USING WARHEAD PLUTONIUM AS REACTOR FUEL DOES NOT MAKE IT UNUSABLE IN NUCLEAR BOMBS

Steven Dolley
Research Director

March 28, 1997

Beware False Claims About "Reactor-Grade" Plutonium!

The nuclear industry and other advocates of using warhead plutonium in "mixed-oxide" (MOX) plutonium-uranium fuel often claim that the MOX option would make plutonium unusable in weapons. Such claims are inaccurate and irresponsible. Using warhead plutonium in MOX fuel neither "burns it up" nor renders it unusable in nuclear weapons. In fact, reactor-grade plutonium is even more desirable than weapon-grade in crude bomb designs that might be used by terrorists because reactor-grade makes initiation of the nuclear chain reaction easier.

The proliferation resistance of the final waste forms largely determines the potential for reversing the disposal process and re-using plutonium for weapons. There are two potential waste forms. One is "spent" MOX fuel that has been irradiated in a nuclear power plant. The other is an immobilized waste form, produced by incorporating weapons plutonium into a glass- or ceramic-based matrix, mixed with intensely radioactive fission products or other contaminants, suitable for geologic disposal.

Reversibility is a function of three factors: the amount of residual plutonium remaining in the final waste forms, how easy it is to retrieve plutonium from the waste, and the quality of the plutonium retrieved from the waste.

Reactors Do Not "Burn Up" Plutonium

The MOX option would leave modestly less plutonium in final waste than the immobilization option. However, it is misleading to speak of MOX "burning" of weapons plutonium as if all or even most of the plutonium is consumed during irradiation. In fact, irradiated weapons-plutonium MOX fuel would contain only about 30 percent less total plutonium per unit fuel than fresh MOX.1

However, even these reductions are not likely to be achieved in practice, because they would require reactors to be loaded entirely with MOX fuel. No light-water reactor anywhere in the world has been operated with a 100 percent MOX core. More realistically, a light-water reactor (LWR) loaded with one-third core of MOX fuel would discharge only about one percent less plutonium than was contained in the MOX fuel originally loaded.2

Warhead Plutonium Can Be Retrieved from Final Waste Forms

The second factor is retrievability of the residual plutonium. Separation of plutonium from fresh MOX fuel is a straightforward chemical process. Reprocessing irradiated MOX fuel by means of PUREX employs proven technology that could recover substantial amounts of plutonium. Chemical separation of plutonium from vitrified waste forms is a similar chemical processes, roughly comparable in difficulty.3

"Reactor-Grade" Plutonium Can Be Used to Make Bombs

The third factor is the isotopic composition of the residual plutonium in the final waste form. Plutonium disposed of in glass remains weapons-grade (about 93 percent Pu-239). Weapons plutonium in irradiated MOX fuel contains a considerably smaller proportion of Pu-239, and a higher proportion of Pu-240, after irradiation than before. Because Pu-240 emits more spontaneous fission neutrons than Pu-239, it is more likely to "predetonate"---that is, begin the chain reaction too early to achieve full explosive yield when the bomb is detonated. Bomb designs utilizing reactor-grade plutonium would also require a somewhat larger critical mass of plutonium, and would need to account for the greater heat generated by Pu-238. These differences between reactor-grade and weapon-grade plutonium are not nearly as important from a non-proliferation perspective as some have argued.

Many MOX proponents emphasize the degree to which the isotopics of the weapons plutonium would be altered by irradiation in a particular reactor technology---that is, the degree to which the Pu-239 proportion can be reduced---as if this factor should be decisive in choosing among disposition technologies. This is an inappropriate criterion by which to assess proliferation risks because it perpetuates a dangerous myth that reactor-grade plutonium cannot be used to make workable weapons.

The ability to construct a weapon from reactor-grade plutonium was demonstrated decades ago. It is dangerous even to consider it an open question. Hans Blix, director-general of the IAEA, informed our Institute that there is "no debate" on this point in the Safeguards Department of the IAEA, and that the agency considers virtually all isotopes of plutonium, including high burn-up reactor-grade plutonium, to be usable in nuclear weapons.4 In June 1994, U.S. Energy Secretary Hazel O'Leary declassified further details of a 1962 test of a nuclear device using reactor-grade plutonium, which successfully produced a nuclear yield.5

In fact, reactor-grade plutonium may be even more desirable than weapon-grade plutonium as a bomb material for terrorist or other sub-national groups. The increased probability of pre-detonation would eliminate the need to include a neutron initiator in the weapon, considerably simplifying the task of designing and producing such a weapon.6 As for nuclear-weapon states, conversion of plutonium to reactor-grade would not pose an impediment to rearmament. Weapons design innovations have largely eliminated the risk that predetonation due to Pu-240 would reduce the yield or reliability of a superpower nuclear bomb.7 In a future breakout scenario, the United States (or Russia) could draw on its historical nuclear test data and predictive capabilities to reconfigure weapons and reconstitute a large arsenal, even from plutonium isotopically degraded to reactor-grade by irradiation in MOX. Also, development of laser isotope separation technology, such as AVLIS, is likely eventually to permit the "mining" of the Pu-239 isotope from reactor-grade plutonium.

It has been alleged that isotopic degradation is desirable because, in a rearmament situation, a nuclear-weapon state would not have sufficient confidence in an untested design using reactor-grade plutonium. This position lacks merit. The U.S. Government plans to spend billions of dollars over the next decade constructing a technical infrastructure for "stockpile stewardship," designed to ensure reliability of the nuclear stockpile in a world without nuclear testing. Such technologies as DAHRTF (the Dual Access Radiographic Hydrodynamic Test Facility) would provide the capability to modify existing nuclear weapons designs to substitute reactor-grade plutonium without the need for testing. In a future rearmament emergency in which the United States or Russia felt compelled to make prompt use of MOX spent fuel as a source of weapons plutonium, even a comprehensive test ban treaty would not prevent testing, even if testing of reactor-grade plutonium weapons were deemed to be required. Article 9 of the CTBT allows a member state to withdraw from the treaty after six months' notice if its "supreme interests" are at stake. However, actual testing probably would not be needed.

Conclusion

For these reasons, neither the United States nor Russia could count on the reactor-grade isotopics of the other nuclear power's dispositioned plutonium stockpile providing a major impediment to remilitarization. In short, the arms-control value of isotopic degradation is negligible. Isotopic degradation does not pose a substantial barrier to re-militarization of warhead plutonium, and therefore does not constitute a compelling argument in favor of the MOX disposition option.

False Claims John C. Ringle, professor of nuclear engineering, Oregon State University: "Weapons-grade plutonium is disabled when converted to MOX and cannot be reused in warheads or pose a security problem." Michael Lawrence, Pajarito Scientific Corporation (a subsidiary of British Nuclear Fuels Ltd.): "[MOX fuel] virtually destroys the weapons plutonium making it impossible to use for future weapons." Joel Cehn, radiation physicist: "Furthermore, the plutonium is totally disabled in the [MOX disposition] process and cannot be reused in warheads." Amarillo National Resource Center for Plutonium: "Although the practical level of isotopic denaturing does not eliminate potential diversion for terrorist nuclear explosive use, it is a significant additional barrier to host nation re-use or rump nation military use."

Accurate Statements J. Carson Mark, former director, Theoretical Division, Los Alamos National Laboratory: "Reactor-grade plutonium with any level of irradiation is a potentially explosive material. The difficulties of developing an effective design of the most straightforward type are not appreciably greater with reactor-grade plutonium than with those that have to be met for the use of weapons-grade plutonium." Hans Blix, Director-General, International Atomic Energy Agency (IAEA): "On the basis of advice provided to it by its Member States and by the Standing Advisory Group on Safeguards Implementation (SAGSI), the Agency considers high burn-up 'reactor grade' plutonium and in general plutonium of any isotopic composition with the exception of plutonium containing more than 80 percent Pu-238 to be capable of use in a nuclear explosive device. There is no debate on this matter in the Agency's Department of Safeguards." T.E. Shea and K. Chitumbo, Department of Safeguards, IAEA: "[E]ven highly burned reactor-grade plutonium can be used for the manufacture of nuclear weapons capable of very substantial explosive yields." Prof. Marvin Miller, Defense and Arms Control Studies Program, MIT: "[W]ith an amount on the order of 10 kilograms, it is now possible for a small group, conceivably even a single 'nuclear unibomber' working alone, to 'reinvent' a simplified version of the Trinity bomb in which the use of reactor-grade rather than weapon-grade plutonium is an advantage." Office of Arms Control and Nonproliferation, Department of Energy: "There is clear scientific evidence behind the assertion that nuclear weapons can be made from weapons-grade and reactor-grade plutonium. Since a central goal of plutonium disposition is to help prevent the re-use of this material in nuclear weapons, isotopic conversion is not a technically valid basis on which to achieve this goal." National Academy of Sciences: "The plutonium in the spent fuel assembly would be of lower isotopic quality for weapons purposes than the still weapons-grade plutonium in the glass log, but since nuclear weapons could be made even with the spent fuel plutonium this difference is not decisive." [emphasis supplied]

End Notes

1. Panel on Reactor-Related Options for the Disposition of Excess Weapons Plutonium, Committee on International Security and Arms Control, National Academy of Sciences, Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options, 1995 [NAS 1995], Table 6-5, p. 270, indicates that a fresh weapons-plutonium MOX fuel element would contain 25 kilograms of plutonium. The same element, after irradiation to a burn-up of 40 megawatt-days per kilogram heavy metal, would contain 18 kilograms of plutonium. Back to document

2. Westinghouse Electric Corporation, Plutonium Disposition in Existing Pressurized Water Reactors, DOE/SF/19683--6, June 1, 1994, p. 2.6-3. Back to document

3. NAS 1995, p. 243. Back to document

4. Letter from Hans Blix, Director-General of the IAEA, to Paul Leventhal, NCI, November 1, 1990; "Blix Says IAEA Does Not Dispute Utility of Reactor-Grade Pu for Weapons," NuclearFuel, November 12, 1990, p. 8. However, Blix made this statement only after the Nuclear Control Institute challenged assertions by IAEA officials earlier that year that reactor-grade plutonium was unsuitable for use in weapons. Back to document

5. U.S. Department of Energy, Office of the Press Secretary, "Additional Information Concerning Nuclear Weapon Test of Reactor-Grade Plutonium," DOE Fact Sheet released as part of the Openness Initiative, June 27, 1994. The fact that the test occurred and produced a nuclear yield was declassified in 1977. Robert Gillette, "Impure Plutonium Used in '62 A-Test," Los Angeles Times, September 16, 1977, part 1, p. 3. Back to document

6. Marvin Miller, professor of nuclear engineering, MIT, private communication, November 24, 1996. Back to document

7. Miller, ibid. Back to document

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