	Threats

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. (8) 
Terrorists or rogue states might acquire nuclear weapons or fissile materials in various ways. (9) It has much fissile material. (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. Securing the Bomb states, "More than 130 research reactors still use HEU as their fuel, in more than 40 countries. Weapon Delivery

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. See also CRS Report RS21729 , U.S. International Borders: Brief Facts, by Marilyn Nelson and [author name scrubbed]. The concern is that if terroristscould place a bomb in a container overseas, they could ship it into the United States and transportit anywhere in the country. The response is often termed "layered defense," reflecting theidea that terrorists would have to proceed through many steps to acquire a nuclear weapon andsmuggle it into the United States, and that attempting to thwart them at each step has a higherlikelihood of success than trying to block one step only. Threat Reduction Programs in the Former SovietUnion. (37) 
Efforts To Secure HEU Worldwide. As noted above, much is said to be poorly guarded. (40) On May 26, 2004, responding to such concerns, Secretary ofEnergy Spencer Abraham announced a new Global Threat Reduction Initiative to secureRussian-origin fresh HEU by the end of 2005; to secure spent fuel of Russian/Soviet origin by 2010,and of U.S. origin within a decade; to convert the cores of civilian research reactors using HEU tobe able to use uranium with a concentration of uranium-235 too low to be used in a nuclear weapon,and to try to identify and secure other nuclear and radiological materials that may pose a threat. (43) 
Control of Former Soviet and Other Borders. (44) 
Container Security Initiative. (50) 
U.S. Border Security. Supporting Capabilities. Technology,intelligence, and forensics cut across and support the foregoing steps to keep terrorists or rogue statesfrom acquiring and delivering a nuclear weapon. Technology Development. Improving and organizing intelligence for homeland security have been sharplydebated. (62) In theevent of a terrorist nuclear attack, forensics might be able to identify the nation that originated thefissile material or weapon, and determine whether terrorists had fabricated the weapon on their ownor obtained it from a nation's stockpile. (67) 

	Role of Congress

Congress funds programs to counter nuclear terrorism through several authorization andappropriations bills. Other bills introduced in the 109th Congress that bear on nuclear terrorism include thefollowing.
It would be difficult for terrorists to mount a nuclear attack on a U.S. city, but such an attackis plausible and would have catastrophic consequences, in one scenario killing over a half-millionpeople and causing damage of over $1 trillion.
Terrorists or rogue states might acquire a nuclear weapon in several ways. The nations ofgreatest concern as potential sources of weapons or fissile materials are widely thought to be Russiaand Pakistan. Russia has many tactical nuclear weapons, which tend to be lower in yield but moredispersed and apparently less secure than strategic weapons. It also has much highly enricheduranium (HEU) and weapons-grade plutonium, some said to have inadequate security. Many expertsbelieve that technically sophisticated terrorists could, without state support, fabricate a nuclear bombfrom HEU; opinion is divided on whether terrorists could make a bomb using plutonium. The fearregarding Pakistan is that some members of the armed forces might covertly give a weapon toterrorists or that, if President Musharraf were overthrown, an Islamic fundamentalist government ora state of chaos in Pakistan might enable terrorists to obtain a weapon. Terrorists might also obtainHEU from the more than 130 research reactors worldwide that use HEU as fuel.
If terrorists acquired a nuclear weapon, they could try many means to bring it into the UnitedStates. This nation has thousands of miles of land and sea borders, as well as several hundred portsof entry. Terrorists might smuggle a weapon across lightly-guarded stretches of borders, ship it inusing a cargo container, place it in a crude oil tanker, or bring it in using a truck, a boat, or a smallairplane.
The architecture of the U.S. response is termed "layered defense." The goal is to try to blockterrorists at various stages in their attempts to obtain a nuclear weapon and smuggle it into the UnitedStates. The underlying concept is that the probability of success is higher if many layers are usedrather than just one or two. Layers include threat reduction programs in the former Soviet Union,efforts to secure HEU worldwide, control of former Soviet and other borders, the Container SecurityInitiative and Proliferation Security Initiative, and U.S. border security. Several approaches underliemultiple layers, such as technology, intelligence, and forensics.
Many policy options have been proposed to deal with nuclear terrorism, such as developingnew detection technologies, strengthening U.S. intelligence capability, and improving planning torespond to an attack. Congress funds programs to counter nuclear terrorism and holds hearings andless-formal briefings on the topic. Several bills have been introduced in the 109th Congress relatedto nuclear terrorism.
This report is intended for background, not for tracking current developments. It will beupdated occasionally. It does not cover radiological terrorism; see CRS Report RS21766 , Radiological Dispersal Devices: Select Issues in Consequence Management, and CRS Report RS21528 , Terrorist 'Dirty Bombs': A Brief Primer.