The Physics and Policy of Plutonium Disposition
Alex DeVolpi
Recent studies by the Office of Technology Assessment (1), the National Academy of Sciences (2), and other organizations encourage temporary measures to condition nuclear materials so they are less susceptible to diversion. Adoption of the NAS recommendation for interim--but indefinite--storage of pits and unadulterated plutonium would, I think, postpone irreversible arms reduction. Though surplus plutonium would be kept in secure storage, it would remain in forms that could be reused in weapons.
A shibboleth of some current policy analysis is that all plutonium is "weapons usable." This is a deceptive oversimplification that could result in delaying effective steps to defuse the caliber of weapons-grade plutonium. Moreover, it could provide a rationale to stall further nuclear arms reductions.
Some serious policy implications hinge on the semantics of weapons-usable plutonium. Some of these are outlined below, leading to a recommendation that the American Physical Society (APS) undertake to clarify certain technical issues and standards.
The arms control agenda
Prevailing problems for international arms control and nonproliferation, in order of urgency, are 1) management of existing weapons and fissile-material stocks, 2) deactivation of missiles and weapons, 3) termination of testing and production of nuclear weapons, 4) dismantlement and verification of warheads, 5) safeguarding of fissile materials, 6) demilitarization of uranium and plutonium, and 7) elimination of weapons-usable nuclear materials.
In terms of constructing an international context for these problems, the NAS report provides a thorough review and identification of significant policy choices. However, some conclusions of the Academy's committee are influenced by issues extraneous to the problems of arms control. In particular, the hypothetical threat of diversion is magnified by the same technique used to sustain the Cold War, namely worst-case analysis. In William Arkin's words (3), "proliferation is methadone for Cold Warriors who can no longer get the real thing."
As Thomas Cochran, Nuclear Program Coordinator for the Natural Resources Defense Council, continues to emphasize (4), the "main problem" is the diversion risk of separated fissile materials in Russia, where unstable conditions exist now. Other problems there include military reactors now needed to produce heat and electricity.
Of the 125,000 nuclear weapons manufactured globally, about 50,000 remain in stockpiles. Over 250 tonnes of weapons-grade plutonium and over 2100 tonnes of highly enriched uranium were produced for these weapons. Civilian plutonium stockpiles are even larger, being created at the rate of 1 kg from every tonne of mined uranium.
Is the task of controlling plutonium so intractable that the only solutions are to curtail all reprocessing and shut down all nuclear reactors?
The physics of isotopic depletion
Technical analysis (5) shows that both weapons uranium and plutonium can be degraded by isotopic depletion, that is, replacing fissile isotopes with fertile isotopes. A natural dilutent exists for uranium, and artificial dilutents can be manufactured for plutonium.
The NAS report has a disappointingly imprecise description of the differences between reactor and weapons plutonium. Moreover, their discussion of isotopics does not go beyond reactor-grade plutonium. Higher burnup can further degrade plutonium, even without recycle. Eleven physical effects deleterious to explosive potential occur when the even-isotope fraction of plutonium is increased. The average explosive yield decreases and the statistical uncertainty in yield worsens.
The means have been demonstrated worldwide to demilitarize plutonium by increasing the fraction of its even-isotopes. Advanced reactors are not necessary for this purpose, as operating reactors can demilitarize plutonium sufficiently to exclude its return to existing nuclear warheads. Durable nuclear weapons have been designed, constructed, and tested to satisfy military objectives. If refitted with sub-grade plutonium, precisely machined military weapons cannot be effective, fusion boosting is likely to be less useful, and secondary fusion of multistage thermonuclear weapons would not be properly triggered.
Of fifteen nations known to have produced nuclear weapons or to have embarked on their development, none have chosen anything less than high-quality fissile materials.
Therefore, both fundamental physics and historical experience reinforce the military inadequacy of poor grades of fissile materials. Of course, safeguards need to be maintained and enhanced to deter diversion and production of all fissile materials, civilian and military.
Reference (5) is a 100-page technical review and analysis of plutonium disposition options, containing specific technical data and estimates of explosive yields.
Demilitarization benefits
Plutonium demilitarization will not only buy time, it might avert policy disappointments. Although military and civilian plutonium should not have fundamentally different material controls and accountability (6), they could be treated in two distinct time phases to conform with national security, economic, and technical constraints. Developing a practical methodology for keeping plutonium from being reused in existing warheads should be a major priority.
One example of a looming policy disappointment is the fissile-material production cutoff. Although agreements have been reached for shutting down three Russian plutonium production reactors, no deadline has been negotiated. Thus, indefinite reactor operation with current fuel and burnup would have the effect of prolonging weapons-grade output and processing. Instead, Russian military reactors could be upgraded in safety and quickly modified to yield low-grade plutonium. Russian resistance to immediate reactor shutdown is primarily based on the need for electricity and heat--lacking deliverable interim alternatives.
Another example related to demilitarization is the "spent-fuel standard" proposed by the NAS: The standard might be a more formidable barrier than credited. Mixing U.S. weapons-grade plutonium with existing spent fuel is likely to give sufficient chemical, radioactive, and isotopic contamination to render it too difficult for reuse in weapons. In fact, if weapons and separated reactor plutonium were blended in equal proportions, the resulting "fuel-grade" mixture might be sufficiently demilitarized without additional reactor burnup.
A consequence of a perceived inability to demilitarize plutonium is the unintended strengthening of the argument that further arms reductions should be delayed. If all plutonium were truly "weapons usable", there would be little incentive to dismantle and demilitarize nuclear weapons.
Elimination of plutonium
Another policy dilemma is emerging over the ultimate disposition of plutonium. If the recommendations in the reports cited above were adopted, a stronger rationale might be created for postponing deep cuts in nuclear arsenals.
The Presidential Nonproliferation and Export Policy Statement of 27 September 1993 establishes an interagency group to review long-term options for plutonium disposition, inviting other nations to participate in the study. The Department of Energy, under special assistant Robert DeGrasse, is organizing its own initiative for "safe, secure, environmentally sound control, storage, and ultimate disposition of surplus fissile materials".
The most realistic options for long-term disposition of U.S. plutonium are storage and fission. For geologic storage, plutonium would be sorted underground as vitrified waste in corrosion-resistant containers. For destruction by fission, either nuclear reactors or accelerators could be used.
Research policy is likely to be more effective if expressed in terms of goals rather than singling out specific means. Some examples regarding vitrification and fission illustrate the stakes involved in prematurely focusing on specific choices.
Vitrification has serious flaws
The most touted form of storage--mixing heavy doses of weapons-grade plutonium into radiologically contaminated vitrified waste--has two afflictions from which it might never be free: the need for perpetual safeguards and the risk of nuclear criticality.
Vitrified weapons plutonium would remain forever recoverable. Chemically re-separating plutonium after vitrification is considered less difficult than deriving cocaine from cocoa leaves (7). The main benefit of radioactive and chemical contamination--without isotopic depletion--is the creation of a rate-limited barrier to quick reconstitution.
The other serious problem with vitrification of plutonium is susceptibility to an uncontrolled nuclear chain reaction. Proposed mixtures of plutonium in borated glass have ranged from 0.1 wt% to more than 4 wt% plutonium in a 1.5 tonne glass log. Because the smaller fraction is considered expensive, consideration has focused on incorporating at least 3 wt%. This amounts to about 45 kg of pure plutonium in a single log; yet much less than 1 kg could become critical if moderated by water. Hampered by the fact that intruding water can readily leach boron differentially, one would have to prove that each log would remain subcritical under foreseeable conditions (7). Both criticality physics and derivative regulatory requirements would stand as serious obstacles to licensing.
These flaws could be fatal for vitrified plutonium, because freedom from these technical concerns might never by provable (8).
Responsibility should not be postponed
Another problem with geologic storage is an extension of the NIMBY (not in my backyard) syndrome, namely NIMT: not in my time: Put it off for future generations. Instead, the national goal should be to eliminate plutonium. Our Cold War generation created the dilemma, and we should not transfer the cost and problems to future generations.
Although recognizing fission as an option for plutonium elimination, the National Academy report proposes that the U.S. limit itself to undefined "conceptual" research for advanced options--foregoing modest development and demonstration of existing, promising ideas. The Academy places the hypothetical risk of reactor-grade material diversion above the contemporary dangers of weapons-grade plutonium.
Because the NAS concludes, "consumption fractions...between 50 and 80 percent...are not sufficient to greatly alter the security risks posed by the [plutonium] remaining in the spent fuel," they advise that "...technologies designed to fission or transmute nearly 100 percent of the plutonium are the only plausible elimination approaches." However, if demilitarization of plutonium at lesser grades were accepted, such extreme burnup measures would be unnecessary.
APS role
As indicated in the foregoing examples, functional definitions can affect policy choices. Toward both interim and ultimate goals, standards need to be formulated for demilitarizing and denaturing weapons-grade plutonium. For this purpose, an independent scientific organization such as the APS could fulfill an important role.
Policy decisions on fissile material disposal options should be based on assessment that have objective criteria. The government will have to establish explicit priorities for environmental impact, cost avoidance, energy recovery, economic subsidies, public risk, nonproliferation concerns, and rearmament peril. A cost-benefit-risk public-policy equation ought to be adopted and its implications understood.
The utility of fissionable materials in the manufacture of nuclear weapons is subject to confusion and obscuration, some rooted in semantics and policy disputes. This is in contrast to the reality that reactor-grade plutonium is yet to be chosen for militarization by any of the acknowledged or suspected nuclear-weapons states.
The semantic issue is centered in part on definitions of nuclear weapons. Those who fear the malevolent use of low-grade nuclear materials stress that a devastating fission explosive can be made with any isotopes of plutonium. This qualitative contention is quite misleading. Policy-makers need to have a more sophisticated understanding of the relative risks and tradeoffs for fissile materials, especially reactor-grade plutonium. Based on the fundamental physics of nuclear reactions, standards for reconstitution of plutonium in existing weapons could be devised.
During its comprehensive treatment of the nuclear fuel cycle in 1978 (9), the APS examined issues related to isotopic denaturing, drawing some qualitative conclusions. Picking up at that point, a limited study could help in developing standards, allowing for information that has been more recently declassified and discovered about domestic and foreign programs.
Until eliminated, international plutonium will need to be cooperatively managed. Meanwhile, research, development, and demonstration could be conducted to benefit sound policy choices. The present administration should not defer action to another generation. Qualified scientists could help in establishing goals and standards for timely and cost-effective plutonium disposition.
- Office of Technology Assessment, "Dismantling the Bomb and Managing Nuclear Materials", U.S. Congress (1993).
- National Academy of Sciences, "Management and Disposition of Excess Weapons Plutonium," National Academy Press, Washington, D.C. (1994); a briefing on this study appears immediately below in this issue of Physics and Society.
- W.M. Arkin, The Bulletin of the Atomic Scientists, March/April 1994, p. 64.
- T. Cochran in International Policy Forum "The Disposition of Weapons Grade Plutonium & HEU," Leesburg, Va. (8-11 March 1994).
- A. DeVolpi, "Whither Plutonium? Demilitarization and Disposal of Fissile Materials," Argonne National Laboratory report ANL/ACTV-93/3 (March 1994).
- P. Leventhal, International Policy Forum (op cit.).
- J.C. Martz and J.M. Haschke, "Technical Issues in Plutonium Storage," presented at DOE Plutonium ES&H Vulnerability Assessment, Working Group Meeting, Gaithersburg, Md. (Mar. 28, 1994).
- N. Oreskes, K. Shrader-Frechette, and K. Belitz, "Verification, Validation, and Confirmation of Numerical Models in the Earth Sciences," Science, Vol. 263, pp. 641-6 (1994).9. APS, Rev. Mod. Phys., Vol. 50, No. 1, Part II (1978).
The author is at 7778 Woodward, Woodridge, IL 60517.