It would be difficult for terrorists to attack a U.S. city using a nuclear weapon, but such anattack is plausible and would have catastrophic consequences. A report of June 2004 by the NationalCommission on Terrorist Attacks upon the United States found that even though an attempt by alQaeda in 1994 to purchase uranium failed, "al Qaeda continues to pursue its strategic objective ofobtaining a nuclear weapon." (1) A book by the Center for Nonproliferation Studies emphasizes theurgency of taking steps to reduce the risk that terrorists could obtain nuclear weapons ormaterials. (2) A May 2004report by Harvard University's Project on Managing the Atom finds that a nuclear attack "would beamong the most difficult types of attacks for terrorists to accomplish," but that with the necessaryfissile materials, "a capable and well-organized terrorist group plausibly could make, deliver, anddetonate at least a crude nuclear bomb capable of incinerating the heart of any major city in theworld." (3) An earlier reportby the same group estimated the consequences of a ten-kiloton weapon (two-thirds the yield of theHiroshima bomb) detonated at Grand Central Station in Manhattan as over a half-million peoplekilled immediately, hundreds of thousands injured, the possibility (depending on wind direction) ofhaving to evacuate all of Manhattan, much of lower Manhattan permanently destroyed, direct costsof well over $1 trillion, indirect costs several times that, and the prospect for nationwide panic andeconomic chaos if terrorists subsequently claimed to have another bomb. (4) The latter two reports, and acompanion website, (5) provide detailed data on U.S. threat reduction programs and argue that there is an urgent need toaccelerate these programs. This report divides the threat into two aspects, the acquisition of a bomb and its delivery toa target. The former could involve obtaining a nuclear weapon, or obtaining fissile material andmaking it into a bomb. Delivery involves more common and less sophisticated skills to move theweapon covertly to its target. Experts have raised concerns that terrorists might try to acquire two types of nuclearweapons. In the simplest, a "gun-type" weapon, a mass of uranium highly enriched in the fissileisotope 235 (highly enriched uranium, or HEU) is shot down a tube (resembling an artillery tube)into another HEU mass, creating a supercritical mass and a nuclear explosion. The Hiroshima bombused this approach; its designers had such high confidence in it that they did not test this type ofweapon prior to using it. The second type, an implosion weapon, typically uses weapons-gradeplutonium (WGPU, composed mainly of the isotope 239). A shell of WGPU is surrounded bychemical explosives arrayed to produce a symmetrical inward-moving (implosion) shock wave thatcompresses the plutonium enough to be supercritical. The Nagasaki bomb was of this type. It ismuch more complex in design and manufacture than a gun-assembly weapon. An implosion devicewas tested in New Mexico prior to use on Nagasaki. A National Academy of Sciences report asserts,"Crude HEU weapons could be fabricated without state assistance." (6) Some experts believe thatterrorists could create an implosion weapon; (7) others disagree. (8) Terrorists or rogue states might acquire nuclear weapons or fissile materials in various ways. The source of greatest concern is Russia. (9) It has much fissile material. A National Nuclear SecurityAdministration (NNSA) document shows that considerable work remains for the disposition of thismaterial, (10) and aNational Academy of Sciences report states that the risk of diversion of special nuclear material(SNM, or fissile material) from Russia is "high" because "large inventories of SNM are stored atmany sites that apparently lack inventory controls, and indigenous threats have increased." (11) A related concern is that Pakistan might be the source of nuclear weapons or materials forterrorists under several scenarios: (1) Islamists in the armed services might provide such assistancecovertly under the current government; (2) if that government were overthrown by fundamentalists,the new government might make weapons available to terrorists; or (3) such weapons might becomeavailable if chaos, rather than a government, followed the overthrow. Other nations are seeking nuclear weapons. President Bush, in his 2005 State of the UnionAddress, said, "Today, Iran remains the world's primary state sponsor of terror, pursuing nuclearweapons ... it must give up its uranium enrichment program and any plutonium reprocessing ..." (12) According to a pressreport, Mohamed ElBaradei, Director General of the International Atomic Energy Agency, "says heis now certain that the nuclear material his agency once monitored there [in North Korea] has beenconverted into fuel for four to six nuclear bombs." (13) And in a statement of February 2005, North Korea declared thatit had nuclear weapons. (14) There is also evidence of transfers of nuclear weapon materials, designs, productionequipment, and expertise to other nations. A.Q. Khan, the father of Pakistan's atomic bomb, ran acovert operation for many years that reportedly provided Libya, North Korea, and Iran withequipment for making HEU and plans for making an atomic bomb. (15) Reports of February 2005pointed to evidence that strongly indicated that North Korea had provided Libya with uraniumhexafluoride, a chemical used as the feedstock to produce HEU, though another report said that the"evidence ... could just as easily point to Pakistan." (16) Transfers to rogue states, or directly or indirectly to terroristgroups, are a source of concern. Nuclear research reactors offer still another route to obtaining a weapon. Securing the Bomb states, "More than 130 research reactors still use HEU as their fuel, in more than 40 countries. Mostof these facilities have very modest security -- in many cases, no more than a night watchman anda chain-link fence." (17) A more recent Government Accountability Office report stated that as of July 30, 2004, "39 of the105 research reactors targeted by DOE [for conversion from HEU to low-enriched uranium, or LEU]have converted to LEU fuel." (18) Six of these reactors are reportedly on U.S. universitycampuses. (19) A gun-assembly weapon need not be particularly large. The Hiroshima bomb, according toone source, weighed 8,900 pounds and was 10 feet long and 28 inches in diameter. (20) Much of that size andweight, however, was taken up by an armored steel shell and stabilizing fins, as well as by arming,fuzing, and firing devices. (21) The gun barrel -- the actual nuclear explosive device -- measured6 feet in length by over 6 inches in diameter and weighed about a half-ton. (22) Simple improvementsmight shrink size and weight. A terrorist-made implosion weapon or a weapon stolen from a nation'sarsenal could be smaller. In short, a weapon could fit in a car, boat, or small airplane, and wouldoccupy a small corner of a shipping container. The United States has many thousands of miles of land and water borders, as well as severalhundred sea, land, and air ports of entry -- 317 by one count -- giving terrorists many pathways tosmuggle a nuclear bomb into this nation. (23) There are many types of borders, as the following table shows-- oceans (tropical to temperate to Arctic), land and river borders with Mexico and Canada, and theGreat Lakes. Each poses its own set of opportunities for smugglers. Legal and illegal crossings into the United States also present terrorists with different risksand opportunities. Legal crossings are points, such as seaports, airports, and border stations on roadsentering the United States. Illegal crossings are lines -- the thousands of miles of unguardedstretches of coasts and land borders. Securing points poses different requirements from securinglines. Points have an immense volume of traffic, almost all of it legal, and a correspondingconcentration of people and resources of U.S. Customs and Border Protection (CBP). The task ofCBP is to find the needle in the haystack while expediting legal traffic. Attempts to smuggle anuclear weapon through a legal crossing would run the risk that the weapon might be detected bycomputerized screening of cargo manifests, imaging devices (similar to x-rays), neutron activationdevices, or physical inspection, as discussed below. That risk is reduced by the need for CBP agentsto process huge numbers of vehicles, vessels, and passengers, leaving little time or attention for thosenot raising suspicions, and by the low radioactivity of fissile uranium-235 -- approximately onehundred-millionth that of radioactive material that might be used in a "dirty bomb." (24) ABC News attempted to highlight the difficulty of detecting smuggled material. OnSeptember 11 of 2002 and 2003, that organization smuggled a 15-pound cylinder of depleteduranium (DU -- natural uranium minus most of its uranium-235) into the United States in cargocontainers; CBP did not detect it. ABC News said that DU is a good surrogate for HEU; CBP saidthat was not the case, and that there was no reason for CBP to have detected the DU because it islegal to import up to 15 pounds of DU as long as it is declared. The DHS Inspector General found"Improvements are needed in the inspection process to ensure that weapons of mass destruction ...do not gain access to the U.S. through oceangoing cargo containers.