The most difficult challenge for a terrorist organization seeking to build a nuclear weapon or improvised nuclear device is obtaining fissile material, either plutonium or highly enriched uranium (HEU). HEU, uranium that has been processed to increase the proportion of the U-235 isotope to over 20%, is required for the construction of a gun-type nuclear device, the simplest type of nuclear weapon. The greater the proportion of U-235 (i.e. the higher the enrichment level), the less material is needed for a nuclear explosive device. "Weapons-grade" uranium generally refers to uranium enriched to at least 90%, but material of far lower enrichment levels, found in both fresh and spent nuclear fuel, can be used to create a nuclear explosive device.
In 2002, the U.S. National Research Council warned that "crude HEU weapons could be fabricated without state assistance," noting that "the primary impediment that prevents countries or technically competent terrorist groups from developing nuclear weapons is the availability of [nuclear material], especially HEU." Creating a nuclear weapon from HEU is technically easier than building a plutonium weapon. Moreover, current technology is unlikely to detect a shielded nuclear device on a truck or boat. Therefore, securing and eliminating stocks of HEU is the surest way to decrease the risk that terrorist groups could use this material to create a nuclear explosion. (Read about HEU and the creation of an improvised nuclear device, at "HEU as weapons material - a technical background," prepared by the organizers of the June 2006 Oslo Symposium on Minimization of HEU in the Civilian Nuclear Sector.)
WHERE IS CIVILIAN HEU LOCATED?
As of 2010, experts estimated that approximately 70 tons of HEU were being used in civilian power and research programs in roughly 30 countries.  Yet as little as 25kg are needed to produce a nuclear weapon; some 40-60kg are needed for a cruder nuclear device. Bomb-grade material can be obtained from both fresh (unirradiated), and irradiated (also referred to as spent), HEU fuel. Fresh and lightly irradiated fuel (such as fuel used in critical assemblies and pulse reactors) is not radioactive, and is therefore relatively safe to handle. Although using nuclear fuel in high-powered reactors initially makes it highly radioactive and thus very difficult to handle safely (often this fuel is referred to as "self-protecting"), spent fuel loses its radioactivity over time, making it easier to handle and therefore potentially more attractive to terrorists.
HEU is currently used in the civilian sphere to fuel research reactors, critical facilities, pulsed reactors, and a few. According to the IAEA, 252 research reactors are in operation or temporarily shut down across 56 countries. A further 414 reactors have been shut down or decommissioned, while five are planned or under construction. The IAEA database does not contain information on the enrichment level of fuel currently in the reactors, but it does note that over 20,000 spent fuel assemblies from research reactors are enriched to levels above 20 percent. Nearly half of these stored fuel assemblies are enriched to levels at or above 90 percent . (There is as yet no comprehensive, authoritative inventory of civil HEU globally, another obstacle to progress in this area). Many of the research reactors that have been shut down, but not decommissioned, have spent HEU fuel on-site.
The United States and Russia supplied much of the HEU fuel used in research reactors world-wide; other producers include China (which sent HEU fuel for research reactors to Nigeria, Ghana, Iran, Pakistan, and Syria, as well as enriched uranium to South Africa, and Argentina); France (to Chile and India); the United Kingdom (to Australia, India, and Japan); and South Africa (which did not export this fuel). Before 1978, when Washington and Moscow became concerned about exports of highly enriched fuels, most of the fuel supplied by the United States (the bulk of which went to North American and the Asia-Pacific), was of very high enrichment levels (90% and above). The Soviet-supplied fuel, chiefly sent to Eastern Europe, was typically 80% enriched. In order to reduce the risk of theft, many countries have returned HEU fuel, both fresh and spent, to its country of origin.
HEU is also used as a target in reactors that produce medical isotopes. HEU is used for this purpose annually in reactors in Belgium, Canada, France, the Netherlands, and Russia. Other countries, including Australia and Indonesia, have begun producing these isotopes with LEU targets. In particular, South Africa—a major exporter—converted its Safari-1 reactor to rely on both LEU targets and fuel for the production of medical isotopes. The first commercial shipment of medical isotopes produced using LEU from South Africa to the United States arrived in August 2010. In October 2010, the United States government awarded the South African company in question, Necsa, a $25 million contract for molybdenum-99 produced using LEU. Most of the other major producers of medical isotopes, including Canada, the Netherlands, and France, utilize LEU fuels in their reactors, but continue to rely on HEU targets. Progress towards fuller use of LEU is not universal, however. A Russian project, for example, aims to produce enough molybdenum-99 using HEU fuel and targets to satisfy 20 percent of global demand by 2015.
A bill under consideration by the U.S. Senate would provide incentives for U.S. production of medical isotopes using LEU fuel and targets through grants, subsidies, and government responsibility for radioactive waste from isotope-producing reactors using LEU. A similar bill passed the House of Representatives by a 400 to 17 vote in 2010, but the Senate version remains stalled. Medical organizations in several countries have expressed interest in halting the production of medical isotopes using HEU. For example, the 17,000-member Society for Nuclear Medicine endorsed the above-mentioned Senate bill.
In addition to these uses of HEU, as of 2010 Russia was operating seven nuclear-powered icebreakers that relied on fuel enriched to levels between 36 and 90 percent. The most recent vessel to join the fleet, Fifty Years of Victory, began operations in 2007.
SECURITY OF CIVILIAN HEU
Many civilian facilities with HEU on-site do not have adequate security. The International Atomic Energy Agency (IAEA) has reported that one of its missions discovered a research reactor with HEU that "was observed to have essentially no physical protection." The IAEA assisted the facility in question in improving its security, but reported that overall, "deficiencies remain in the legal, administrative, and technical arrangements for controlling and protecting nuclear materials ... in many countries." The U.S. Department of Energy has been assisting in physical protection upgrades for 22 foreign research reactors through the Global Research Reactor Program. A September 2009 GAO report found that while most sites that have received upgrades generally met IAEA security guidelines, in some cases, critical security weaknesses remained.
It is not a simple matter to upgrade security measures adequately, since the majority of the world's research reactors are located in universities or other research centers that tend to be quite open to the public. While security concerns have dramatically increased since 9/11, it is difficult to reconfigure a site that was not built with physical protection in mind. Storage of spent fuel stocks is generally even less secure than fresh fuel stocks, as until a few years ago spent nuclear fuel was considered "self-protecting" and few facilities wanted to spend money securing a material that was no longer of economic value. It is far more effective to remove this material from vulnerable locations than to attempt to increase security on-site.
PROGRAMS TO REDUCE AND ELIMINATE HEU
There have been efforts to reduce the amount of HEU at civilian facilities since 1978, when Washington initiated the Reduced Enrichment for Research and Test Reactors (RERTR) Program. Moscow also began its own program to reduce enrichment at Soviet-built research reactors outside of the Soviet Union, and changed its HEU export policies, supplying these reactors with 36% HEU in lieu of 80% HEU. In the past 25 years, many countries have cooperated with the RERTR program or initiated their own, similar programs. In May 2004, the U.S. Department of Energy launched the Global Threat Reduction Initiative (GTRI), which the IAEA, Russia, and others have since joined. Among its goals, the GTRI seeks to "minimize and eventually eliminate any reliance on HEU in the civilian fuel cycle, including conversion of research and test reactors worldwide from the use of HEU to the use of LEU fuel and targets."
In addition to converting research reactors that use HEU fuel, the RERTR program is also working on the conversion of six medical isotope producers that use HEU targets in their reactors. The program includes the four largest producers of medical isotopes, located in Belgium, Canada, the Netherlands, and South Africa. The RERTR program helped convert an isotope production reactor in Argentina to LEU in 2003, however, the Argentine reactor only produced medical isotopes on a relatively small scale. There are no longer any technical barriers to conversion to LEU, as the conversion of the South African reactor demonstrates; only political and financial issues remain.
Besides converting facilities to use LEU fuel, there have also been efforts to consolidate fresh and spent HEU fuel at a smaller number of relatively secure locations. This has involved removing the fuel, mostly to the United States and Russia, from other countries, as well as consolidating the fuel within countries. U.S. programs in this area (the Russian Research Reactor Fuel Return program to assist in the repatriation of fuel to Russia, and the Foreign Research Reactor Spent Nuclear Fuel Acceptance Program, whereby U.S.-origin fuel is repatriated to the United States), have all been subsumed under the 2004 GTRI initiative. Together, the two programs have returned over 1,820 kilograms of spent and fresh HEU fuel to the United States and Russia since 2004. According to the IAEA's definition of the quantity of HEU necessary to construct a nuclear explosive device, the amount of repatriated HEU is equivalent to over 70 weapons. Despite this progress, however, many HEU sites persist worldwide. A related program, the Material Consolidation and Conversion (MCC) project, established in 1999, reduces excess Russian civilian HEU by blending it down into LEU. As of January 2009, 11.1 of an estimated 17 tons of U-235 in excess Russian civilian HEU had been blended down.
Both the United States and Russia have large quantities of HEU that are no longer needed in their defense programs. In Russia, excess HEU from weapons is blended down to LEU within the framework of the "Megatons to Megawatts" program (also known as the HEU-LEU program). The resulting LEU is then released for civilian use. The program will end in 2013, at which point 500 tons of HEU will have been downblended. The United States initially declared some 174 metric tons of HEU as excess to military needs, designating this material as civilian. An additional 200 metric tons were officially removed from the U.S. weapons stockpile in November 2005; of this amount, approximately 70 metric tons will be downblended to LEU.
Since the amount of HEU that is actually excess to military needs is likely far greater than the amount that has officially been declared excess to date, there have also been calls to speed up the various downblend programs. Despite the Obama Administration's focus on nuclear security efforts, funding for downblend programs decreased from 2009 through 2011.
PROPOSALS TO ELIMINATE CIVILIAN USE OF HEU
Many national governments are beginning to call for the elimination of HEU in the civilian sphere. Indeed, former IAEA Director General Mohamed El-Baradei called on countries "to minimize, and eventually eliminate, the use of high enriched uranium in peaceful nuclear applications." At the 2005 Non-Proliferation Treaty (NPT) Review Conference, the opening statement from Kyrgyzstan noted that "the Kyrgyz Republic believes this Review Conference should consider means to enhance the security of existing stockpiles of highly enriched uranium, while consolidating them, reducing their size, and moving toward the elimination of the use of highly enriched uranium in the civilian nuclear sector." This call was taken up by other countries, with Iceland, Lithuania, Norway, and Sweden submitting a working paper entitled "Combating the risk of nuclear terrorism by reducing the civilian use of highly enriched uranium" in an effort to seek an international consensus on this issue. Norway has been particularly active in this regard, issuing a Position Paper at the Review Conference that called for the Conference to adopt "a moratorium on the production and use of [civilian] highly enriched uranium (HEU), like the moratorium on the production of weapons grade material declared by certain [nuclear weapon states]. The long-term objective should be the establishment of a total ban." Norway reiterated this call in its statement to the IAEA General Conference in September 2005, as well as calling for the IAEA to develop guidelines for the management of HEU in the civilian sector. The U.S. statement, too, called to "phase-out the commercial use of highly enriched uranium," a policy the United States has been promoting since 1992, when it restricted exports of HEU in order to promote conversion to LEU.
Civilian use of HEU did not figure prominently in the next NPT Review Conference, held in May 2010, but states did agree to place the issue in the consensus action plan. Action 61 of the plan "encourages" states to further minimize HEU in civilian stocks, voluntarily, where technically and economically feasible.
In April 2010, 47 heads of state or government attended the Washington Nuclear Security Summit, an unprecedented high-level meeting on the issue. Participating states agreed to consider, "where appropriate," the conversion of nuclear facilities utilizing HEU to LEU, and to collaborate on the development of LEU-based technologies for the production of medical or other isotopes. In addition, some states pledged individual measures to reduce their use of HEU or secure existing supplies. Among others, Canada announced that it would return spent HEU fuel to the United States, Chile returned all of its HEU (18kg) prior to the summit, Mexico and Vietnam agreed to convert HEU-based research reactors to LEU, and Ukraine pledged to return all of its HEU to Russia by 2012. Substantial progress has been made on many of these commitments; Ukraine is on track to complete the return of its HEU by the deadline, for example. A second summit will be held in 2012 in Seoul, South Korea.
NEED FOR A COORDINATED INTERNATIONAL APPROACH
Current programs that reduce the use of HEU are laudable, but piecemeal efforts. Many uses, such as pulsed reactors, critical assemblies, and reactors for naval propulsion, are not covered by the current programs. Indeed, there is no current, accurate, consolidated global inventory of HEU in civilian use that would enable states to prioritize their activities in this sphere. This is critical both in the short-term, so that security upgrades are first initiated where they are most urgent, as well as in the long-term, in order to locate all of the HEU that should be consolidated into safe and secure storage, and to decide which reactors to convert to LEU and which to shut down. Consolidating material and activities that require high levels of security demands the macro perspective that such a database would make possible. Furthermore, it would help states to ensure that they are not expending time and money to remove materials from one site, and leaving yet more vulnerable materials in some nearby location.
The inaugural Nuclear Security Summit and its 2012 successor together form an important mechanism for drawing high-level attention to the issue of fissile material security, facilitating information sharing, and providing an incentive for states to follow through on their commitments. The continuation of such meetings beyond 2012 would help institutionalize these activities, and could also help insulate nuclear security work from budget cuts at a time when many governments are interested in curtailing spending.
An international approach is also needed to make HEU reduction programs attractive to all states. For example, if one state funds conversion of a medical isotope production reactor while another does not, the latter may have a financial advantage that gives it an incentive to avoid conversion. Yet more problematic, there is no guarantee that if a country converts its reactors to LEU, a neighboring country will not commence a new type of nuclear activity using HEU on its borders. Current research into future reactor designs suggests that none of the future generation power reactors would benefit from the use of HEU; nor is there any evidence that HEU is necessary in future research or other reactors. Laudably, Russian officials announced in 2005 that new floating nuclear plants will use LEU fuels. However, Germany launched a new research reactor in 2003 that uses HEU fuel, despite international protests and scientific studies indicating that an LEU-fueled reactor would have allowed for the same research. Russia continues to contemplate construction oficebreakers that use HEU fuel. Only an international agreement to reduce and eventually eliminate the use of HEU will ensure that countries do not build such reactors.
Office of Global Threat Reduction U.S. National Nuclear Security Administration
"Reduced Enrichment for Research and Test Reactors," Argonne National Laboratory
"The 2010 Nuclear Security Summit: A Status Update," Arms Control Association, April 2011
"National Nuclear Security Administration Has Improved the Security of Reactors in its Global Research Reactor Program, but Action Is Needed to Address Remaining Concerns," GAO Report GAO-09-949, September 2009
International Medical Isotope Summit Communiqué, June 15, 2009, www.nti.org
Medical and nonproliferation experts group letter to Congress, "Medical and Nonproliferation Groups Unite to Confront Dire Shortage of Medical Isotopes; Millions of Americans May Lose Access to Treatment & Diagnosis of Cancer, Heart Disease; Congress Urged to Expedite Domestic Isotope Production but Avoid Bomb-Grade Uranium," June 15, 2009, www.nti.org.
Letter from Covidien to Nuclear Medicine Professionals regarding Mo-99 shortage, May 2009. The letter provides additional information on the global Mo 99 supply and generator production process.
Medical Isotope Production without Highly Enriched Uranium, National Academy of Sciences, February 2009.
Future of the Nuclear Security Environment in 2015: Proceedings, National Academy of Sciences and Russian Academy of Sciences, February 2009.
"Highly Enriched Uranium in Pharmaceutical Production," resolution passed by the California Medical Association, October 6, 2008.
"Eliminating Highly Enriched Uranium from Radiopharmaceutical Production," resolution passed by the Malaysian Medical Association, June 2008.
Cristina Hansell (Chuen), "Developing HEU Guidelines," paper presented at RERTR-2007 International Meeting, September 2007, www.nti.org.
Charles Ferguson and William Potter, eds., The Four Faces of Nuclear Terrorism (Abindgdon, Oxfordshire, UK: Routledge, June 2005), www.nti.org.
U.S. Department of Energy, "Highly Enriched Uranium: Striking A Balance," January 2001.
 Committee on Science and Technology for Countering Terrorism, Making the Nation Safer: The Role of Science and Technology in Countering Terrorism (Washington, DC: National Academy Press, 2002), pp. 40, 45, as cited in Charles Ferguson and William Potter, eds., The Four Faces of Nuclear Terrorism, p. 132.
 "Global Fissile Material Report 2010," International Panel on Fissile Materials, 2010, pp. 12 and 16, www.fissilematerials.org.
 Research Reactor Database, International Atomic Energy Agency, http://nucleus.iaea.org, 6 April 2011.
 Chinese exports consist of approximately 1kg. of 90% HEU fuel for use in Miniature Neutron Source type reactors. Sources: Greg Webb, "Nigeria Commissions Research Reactor; HEU-Fueled Facility Goes Against U.S.-Led Nonproliferation Effort," Global Security Newswire, 1 October 2004, www.nti.org; Ann MacLachlan, "Operators of small reactors to meet to discuss conversion to LEU fuel," NuclearFuel, 25 April 2005, p. 5; "Research Reactors" Briefing/Information Paper, December 2004, World Nuclear Association, www.world-nuclear.org; Judith Miller, "U.S. is Holding up Peking Atom Talks," New York Times, 19 September 1982; Michael Brenner, "People's Republic of China," in William Potter, ed., International Nuclear Trade and Nonproliferation, 1990, p. 253; and Leonard Spector, Nuclear Ambitions, 1990, p. 274, as cited in Steven Dolley, "China's Record of Proliferation Misbehavior," 29 September 1997, Nuclear Control Institute, www.nci.org.
 Office of Nonproliferation, National Nuclear Security Administration, "RERTR Program Project Execution Plan," 16 February 2004.
 "U.S. Receives LEU-Produced Medical Isotopes," Global Security Newswire, 24 August 2010, www.globalsecuritynewswire.org.
 "Nesca consortium celebrates $25m contract," World Nuclear News, October 29, 2010, http://188.8.131.52/RS-Necsa_consortium_celebrates_25m_award-2910107.html.
 For information on the above reactors see: www.aecl.ca (Canada); www.emtr.eu (the Netherlands); and http://den-dans.extra.cea.fr (France).
 "Applied and Basic Science," Rosatom, 2010, www.rosatom.ru.
 Miles A. Pomper and William C. Potter, "Medical Isotope Production: The U.S. Must Follow South Africa's Lead," Bulletin of the Atomic Scientists, 17 December 2010, www.thebulletin.org.
 "Senate committee seeks to create source for widely used medical isotope," Physics Today, 4 February 2011, http://blogs.physicstoday.org.
 "American Medical Isotopes Production Act of 2011 Introduced in U.S. Senate," SNM, 8 February 2011, http://snm.org.
 "Global Fissile Material Report 2010," International Panel on Fissile Materials, 2010, p. 62, www.fissilematerials.org.
 "Nuclear Icebreakers," Rosatom, 2010, www.rosatom.ru.
 "Promoting Nuclear Security: What the IAEA is doing," IAEA.org.
 "Promoting Nuclear Security: What the IAEA is doing," IAEA.org.
 "National Nuclear Security Administration Has Improved the Security of Reactors in its Global Research Reactor Program, but Action Is Needed to Address Remaining Concerns," GAO Report GAO-09-949, September 2009, www.gao.gov.
 "DOE Needs to Take Action to Further Reduce the Use of Weapons-Usable Uranium in Civilian Research Reactors," GAO-04-807, July 2004, www.gao.gov.
 "GTRI: Reducing Nuclear Threats," National Nuclear Security Administration, 1 February 2011, www.nnsa.energy.gov.
 "IAEA Safeguards Glossary," International Atomic Energy Agency, 2001, p. 23, www.iaea.org.
 "NNSA: Working to Prevent Nuclear Terrorism," NNSA, January 2009, http://nnsa.energy.gov. Tom Wander, director of the DOE's Materials Consolidation and Conversion program, private communication, July 14, 2005, as cited in Alexander Glaser and Frank N. von Hippel, "Global Cleanout: Reducing the Threat of HEU-Fueled Nuclear Terrorism," Arms Control Today, January/February 2006, www.armscontrol.org.
 "Highly Enriched Uranium Disposition," National Nuclear Security Administration, accessed 12 April 2011, http://nnsa.energy.gov.
 For information on the U.S. program, see "Reducing Excess Stockpiles: U.S. Highly Enriched Uranium Disposition," in Securing the Bomb 2005, www.nti.org.
 "Global Fissile Materials Report 2010," International Panel on Fissile Materials, 2010, p. 34, www.fissilematerials.org.
 Peter Stockton and Ingrid Drake, "From danger to dollars: What the U.S. should do with its highly enriched uranium," Bulletin of the Atomic Scientists, vol. 66 no. 6, p. 66, www.politik.uni-kiel.de.
 Mohamed El-Baradei Statement, Treaty on the Non-Proliferation of Nuclear Weapons 2005 Review Conference, United Nations, New York, 2 May 2005, IAEA.org.
 Statement by H.E. Nurbek Jeenbaev, Permanent Representative of the Kyrgyz Republic to the UN at the 2005 Review Conference of the Parties to the Treaty on the Nonproliferation of Nuclear Weapons (New York, 3 May 2005), www.un.org.
 Norwegian Position Paper, Treaty on the Non-Proliferation of Nuclear Weapons, 2005 Review Conference, 5 May 2005.
 Final document of the 2010 Review Conference of the Treaty on the Nonproliferation of Nuclear Weapons, May 2010, p. 29, www.reachingcriticalwill.org.
 "Highlights of the National Commitments Made at the Nuclear Security Summit," The White House, 13 April 2010, www.whitehouse.gov.
 Robert Golan-Vilella, Michelle Marchesano, and Sarah Williams, "The 2010 Nuclear Security Summit: A Status Update," Arms Control Association, April 2011, p. 6, www.armscontrol.org.
 Interview of Valentin Ivanov, Russian State Duma Deputy, 8 November 2005; interview of Rosatom Nuclear Power Department Head Valery Rachkov, 18 November 2005; Vyacheslav Belyayev and Konstantin Leontyev, Reactor Out to Sea, Nuclear Engineering International, Vol. 49, No. 594 (January 2004), also states that the KLT-40S will use ceramic metal fuel and <20% enriched uranium, meeting nuclear non-proliferation requirements.