A new joint report by NTI and the Russian-based Center for Energy and Security Studies highlights key projects the United States and Russia can take on to innovate and build trust in the nuclear sphere.

Our Priorities

Proliferation Risks of Nuclear Power Programs

Introduction: The Clear and Present Danger of Nuclear Proliferation Risks

While for more than fifty years the problem of using peaceful nuclear power to fuel nuclear weapons programs has threatened international security, in recent years four issues have brought renewed attention to proliferation dangers from states and non-state groups. First, the increasing real and perceived threats of nuclear terrorism after September 11, 2001, raised concerns that terrorist groups could potentially obtain nuclear explosive materials from either civilian or military nuclear programs. Second, the revelations by an Iranian dissident group at a press conference in August 2002 of Iran's progress in enriching uranium and building a nuclear research reactor that could produce plutonium - two activities that could also fuel weapons programs - demonstrated the shortcoming of traditional nuclear safeguards in detecting these activities. Although it is uncertain whether Iranian political leaders will decide to order production of nuclear weapons, it is clearer that Iran is nearing a break-out capability to make nuclear weapons materials because of the ongoing uranium enrichment program. Third, the unveiling in December 2003 of the A. Q. Khan nuclear black market showed the leakiness of many governments' export controls in stopping the flow of weapon-usable nuclear technologies.

Finally, the buzz about a nuclear power renaissance has stimulated about two dozen countries to renew or express new interest in nuclear energy for peaceful purposes. These peaceful activities might serve as a cover for weapons programs for some of these countries. Currently, 31 countries use about 440 commercial nuclear reactors to generate about 16 percent of the world's electricity. Concerns about energy security and global warming could spur a significant increase in nuclear energy use in the coming decades. (Nuclear energy emits very few greenhouse gases, which contribute to global warming.) If greater national and international efforts are not focused on increasing controls on sensitive nuclear technologies, this potential boost in nuclear power could increase dangers in the three threats of black markets, latent state-level weapons programs, and terrorist acquisition of nuclear explosive materials.

To better understand what these developments portend for international security, this issue brief will explain the dual-use dilemma of the nuclear fuel cycle and discuss proposals to control the proliferation risks of nuclear power programs.

Nuclear Fuel Cycle: Dual-Use Dilemma

The same technologies that make fuel for nuclear reactors can also produce explosive material for nuclear bombs. Two pathways are available to either make fuel or bomb-material. These are the uranium and plutonium pathways.

Uranium Pathway

The uranium pathway starts with mining natural uranium. Natural uranium consists of different isotopes or nuclear forms of the element uranium. All isotopes of uranium have 92 protons in their nuclei. The number of positively charged protons determines an element's chemical properties. Each isotope varies in the number of neutrons in its nucleus. Although neutrons are uncharged and thus do not exert an electromagnetic force, they exert nuclear forces, which determine whether an isotope will exhibit radioactive decay or remain stable and whether the isotope will undergo nuclear fission.

In fission, after absorbing a neutron, the nucleus of a massive isotope will split into two lighter mass isotopes, emit additional neutrons, and release an antineutrino (which is a very light particle and is unimportant from the standpoint of reactors and bombs because it rarely interacts with other matter). The additional neutrons are needed to propagate a chain reaction in which more and more fissions occur. Fission also releases energy. While each fission emits a tiny amount of energy, after about 80 doublings (or two raised to the power of 80 fissions) of the fission chain reaction, the total energy is equivalent to the explosive yield of the Hiroshima bomb. To make this explosive chain reaction happen with uranium, a large concentration of the isotope uranium-235 is required. (The 235 indicates the atomic mass or total number of protons and neutrons in the isotope's nucleus, and uranium-235 has 92 protons and 143 neutrons.)

But natural uranium does not contain a large enough concentration of uranium-235 to power an explosive chain reaction. Natural uranium consists of 0.72 percent uranium-235 and 99.275 percent uranium-238 as well as a tiny fraction of uranium-234. Uranium-235 is called a fissile isotope because it easily fissions if it absorbs a neutron of almost any energy. In contrast, uranium-238 is called a fissionable isotope because it has only a relatively small probability of fission if it absorbs high energy neutrons. Enrichment is the process to increase the concentration of uranium-235.

Uranium has different enrichment grades, which vary in their suitability for powering nuclear weapons. Weapon-grade uranium is usually defined as a mixture of uranium isotopes consisting of more than 90 percent uranium-235. This type of uranium is most suitable for weapons because of its high concentration of fissile uranium-235. Weapon-grade uranium is a subset of the larger class of highly enriched uranium. Highly enriched uranium (HEU) is defined as consisting of a 20 percent or greater concentration of uranium-235 and can power explosive chain reactions. The greater the concentration of uranium-235, the less material is needed to make a nuclear bomb. For example, about 25 kilograms of weapon-grade uranium would be needed for a first generation nuclear weapon. In comparison, more than 200 kilograms of 20 percent enriched uranium would be needed to make a nuclear explosive. In contrast, low-enriched uranium (LEU), which is suitable for fueling reactors but not for powering bombs, is defined as consisting of greater than 0.72 percent, but less than 20 percent uranium-235. The same enrichment technology can be used to make either LEU or HEU.

Enrichment technologies separate lighter mass isotopes from heavier mass isotopes. Because isotopes of the same element have the same chemical properties (same number of protons), enrichment methods cannot use chemical reactions to separate the different isotopes. Instead, these methods must apply physical forces that take advantage of the difference in masses to separate the isotopes. For example, the increasingly popular gas centrifuge enrichment method applies centripetal force to uranium hexafluoride gas (which consists of one uranium atom bound to six fluorine atoms). The major component of the centrifuge is a rapidly rotating cylinder that contains the uranium hexafluoride. The rapid rotation creates the centripetal force. Because centripetal force depends on the mass of the object feeling the force, the lighter uranium-235 hexafluoride molecules will separate from the heavier uranium-238 hexafluoride molecules. Each individual rotating centrifuge cylinder only produces a relatively slight increase in the concentration of uranium-235. Thus, many cylinders are required to be connected to each other to boost the enriched uranium product to the desired enrichment level.

The connected assemblage of centrifuges is called a cascade. A cascade optimized for LEU production differs from a cascade optimized for HEU production. To visualize the cascade structure, think of two pyramids with one upright and the other inverted. (Pyramids are three-dimensional structures. Here, to picture the approximate shape of the cascade, visualize a two-dimensional cross-section of a pyramid slicing from the apex to the base.) The two pyramids are connected at their bases. Each layer of a pyramid consists of a row of centrifuges connected to each other. Each row is connected to the row immediately above it and the row immediately below it. At the base row of the centrifuge pyramid, where uranium hexafluoride is initially fed into the cascade, there are more centrifuge units than in any other row. In the upright pyramid, uranium becomes increasingly enriched in uranium-235 as it makes its way up to the top of the pyramid. In the inverted pyramid, uranium becomes increasingly depleted (reduced concentration) in uranium-235 as it makes its way to the bottom of the inverted pyramid. LEU and HEU cascades differ in the shapes of their pyramid-like cascades. That is, LEU cascades would have wider bases (more centrifuges in the base rows of the cascades) and shorter heights (fewer rows of centrifuges) than HEU cascades. The inverted depleted-uranium pyramid for both LEU and HEU cascades has fewer rows than the upright enriched-uranium pyramid.

A centrifuge enrichment plant could be designed to allow the operator to change the connections among the centrifuge units to shift cascades from LEU to HEU production. Depending on the plant design, rearranging these connections could take little more than several days to a few weeks. This relatively rapid changeover poses challenges for safeguard inspectors who are trying to determine if an enrichment plant has produced weapon-usable uranium before use of that uranium in a bomb. Another safeguard challenge arises from the fact that an operator would not have to change the connections among the centrifuge units to produce HEU. The operator could use the less efficient process of batch recycling of LEU in an LEU cascade to boost the enrichment levels to HEU. That is, the LEU product from one pass through the plant could be used as the feed for another pass through the plant and so on until enrichment levels are increased to the desired concentration of uranium-235. Only a handful of passes, typically four or five, are needed to boost LEU to weapon-grade levels. Therefore, an LEU enrichment plant is a latent nuclear explosive material factory. However, as long as safeguard inspections are applied to the plant, the operator would have to be concerned that HEU production could be detected. But if the government that owns the enrichment plant wanted to produce HEU, it could kick out inspectors and abrogate its safeguard agreement.

Plutonium Pathway

Like uranium, plutonium is a heavy element with certain types of isotopes that are very suitable for nuclear fission. In particular, plutonium-239 is the most suitable fissile isotope. Unlike uranium-235, which has a radioactive half-life of several hundred million years, plutonium-239 has a relatively short half-life of about 24,000 years. (Half-life measures how long it takes for one half of a sample of a radioactive isotope to decay to a different isotope.) On the geological timescale of billions of years, uranium-235 lasts a long time, and plutonium-239 decays rapidly. Thus, any plutonium-239 that was present when the earth formed about four billion years ago is no longer present. Consequently, to have plutonium-239 available for fueling bombs, manmade processes are needed to produce this isotope. Nuclear reactors produce plutonium-239 when uranium-238 in the fuel mixture absorbs neutrons. After a uranium-238 nucleus absorbs a neutron, it turns into uranium-239. After two relatively rapid radioactive decays, the uranium-239 becomes plutonium-239.

Because a fuel mixture that has been in an operating reactor includes a large amount of highly radioactive fission products, rigorous safety precautions are required to ensure workers are not exposed to harmful radiation from the spent fuel. A reprocessing plant is designed to safely remove or extract the plutonium from the fuel mixture.

Reprocessing involves a series of physical steps and chemical reactions to extract plutonium. The PUREX technique is the only commercialized reprocessing technique. The first physical step is to chop up the spent fuel into pieces. These pieces are then sent into a vat of hot nitric acid to remove the fuel cladding from the fuel and fission products. The fuel consists of uranium-235, uranium-238, and plutonium in different isotopic forms. In addition to the fission products, uranium, and plutonium in the fuel, the mixture of materials includes other transuranic (heavier than uranium) isotopes such as americium and neptunium. (Fission products are lighter than uranium and plutonium.) After removal of the fuel cladding, the mixture of materials is combined with kerosene and tri-butyl-phosphate (TBP) to remove uranium and plutonium from the fission products and other radioactive materials. The separated plutonium can then be combined with depleted uranium to make mixed oxide (MOX) fuel.

The proliferation hazard in PUREX reprocessing is that plutonium is separated from the highly radioactive fission products. These fission products provide a protective (lethal) barrier against theft of unshielded spent fuel containing plutonium. Because plutonium is not very radioactive, separating plutonium from the fission products leaves it potentially vulnerable to theft. Thus, separated plutonium should be guarded as if it were a nuclear weapon because it is directly usable in such a weapon.

Like uranium, plutonium has different grades that vary in their suitability for weapons. Essentially, the greater the concentration of plutonium-239, the more suitable is a plutonium mixture for weapons. Weapons-grade plutonium has less than six percent of its isotopes as non-plutonium-239. Fuel-grade plutonium is defined as having more than six but less than 18 percent of non-plutonium-239 isotopes. Reactor-grade plutonium has greater than 18 percent of these isotopes. Many of the non-plutonium-239 isotopes tend to undergo spontaneous fission relatively often and thus a large concentration of these isotopes can result in a greater likelihood of a nuclear bomb producing a fizzle yield (less than design yield) because the explosive chain reaction would start prior to full assembly of the fissile material. Nonetheless, some nuclear explosive yield would likely occur in a nuclear bomb made from reactor-grade plutonium. While some nuclear industry officials have questioned reactor-grade plutonium's usefulness in nuclear weapons, the U.S. Department of Energy clearly stated in 1997 that this type of plutonium is weapon-usable.[1] For more discussion and references on this issue, see the Issue Brief on "Risks of Civilian Plutonium Programs."[2]

Controlling the Proliferation Risks

There is no prospect of security against atomic warfare in a system of international agreements to outlaw such weapons controlled only by a system which relies on inspection and similar police-like methods...rivalries are inevitable and fears are engendered that place so great a pressure upon a system of international enforcement by police methods that no degree of ingenuity or technical competence could possibly hope to cope with them.[3]

This conclusion came from the Acheson-Lilienthal Report of 1946, issued by the United States government and mainly drafted by J. Robert Oppenheimer, the scientific leader of the Manhattan Project, which produced the first nuclear bombs. This report became the basis of the Baruch Plan, which was placed before the United Nations in 1946 and offered a proposal for international control of nuclear energy. While this proposal was not passed by the UN, it has periodically been resurrected in various incarnations throughout the next sixty plus years. Here, the focus is not to provide a detailed discussion of these various proposals and other options for controlling proliferation risks. Instead, this Issue Brief provides a concise tutorial about the concepts behind these proposals.

The proliferation-control concepts involve political, financial, and technical solutions. With respect to the political dimension, the first issue to be understood is that national governments want to protect their right to control what happens inside their states' territories. They also want to ensure they keep all of their rights assigned to them under international treaties.

Relevant to the proliferation risks of nuclear power programs is the nuclear Non-Proliferation Treaty (NPT). In article IV of the NPT, it is declared that a state has the "right" to peaceful nuclear technologies as long as the state maintains safeguards on its peaceful nuclear program and does not manufacture nuclear explosives. While this article does not specifically mention uranium enrichment and plutonium reprocessing technologies as part of a state's right to peaceful nuclear technologies, it does not explicitly exclude enrichment and reprocessing technologies. Recently, there has been considerable renewed debate about whether this right should be interpreted to include these bomb-usable technologies. [4] Nevertheless, the right has usually been interpreted to include these technologies. Thus, non-nuclear-weapon states such as Argentina, Brazil, and Japan, for example, have pursued enrichment or reprocessing or both and have maintained safeguards on these programs. Iran claims that it wants to be like Japan and have a peaceful nuclear program that includes enrichment and possibly reprocessing. However, the IAEA and the UN Security Council have ruled that Iran is not in compliance with its safeguards commitments.

While Iran does not at this time appear willing to give up its uranium enrichment program, the dilemma this program poses to international security has renewed interest in political and financial incentives that would try to dissuade countries from engaging in enrichment and reprocessing. One option is to offer fuel services contracts that are very economically competitive. Under this scheme, a country or group of countries would guarantee that a state in need of nuclear fuel would always have that fuel provided as long as that state did not enrich uranium or reprocess plutonium. The fuel services could also include spent fuel management in which the service providers would agree to remove the spent fuel and safely and securely store it. Such action would remove the material the state would need to extract plutonium for a weapons program. To assuage the state's concerns about its sovereign rights, the service contract could be worded to ensure that the state would not forego its right to enrich or reprocess but would choose not to do these activities as long as it is under the contract.

The state might decide that it still wants to enrich and reprocess especially because it is planning for a large nuclear power program. Generally, unless a state does not have more than eight large nuclear power reactors, it does not make economic sense for the state to invest in making its own nuclear fuel. In this case, a related proposal would come into play. That is, a group of countries could offer to form a multinational partnership with the state to make fuel. Thus, nuclear-fuel-making activities would take place in that state, but more than one country would be involved in those activities. Added proliferation risk reduction enters into this scheme because there would be more than one state involved in the operation of the fuel facility. This extra monitoring would increase the likelihood of catching clandestine nuclear weapons activities in the state of concern.

As with the fuel services proposal, states can accept or reject multinational ownership and control of fuel making facilities. The challenge is to make the incentives great enough. But an unintended consequence of too great an incentive could be stimulation of large nuclear power programs in countries where other energy sources could offer similar benefits of electricity production without the added risk of possible nuclear proliferation. For example, the situation could emerge in which a state expresses interest in nuclear power and is offered fuel services. For many years, it could dutifully avail itself of these services, but then it could decide that its nuclear program had grown so large that economics favor construction of fuel facilities on its territory. It also decides that it does not want to have multinational control of these facilities. Thus, this state could, partly as a result of the stimulus of the fuel services offer, reach a place in its development in which it would eventually make its own fuel and consequently have a latent nuclear weapons capability.

Recognizing that certain countries will enrich or reprocess, technical proposals aim to increase the proliferation-resistance of these activities. For instance, there is ongoing research into reprocessing methods that would not completely separate plutonium from fission products or other radioactive materials such as transuranics. However, enough of the highly radioactive isotopes would be removed such that the mixture of plutonium and the remaining isotopes would not pose an immediately lethal barrier to theft. Also, there is the risk that a proliferant state could take this mixture and use it as input to a clandestine PUREX reprocessing facility. In sum, although proliferation-resistance technologies can offer added barriers to use of fissile material in weapons programs, they are not proliferation-proof. Thus, there would be a continuing need for safeguards and monitoring of peaceful nuclear programs that use proliferation-resistance technologies.

The biggest barrier to preventing proliferation would be to stop the use of nuclear power. In reality, just the opposite trend is taking place. More, but not all, states are trying to expand use of nuclear power programs. With concerns about energy security and climate change, many states are looking toward nuclear energy. However, some states such as Italy, Germany, and Spain have committed to phasing out nuclear power programs or deciding not to acquire them. In the risk-benefit analysis, analysts are still pondering whether climate change or nuclear proliferation poses a greater threat and whether the world can reduce greenhouse gas emissions without using nuclear energy. Faced with this uncertainty and with the continued use of nuclear energy in the foreseeable future, the international community will have to remain increasingly vigilant about controlling the risks of proliferation.

Tariq Rauf and Fiona Simpson, "The Nuclear Fuel Cycle: Is It Time for a Multilateral Approach?" Arms Control Today, December 2004, www.armscontrol.org.

Lawrence Scheinman, "The Nuclear Fuel Cycle: A Challenge for Nonproliferation," Disarmament Diplomacy, March/April 2004 (reprint of his article that was originally published in International Organization in 1981), www.acronym.org.uk.

Brice Smith, Insurmountable Risks: The Dangers of Using Nuclear Power to Combat Global Climate Change, Institute for Energy and Environmental Research and RDR Books, 2006, www.ieer.org.

About the Threats

This material is produced independently for NTI by the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey and does not necessarily reflect the opinions of and has not been independently verified by NTI or its directors, officers, employees, or agents. Copyright 2017.