that could be extracted from spent fuel by a rogue state or perhaps even a well-organized terrorist organization.

Officials stress that there is no evidence that al-Qaeda or any other terror group has the skills or tools to build an atomic bomb using low-enriched uranium or spent fuel. There's a big gap, they say, between knowing such things are possible and being able to do them. Rogue states are a bigger concern: U.S. officials believe that Iran and North Korea (news - web sites) are trying to develop the capability to make nuclear weapons using spent fuel.

Yet U.S. efforts to stop the spread of nuclear weapons still focus almost exclusively on protecting plutonium and highly enriched uranium, the traditional ''weapons usable'' nuclear materials. That atomic bombs can be made with little or none of those substances reveals significant gaps in current programs to keep rogue states and terrorists from developing a nuclear capability.

Under U.S. and international protocols for protecting nuclear materials, facilities handling low-enriched uranium or spent fuel are not obliged to have armed guards or security systems to stop break-ins or insider thefts. Such measures are expected of nuclear installations holding plutonium or highly enriched uranium. Accounting and inventory rules also are far less stringent for material not deemed ''weapons usable.''

Officials at the International Atomic Energy Agency, the arm of the United Nations (news - web sites) that monitors nuclear stocks worldwide, say such distinctions are appropriate because plutonium and highly enriched uranium remain the most effective and easy-to-use materials for making nuclear weapons.

''We work on the assumption that rogue states, or terrorists for that matter, know how to make (nuclear) weapons with small amounts of material and different types and combinations of material. But we've been advised by experts with the nuclear weapons states that it would be very difficult,'' says Davis Hurt, a senior safeguards expert at the agency.

The agency, based in Vienna, has to focus its oversight on the biggest threats, Hurt says. Expanded monitoring of other materials, such as low-enriched uranium, wouldn't be possible unless the agency's member states provided money to boost its budget, he says.

Cracking nuclear myths

From the dawn of the atomic age, nuclear weapons have relied on plutonium or highly enriched uranium for their explosive punch.

Highly enriched uranium is easier to make and to use. It is created by processing natural uranium to boost its concentration of uranium-235, the element's most fissionable isotope. Once that concentration is sufficient, it's relatively easy to make a ''gun-type'' atomic bomb, which slams masses of enriched uranium together in a gun barrel-like tube. That was the type of bomb the U.S. dropped on Hiroshima in 1945.

Plutonium is more of a challenge to make because it requires running uranium through a nuclear reactor. It's also harder to fashion into a bomb: a plutonium ''pit'' has to be surrounded with conventional explosives that are precisely detonated to compress the material and create a critical reaction. But plutonium weapons yield much bigger blasts with far less material.

In the 1960s, as nuclear power went global, U.S. officials successfully pushed for international standards to keep plutonium and enriched-uranium stockpiles secure. The International Atomic Energy Agency monitors compliance with voluntary measures through inspections and inventory checks. Physical protections -- guards, gates and guns -- are the responsibility of each nation, and the agency cannot mandate such measures.

The agency's protocols and inspection programs fail in several ways to account for types and amounts of materials that can serve as fuels for a nuclear device:

* Low-enriched uranium, or uranium containing less than 20% of the U-235 isotope, is not labeled ''weapons usable'' under either U.S. or international standards. Reactors using the fuel are inspected once a year versus once a month for those using plutonium or highly enriched uranium. They also have more leeway in accounting for material that goes missing.

Many of the world's research reactors use low-enriched uranium with U-235 concentrations just below 20%. That material can, as Los Alamos proved, create a nuclear blast, though making a bomb with it requires substantial skill and a relatively large amount of material. The United States helped convert many of those reactors from highly enriched uranium.

Some reactors run on uranium enriched to less than 5%, which cannot sustain the critical reaction for nuclear weapons. But even that material would carry risks in the hands of a rogue state because it is relatively easy to boost its enrichment enough for weapons use.

''If you got a stack of uranium enriched to 4-5%, which as a rule is not seriously protected, the plant needed to convert it to 90% enrichment is potentially small and easy to hide,'' Harvard University physicist Matthew Bunn says.

* Spent reactor fuel, which cannot be used directly to make an atomic bomb, is a growing concern among authorities because it can be processed to extract materials that can be used for nuclear weapons. The work demands sophisticated skills and equipment, but it's not the challenge it once was.

The United States ''reprocessed'' spent fuel from nuclear reactors for decades to recover plutonium for weapons. And U.S. scientists have proved recently that other, less-recognized elements in spent fuel, such as neptunium and americium, also can sustain the chain reaction needed for nuclear blasts.

Security for spent fuel often is a low priority because it is seen as too radioactive to handle. But U.S. scientists have warned for years that poorly guarded spent fuel caches in some countries have sat for so long that radioactivity has dissipated and poses less of a risk.

A recently declassified study by Lawrence Livermore National Laboratory in 1995 found that rogue states or terrorists would need only ''modest facilities and equipment'' to extract weapons-ready nuclear material from spent fuel.

* The amount of nuclear fuel needed to build a bomb is far less than what is officially stated.

The International Atomic Energy Agency says it takes 17.6 pounds of plutonium or 55.1 pounds of highly enriched uranium -- amounts that could fit in a suitcase -- to build a nuclear weapon. Reactors holding those amounts or more are inspected with greater frequency.

Yet U.S. officials acknowledge that nuclear weapons can be built using far smaller quantities of those materials.

A 1995 study by the Natural Resources Defense Council found that terrorists with ''low'' technical ability could build a small nuclear weapon with about nine pounds of plutonium or 20 pounds of highly enriched uranium. A more expert program, such as those run by Iraq, Iran or North Korea, would need about half those amounts, the study said. Such a bomb would have about a quarter the power of the one that destroyed Hiroshima.

''It's a lot easier to make a nuclear explosive device -- and some simple designs require a lot less fissile material -- than the public has been led to believe,'' says Thomas Cochran, a physicist who co-authored the study.

New threats perceived

There's debate among U.S. officials over the gravity of the nuclear threat terrorists or rogue states could pose without significant amounts of plutonium or highly enriched uranium. In August, the debate clouded a much-touted mission by U.S., Russian and international officials to remove more than 100 pounds of highly enriched uranium from a closed and poorly secured Serbian research reactor. With Serbia's blessing, the material was taken under heavy guard to a Russian site where security had been improved with U.S. help.

Officials never revealed in news briefings that they left behind a cache of spent fuel laced with at least 10 pounds of plutonium that could be extracted for weapons. The material remains at the site.

''Some people wanted to take it -- there was a lot of debate,'' says a U.S. official involved in the mission. ''It would have been a lot of work -- more than two tons of additional material in a lot of containers. But our understanding is that the plutonium may be concentrated in certain containers, so if a bad guy got the right ones, he could get some good stuff.''

Assessing the risk is tough because officials don't know what rogue states and terrorists are capable of doing with different types of nuclear material. U.S. officials worry, for example, that al-Qaeda picked up nuclear secrets from sympathetic Pakistani scientists.

The other problem is that no one knows how much nuclear material may be missing around the world. Many research and power reactors keep shoddy fuel inventories, particularly in former Soviet states and developing nations. And reports of thefts or losses tallied by the International Atomic Energy Agency are notoriously spotty.

''No one doubts that there are a number, if not many, instances of diversion or theft of nuclear material that we're not aware of,'' says William Potter, a non-proliferation expert at the Monterey Institute of International Studies. Many countries don't disclose losses, he adds, and intelligence sharing is limited.

Data gathered by the International Atomic Energy Agency and the Monterey Institute reveal several thefts or losses over the past few years involving low-enriched uranium and small amounts of traditional weapons fuel.

A daunting set of solutions

Many officials fear they will undermine the critical mission of securing big stockpiles of plutonium and highly enriched uranium if they draw attention to the risk posed by lesser grades or amounts of nuclear material.

Economic woes in the former Soviet states, especially Russia, have left little money for securing numerous sites storing weapons-ready nuclear fuel left from the Cold War. U.S. assistance programs to help consolidate and protect that material have reached fewer than half the sites of concern.

''We need to have more countries throwing money into the pot,'' says Rose Gottemoeller, a former assistant secretary of Energy now at the Carnegie Endowment for International Peace.

U.S. officials recently won agreement from the seven other top economic powers to collectively match a U.S. pledge of $10 billion over 10 years for non-proliferation efforts. Meanwhile, many experts say, safeguards must be tightened.

The rules should reflect that information on making bombs with low-grade nuclear fuel or small amounts of traditional material ''has leaked out more in recent years,'' says Laura Holgate of the Nuclear Threat Initiative, a non-profit group working to curb the spread of nuclear arms.

Some critics say they expect no action because the International Atomic Energy Agency's member states fear that their nuclear industries would be hurt by the costly measures needed to secure all the types and quantities of nuclear materials that might be useful to rogue states or terrorists.

Those nations are ignoring the growing capabilities of those who would steal material for nuclear weapons, says Cochran of the Natural Resources Defense Council. ''They're living in 1945,'' he says.