Preventing Nuclear-explosive Terrorism by Eliminating Fissile Material:A Progress Report

Frank von Hippel, co-chair, International Panel on Fissile MaterialsProgram on Science and Global Security, Princeton University

The mission of the International Panel on Fissile Materials is to develop the basis of initiatives to secure, to consolidate, and to reduce global stocks of nuclear weapon usable fissile materials. Fissile materials (primarily highly enriched uranium (HEU, uranium enriched to more than 20% U-235) and separated plutonium) are essential in all nuclear weapons, and controlling and reducing their stocks is critical to all efforts to reduce nuclear-weapon stockpiles, increase barriers to proliferation and prevent nuclear terrorism. This paper is a progress report on this effort.

Global stocks of fissile material

The fission of 1 kg of plutonium or HEU releases energy equivalent to the explosion of about 17,000 tons of chemical explosive – approximately the yield of the explosions that destroyed Hiroshima and Nagasaki. Figures 1 show national declarations and non-governmental estimates (indicated by asterisks) of stocks of HEU and plutonium of the weapon states, some key non-weapon states and the other non-weapon states collectively for 2011. The total is about 2,000 tons of fissile material, enough for over 100,000 nuclear explosives. In fact, in 1967, the U.S. had about 31,000 nuclear warheads and, in the mid 1980s, the Soviet Union is believed to had about 40,000 warheads.1

Figure 1a. Global stocks of HEU currently total about 1500 tons– sufficient (at 25 kg per warhead) for 60,000 nuclear warheads (Ref. Global Fissile Material Report 2011, GFMR2011).

Figure 1b. Global stocks of separated plutonium currently total about 500 tons – sufficient (at 8 or 4 kg per warhead for 60,000 or 120,000 warheads. (Ref. GFMR2011).

Securing, consolidating and eliminating nuclear material

Securing. Russia accounts for about half of the global HEU and one third of the separated plutonium stock. After the collapse of the Soviet Union, the U.S. was greatly concerned about the security of fissile materials and nuclear warheads in Russia and has spent about $10 billion dollars in upgrading security there. This effort continues today.

Figure 2. Consolidation at the Y12 HEU site in Oak Ridge, Tennessee. Historically, all U.S. HEU weapon component fabrication, dismantlement and HEU central storage has taken place in the complex of buildings within the heavy line. Two fortified buildings, will replace this complex: one for HEU storage (complete) and one for processing (under construction).

Consolidating. The U.S. Department of Energy (DoE) spends more than $1 billion per year on nuclear-material security at its own facilities. This has motivated a considerable amount of consolidation. The DoE is cleaning plutonium and HEU out of two of the three U.S. nuclear weapons labs (Livermore, California and Sandia, New Mexico) as well as other DoE sites. On sites where significant quantities will remain, the material is being consolidated the into high-security buildings (Figure 2).

Eliminating. Russia and the U.S. have eliminated a great deal of HEU and are planning on eliminating a considerable amount of excess weapons plutonium as well.

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The white areas in Figure 1a show that Russia and the U.S. together have blended down about 600 tons of excess HEU to 4 to 5 percent low-enriched uranium (LEU). They are committed to blend down about 100 tons more. The LEU is being used mostly to fuel U.S. nuclear power plants (more than half of U.S. nuclear capacity). The blend-down of the 500 tons of HEU that Russia has committed to dispose has been at a level of 30 tons per year for about 15 years and will be completed in 2013. The blend-down rate of excess U.S. HEU has dropped from about 10 tons per year to about 3 to 4 tons a year, due to bottlenecks in the warhead and HEU-component dismantlement process and currently is expected to take a decades or more to complete.

Much more HEU could be blended down. A global nuclear arsenal of about 10,000 nuclear warheads – roughly the situation today, not counting warheads in the dismantlement queue – would require only about 250 tons of weapon-grade (90+% U-235) HEU, vs. the approximately 1000 tons that are in the weapon-state military stockpiles.

Russia and the U.S. also each are committed to converting into fuel 34 tons of weapon-grade plutonium. Both are building “mixed-oxide” (MOX, uranium-plutonium) fuel-fabrication facilities. The U.S. plans to produce MOX for once-through use in light-water reactors while Russia plans to produce MOX for fast-neutron breeder reactors, which require plutonium recycle. Russia therefore will not be permanently disposing of its separated weapons plutonium but rather simply will be passing this plutonium through breeder reactors into its stockpile of separated civilian plutonium.

Much more weapons plutonium could be declared surplus. A global stockpile of about 10,000 warheads would require only about 40 tons of weapon-grade plutonium, vs. approximately 140 tons in the weapon stockpiles today.

Reducing the use of HEU and separated plutonium for reactor fuel

The principal non-weapon use of HEU and plutonium is in nuclear fuel.2 HEU is used in research reactor fuel in about 30 countries, and in naval reactor fuel in the US, UK, Russia and India. Plutonium is recycled in MOX fuel in light-water reactors in France, and soon, in prototype breeder reactors in India and Russia. Japan has been planning to recycle plutonium in light water reactors. China has breeder reactor plans.

In all of these uses, there is a risk of diversion to weapons. After India’s use of civilian plutonium for a “peaceful nuclear explosion” in 1974, the U.S. abandoned civilian reprocessing at home and actively tried to prevent the spread of plutonium separation to new countries and to minimize HEU fuel use in research reactors. Only in the case of HEU-fueled civilian research reactors, however, is there a real international consensus on minimization of the use of weapon-usable fissile materials in reactor fuel. Plutonium separation continues in Europe, Japan, Russia, India and China.

Research reactors. Since 9/11, the U.S. has led an intensified campaign to convert the world’s HEU-fueled research reactors and medical fission isotope production targets to LEU (in practice uranium enriched to 19.75% U-235). This is an appropriate priority, since research reactor facilities are often not as secure as are facilities where nuclear weapons are assembled and disassembled and where naval reactor fuel is manufactured.

The technical approach that is being used is to develop fuels with roughly five times higher uranium density than the current fuels containing weapon-grade (> 90% U-235) uranium. This density increase makes it possible to add U-238 to reduce the enrichment of the uranium without reducing the concentration of the U-235. The HEU densities mostly are below 1.5 gms/cc. Thus far, fuels with densities up to 4.8 gms/cc have been developed and fuels with LEU densities up to 15 gm/cc are under development.

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HEU has been “cleaned out” (reduced to less than 1 kg) in more than 20 non-weapon states (Figure 3). Most of the remaining HEU-fueled research reactors are in the weapon states – Russia, in particular. Also, as steady-power research reactors are converted to LEU (61 so far, with about 50 left to convert or retire), reactors with lifetime cores – especially HEU-fueled critical-facilities and pulsed reactors are becoming an increasingly large fraction of the HEU-fueled research reactor population (see Table 1).

Figure 3. Number of non-weapon states with plutonium (updated from GFMR2011).

There are at least three challenges to completing the task of phasing out HEU use in a research reactor fuel:

1. Make it a priority for the Russian government. Russia’s Government has been happy to support the U.S. financed effort to convert Soviet-designed HEU-fueled reactors and retrieve Soviet and Russian-origin HEU fuel outside Russia – but has not given any priority to converting its own HEU-fueled reactors. In 2010, however, after repeated requests from the U.S. Government at the highest levels, Russia’s Government agreed to allow the U.S. Global Threat Reduction Initiative (GTRI) to fund conversion feasibility studies for six of its civilian reactors. Also, since the March 2012 Nuclear Security Summit in Seoul, Rosatom has been showing more committed to the agenda of phasing out civilian HEU use.

2. Higher-density uranium fuels. To convert high-power reactors that require the highest uranium-fuel densities, the GTRI program is focusing on developing the most dense fuel possible: metallic uranium, alloyed with up to 10 weight-percent molybdenum to improve its mechanical properties. Such fuel would have a uranium density of about 15 gm/cc.

Table 1. The global population of HEU-fueled reactors includes much more than the steady-power research reactors that thus far have been the focus of the cleanout effort.3

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3. Critical assemblies and pulsed reactors. Military research reactors include critical assemblies that mock up naval reactors and pulsed reactors to generate neutron bursts that can be used to test the effect on electronics of nearby nuclear explosions. Fortunately, computer simulations have made critical assemblies almost obsolete except for experiments need to benchmark the codes. Computer simulations probably also have taken over in Russia but RosAtom is retiring obsolete critical assemblies only slowly and still has them at about ten sites.

In the US, four of the five remaining HEU critical assemblies have been moved to the high-security Device Assembly Facility (DAF) on the Nevada Test Site from a location in Los Alamos that was not considered adequately secure (Figure 4).4 Sandia National Laboratory also has shipped to the DAF the HEU fuel of its last pulsed reactor and is developing computer simulations of the effects of neutron bursts on the transistors in chips.

Most of Russia’s dozen pulsed reactors are at its two weapons design laboratories. The U.S. had a similar number5 but today, due to security concerns, reduced relevance of nuclear war-fighting scenarios and improved computational capabilities,6 only has one pulsed reactor, the Army’s Fast Burst Reactor at the White Sands Proving Ground in New Mexico. Recently, the Army put out a request for proposals for alternative ways to produce neutron pulses so that it can retire this last pulsed reactor.7 Most military pulsed reactors have annular cores made up of massive pieces of solid uranium alloy.8

Figure 4. The U.S. has consolidated all of its weapon-design-related critical assemblies in the high-security Device Assembly Facility at the Nevada Test Site.

More information is available about the designs of fast critical facilities than about military critical facilities. These designs are therefore useful for illustrating the security problem with critical facilities fueled with HEU and plutonium. The two BSF facilities at the Institute of Physics and Power Engineering, which are designed for modeling the cores of large fast breeder reactors, for example, are built up of about 100,000 different disks, including tens of thousands containing plutonium and HEU. These disks contain a total of 700 kg of HEU(90%), 500 kg of plutonium and 2800 kg of 36% HEU(36%) (Figure 5).

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Figure 6. In the BSF-2 fast critical assembly at the Institute of Physics and Power Engineering in Obninsk Russia, disks of plutonium, HEU and depleted uranium can be stacked in tubes to test the criticality of breeder-reactor cores. The diagram inset shows the arrangement of tubes in the core into which the disks are stacked.9

Naval reactors. The U.S. and U.K. fuel their naval propulsion reactors with weapon-grade uranium. Russia and India are believed to use HEU that is less highly enriched. France and China are believed to use low-enriched uranium. Russia’s Artic fleet of civilian nuclear-powered icebreakers and transports also uses HEU fuel.

The U.S. fabricates an average of about 2 tons of weapon-grade uranium into naval fuel per year and has established a reserve of 130 tons of weapon-grade uranium for future naval-reactor use. Although naval fuel cycle facilities are more secure than research reactors, thefts of HEU have occurred. It would be desirable to shift to LEU-fueled reactors. Also, if much lower levels of nuclear weaponry can be achieved, large stocks of HEU justified by the future needs of naval reactors would raise concerns about breakout.

In 1994, the U.S. Congress required the U.S. Navy to study the possibility of moving to LEU-fuel. The report from the Director of Naval Nuclear Propulsion argued that the costs would be high and the benefits insignificant.10 In 2008, the Senate Armed Services Committee, in its report on the Defense Authorization Act for Fiscal 2009, stated that:11 

“The committee directs the Office of Naval Reactors to review carefully options for using low enriched uranium fuel in new or modified reactor plants for surface ships and submarines.”

The Office has ignored this language. As of 2012, the design of a new aircraft propulsion reactor was about 96 percent complete and the design of the reactor for a new ballistic-missile submarine was launched in fiscal year 2012. 12 Both designs are HEU fueled. The HEU-fueled Virginia-class attack submarine will be in production for at least another decade.

Nevertheless, it would be desirable to continue the effort to establish the use of low-enriched uranium as a norm for naval as well as other reactors. If, in the future, the production of HEU for weapons is banned but production for naval fuel is not by a Fissile Material Cutoff Treaty, one of the most difficult verification challenges will be difficult to detect governmental diversion of HEU from naval fuel cycles.

What are the objections of the U.S. nuclear navy to shifting to LEU? The objections in the 1995 related primarily to cost. It was assumed that conversion to LEU would require either:

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1. Retreat from lifetime cores to refueling (3 times in a submarine lifetime and two times in an aircraft carrier lifetime). The cost of purchasing the extra cores was estimated at about $100 million each for submarines and $600 million for twin aircraft carrier cores. It also was estimated that refueling would extend the duration of major overhauls in port by 10 months for submarines and 22 months for the aircraft carriers and the cost of each overhaul by about $60 million for the submarines and $300 million for aircraft carriers. Because of the extra time in port, it was assumed that the Navy would have to buy an additional four attack submarines, one ballistic-missile submarine and one aircraft carrier to maintain the same at-sea strength.

2. Build reactors with bigger cores. It was estimated that this would cost about $300 million extra for the submarine reactors and $1.2 billion extra per aircraft carrier. In addition, it was estimated that the size of attack submarines would have to be increased to accommodate the larger cores and reactors at a cost of $150 million each. (The estimated cost increases for the larger ballistic missile submarines and aircraft carriers were relatively minor.)

Thus the lifetime core option was found to be less costly at about an extra $1 billion/year vs. about $2.5 billion/year for the return to refueling (1995 $). The report concluded, however, that, “[n]either option for using LEU in place of HEU offers the Navy a technical, military or economic advantage. Either option would be extremely costly.”

The UK, which obtains HEU and its fuel design from the U.S., would change to LEU only if the U.S. did. France and China, however, have made different decisions.

In the case of France, there appear to be three differences in the cost-benefit calculation:

1. France, unlike the U.S. does not have a huge stockpile of excess Cold War HEU and did not want to replace its costly HEU-production plant, which was based on gas-diffusion technology and was expected to expire around 2000. 13 In its newest nuclear submarine, it will move to fuel using the same enriched used in power reactors, about 5% enriched uranium;14

2. French submarines have large hatches that allow refueling without an extension of refit time, 15 whereas refueling U.S. submarines requires cutting a hole in the hull; and

3. The fabrication cost for U.S. naval-reactor fuel is very high.16

China too may not have a large stockpile of HEU in excess to its weapons needs. Also, its first submarine reactor design was based on published descriptions of the propulsion reactors of Russia’s first nuclear-powered icebreaker, whose fuel was 5 percent enriched.17

Plutonium use in power reactors. Separation of civilian plutonium was launched by the major industrialized countries in the 1960s and 1970s because of twin beliefs: i) Global nuclear power capacity would quickly grow to thousands of gigawatts, and ii) The world does not have enough low-cost uranium to support on that scale nuclear power based primarily on a once-through fuel cycle not involving plutonium recycling (Figure 6).

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Figure 6. IAEA projections in 1975 and in 2011 of global nuclear power growth and of global resources of low-cost (< $130/kgU) uranium.

On that basis, several countries decided to launch major efforts to develop plutonium breeder reactors whose ultimate fuel would be the U-238 that constitutes 99.3% of natural uranium. Despite the expenditure of about $100 billion on attempts to commercialize fast-neutron sodium-cooled breeder reactors, however, the effort failed. And, because of slow growth of global nuclear generating capacity and growing estimates of global uranium resources, most countries concluded that breeder reactors would not in any case be required for more than a century.

One legacy of this effort is about 250 tons of separated civilian plutonium from reprocessing programs that were originally launched to obtain plutonium for initial breeder reactor cores. In India and Russia plutonium separation continues on a small scale (1 to 2 tons per year) with this rationale and China has built a pilot reprocessing plant (1 ton/year if it is operated at full capacity) and is considering building a larger reprocessing plant. All three countries are still concerned that available uranium supplies may not be able to keep up with their ambitious plans for nuclear power expansion.

The large reprocessing plants are in France, Japan and the UK, however– although the two U.K. reprocessing plants are only operating at partial capacity and Japan’s reprocessing plant is not operating at all. France recycles its separated plutonium in “mixed-oxide” (MOX, uranium-plutonium) fuel into the light water reactors from whose spent uranium fuel the plutonium was separated. Spent MOX fuel is stored pending decisions on whether to dispose of it in an underground repository or to reprocess it and use the recovered plutonium for either startup fuel for future breeder reactors or perhaps repeated recycle in light-water reactor fuel. In the wake of the Fukushima accident, Japan is currently debating whether or not to continue with its reprocessing plans. The UK has decided to end reprocessing when its foreign and domestic contracts have been completed – currently estimated in 2018.

Japan is starting to build a MOX plant, despite the fact that its efforts to recycle MOX fuel coming back from France had been delayed by a decade by public opposition even before the Fukushima accident. The U.S. is mid-way in building a MOX plant for its excess plutonium but has encountered huge cost overruns and schedule delays. The UK has accumulated approximately 100 tons of separated civilian plutonium that it now has to dispose of. It built one MOX fuel fabrication plant that only operated at about one percent of its design capacity for ten years before being abandoned in 2011. It is currently planning to build a second MOX plant to produce fuel that would be used in light-water reactors that it hopes will be built in the UK (the UK currently as only one LWR). Some of us have suggested direct disposal of plutonium as an alternative to MOX.18

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Conclusions

1. Reduction of the Cold War stockpiles of HEU has begun on a substantial scale and reductions of the weapons stockpiles of plutonium are planned.

2. There is an emerging consensus to phase out the civilian use of HEU.3. There has been an intense debate over plutonium separation and recycle for almost 40 years.

Most countries have abandoned reprocessing because of its large costs and few benefits, but a few major countries remain committed to reprocessing.

4. A policy debate over whether to move away from the use of HEU fuel in naval propulsion reactors has not really begun.

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1 United States, US Department of Defense, “Increasing Transparency in the U.S. Nuclear Weapons Stockpile,” Fact Sheet, 3 May 2010; Soviet Union/Russia, Natural Resources Defense Council estimate, http://www.nrdc.org/nuclear/nudb/datab19.asp.2 About 50 kg of weapon-grade uranium annually has been used in neutron “targets” to produce fission radioisotopes for medical purposes. This use is being phased out.3 Update of table in Ole Reistad and Styrkaar Hustveit, “HEU Fuel Cycle Inventories and Progress on Global Minimization,” Nonproliferation Review, Vol. 15, # 2, 2008, p. 265.4 The fifth U.S. critical assembly is associated with the Advanced Test Reactor at the Idaho National Laboratory. It is also possible that the U.S. has a naval critical facility.5 C. Paxton, A History of Critical Experiments at Pajarito Site (Los Alamos National Laboratory, LA-9685-H, 1983 H.), p. 13.6 In 2004, U.S. Secretary of Energy Spencer Abraham announced that “the Sandia Pulsed Reactor [at the Department of Energy’s Sandia National Laboratory, where nuclear-weapons work is done] will no longer be needed because computer simulations will be able to assume its mission…When its mission is complete, this reactor’s fuel will be removed…allowing us to reduce security costs at Sandia and further consolidate our nuclear materials”, “Remarks prepared for Energy Secretary Spencer Abraham for the Security Police Officer Training Competition,” May 7, 2004, reprinted in U.S. Nuclear Weapons Complex: Homeland Security Opportunities, Project on Government Oversight, May 2005, Appendix B. Sandia has a program on Qualification Alternatives to the Sandia Pulsed Reactor (QASPR), which focuses on computer modeling of neutron effects on transistors. Predictions are compared to experimental results obtained by the Ion Beam Laboratory.7 US Army, “Alternative Source for Neutron Generation,” 2012), http://www.dodsbir.net/sitis/display_topic.asp?Bookmark=426618 The Idaho National Laboratory has not used the last U.S. civilian pulsed reactor, the Transient Reactor Test (TREAT) reactor, for 20 years. This reactor is a lesser security concern, however, because its HEU is in solution-impregnated graphite – one atom per ten thousand carbon atoms.9 BFS-2 image captured from the website of the Institute of Physics and Power Engineering, Obninsk, Russia. Inset diagram from Alessandro Marinoni et al, “Analysis of the BN-600 fast-spectrum core mock-up at BFS-2 zero-power facility using MCNPX,” Annals of Nuclear Energy, Vol. 44 (2012) p. 2610 Report on Use of Low Enriched Uranium in Naval Nuclear Propulsion, Director, Naval Nuclear Propulsion, 1995.11 U.S. Senate Armed Services Committee fiscal year 2009 Authorization Report (p. 515)12 National Nuclear Security Administration, FY 2013 Congressional Budget Request, p. 484.13 Y. Girard, Techmcatome of France, presentation, Conference on The Implication of the Aquisition of Nuclear-Powered Submarines (SSN) by Non-Nuclear Weapons States, MIT, 27-28 March 1989; 14 “France’s Future SSNs: The Barracuda Class,” Defense Industry Daily, 21 Dec. 2011.15 Y. Girard, op. cit. 16 A U.S. Trident submarine contains about 1 ton of HEU and a Virginia-class submarine about 0.7 tons, IPFM, Global Fissile Material Report 2010, endnote 131. On this basis, the figures quoted above from the 1995 report by the Director, Naval Nuclear Propulsion suggest that the fabrication costs for these cores were about $100 million or about $100,000/kgU.17 John Lewis and Zue Litai, China’s Strategic Seapower (Stanford University Press, 1994), p. 31; Global Fissile Material Report 2010, op. cit., endnote 282.18 Frank von Hippel, Rodney Ewing, Richard Garwin and Allison Macfarlane, “Time to Bury Plutonium,” Nature, 10 May 2012, p. 167