Demilitarizing Weapon-grade Plutonium: Part I

Alexander DeVolpi

Reducing Risk of Nuclear Proliferation

Ongoing multinational demilitarization of stockpiled nuclear weapons has been reducing proliferation risk and offsetting those government expenditures that result from conversion (demilitarization) of uranium and plutonium to non-weapon-grade.

All together, ten nations (US, USSR, G.B., France, China, India, Pakistan, Israel, South Africa, and North Korea) have militarized nuclear materials. Several other nations once made technical inquiries into nuclear weaponization. There are now 9 nuclear-weapons states and 185 non-nuclear-weapon states.

As of mid-January 2015, 439 nuclear power plant units with an installed net electric capacity of about 377 GW are in operation in 31 countries. Under construction are 69 nuclear plants with an installed capacity of 66 GW in 16 countries. In addition there are numerous maritime reactors, as well as education, testing, and development reactors — perhaps 1000 in operation or standby, and others under construction or planned.

The Value of Demilitarized Plutonium
Civilian nuclear-power reactors are burning up weapons plutonium (and uranium), helping reduce federal budget expenses. In addition, the fissile demilitarization sequence provides enormous public value in terms of nuclear arms reduction and nonproliferation. Many billions of taxpayer dollars can be recovered from commercial sales of fissionable materials no longer needed for weapons.

This forum for Physics and Society has witnessed a half-century of ongoing discourse about issues dealing with the theoretical potential for making nuclear explosives out of “weapon-grade” plutonium.1

Strongly inclined against weaponization of reactor-grade plutonium are a number of factors, such as fundamental nuclear physics and engineering calculations, supported by experimental data and public information. It is well understood that low-enrichment grades of uranium also do not constitute the fissile core of nuclear weapons because they would become heavy, unwieldy devices.

Nine countries altogether now possess about 16,000 nuclear weapons, reduced significantly since the peak of the Cold War. Both the US and Russia each still maintain roughly 1,800 of their nuclear weapons on high-alert status — ready to be launched for long-range attack within minutes of a warning. Most of the weapons are many times more powerful than the devastating atomic bombs dropped on Japan in 1945.

The failure of Cold-War powers to further reduce their devastating arsenals had evidently incentivized a couple of other countries to acquire nuclear weapons. However, the rate of proliferation has indeed flattened out since North Korea independently developed nuclear-weapon technology.

Weaponizability of Reactor-Grade Plutonium
A long-exercised dispute about “weaponizability” of reactor-grade plutonium revolves about semantic differences (see Part II, to be published in the next issue of Physics and Society). My own periodic reassessments since the late 1970s indicate that nuclear-source material qualifies only for a nuclear weapon if it can be manufactured, field-tested, and produced for the nation’s military arsenal. Nuclear materials of inferior quality are effectively demilitarized: They have not been usable in military-quality weapons.

The Cold-Warrior nations, by mutual agreement, have found it in their interest to reduce nuclear-weapon arsenals and to demilitarize stockpiles of fissile materials. Nuclear warheads in arsenals have turned out to be devices subject to precise and carefully managed explosive qualification for predetermined yield and reliability. Weapon-grade plutonium is typically about 93% Pu-239. Lacking confirmation is the allegation that lower-quality “reactor-grade plutonium” could be weaponized. That any military arsenal that would contain “poor” quality fissile material is far-fetched: Anything less than premium fissile material is unlikely to be utilized.

Regarding weaponization of “reactor-grade plutonium,” the US DOE (AEC/ERDA) has equivocated for nearly a half-century.2 The explosive yield of a 1962 test in Nevada still remains secret; however, a great number of other test yields and details have been declassified. One must now consider the 1962 explosive to have been very low and disappointing from the viewpoint of weaponization.

In any event, all nuclear-weapon states are universally understood to have gone to the trouble and expense of producing weapon-quality fissile components in their arsenals. As for other scenarios using low-grade plutonium, some speculation includes radiation-dispersal devices with limited burst range. Remaining essential are national and internationalized nuclear treaties, safeguards, inspections, and surveillance on all radiative materials.

Demilitarizing Uranium and Plutonium
A 15-year Megatons-to-Megawatts US-USSR program for demilitarizing 500 tonnes of weapon-grade uranium (HEU) has proven to be an on-schedule winner — in mutual arms control, economics, and non-proliferation. About 20,000 Soviet nuclear warheads have been effectively converted to civilian reactor use, fueling half of our nuclear-power plants, which produce as much as 10% of US electricity. The program has readily paid for its federal budget outlays, and it has been reducing national and international nuclear risk (a swords-into-plowshares paradigm).

Under a subsequent bilateral agreement, the US and Russia were to dispose of surplus weapon plutonium from approximately 17,000 nuclear warheads. When operational, a mixed-oxide (MOX) conversion facility will be capable of turning 3.5 metric tons/yr of weapon plutonium and uranium into civilian-use fuel assemblies.

MOX is a blend of plutonium and natural uranium or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX is an alternative to low enriched uranium (LEU) fuel used in the light water reactors that predominate in international nuclear-power generation. MOX also provides a means of using excess weapon-grade plutonium (from military sources) to produce electricity.

In the wholesale market, 34 tonnes of converted weapon plutonium might eventually fetch as much as $3B, equivalent to $30B in taxable retail sales in the United States. Even if costs did inflate for the 60+%-completed South Carolina MOX facility, there are multiple ancillary national and international benefits of plutonium conversion (to reactor fuel).

As long as the US carries out its previously negotiated treaty reduction, the RF is now on track to convert a matching amount of weapon plutonium into peacetime nuclear energy. That would correspond to irreversible reduction of ~10,000 US nuclear weapons. The plutonium is to be permanently diluted, irradiated, and rendered unusable for future weapons use. Relevant experience has been gained in Europe for burnup of both reactor and weapon plutonium.

Demilitarization, denaturing, and degradation are terms applicable to fissile materials rendered unsuitable for use in military-quality weapons. For practical purposes, extensively irradiated reactor-grade uranium and plutonium are effectively demilitarized, despite Jimmy Carter’s unwarranted 1977 ban on US reprocessing. The unilateral decision had essentially no influence overseas.

Demilitarization Through MOX Burnup
The US has three times more weapon-grade plutonium in its national nuclear-weapon inventory than now needed, as well as much weapon-grade uranium. To move toward irreversible nuclear disengagement, both the US and Russia could continue their leadership, irreversibly demilitarizing weapon-grade fissile inventories.

Demilitarization of weapon plutonium gains other substantial benefits, reducing risk of international proliferation and nuclear terrorism. Burnup adds physical, chemical, radiological, and isotopic barriers that reduce accessibility and utility. Once converted to MOX, reactor fuel is no longer usable in nuclear weapons, a technical proposition never contradicted by specific nuclear-test data.

No other proposed demilitarization technology has proven to be as effective and beneficial as conversion to and burnup of MOX. Earlier suggestions and inquiries into vitrification and/or burial did not withstand critical analysis.

US (and some overseas) reactors routinely consume weapons uranium that has been downblended to reactor grade, as well as some weapon plutonium converted to MOX. The industrial technology for demilitarizing weapon-grade materials is well established. For degrading fissile materials removed from nuclear weapons, commercial reactor burnup provides a means that is technologically realistic, proliferation secure, economically viable, and militarily irreversible. Using a rough estimate of 2000 tonnes HEU and 260 tonnes plutonium remaining in the world, demilitarization and utilization might net tens of billions of dollars on the open market.

Megatons to Megawatt Conversion
The US-RF MOX conversion program would help both nations comply with international obligations toward worldwide reciprocal nuclear disarmament under Article VI of the Nuclear Non-Proliferation Treaty. Such a move would present a commendable example for other weapon states — the UK, China, and France.

Russia, Britain, and France have commercialized their excess MOX, so they don’t necessarily have to draw upon substantial government subsidies. Demilitarization is a rather straightforward technology that has little technical risk, although environmental restrictions might indeed artificially elevate its cost. An early-generation reprocessing plant was built for extracting plutonium from spent fuel in the UK's atomic weapons program, partly under a US-UK Mutual Defence Agreement. It operated from 1951 until 1964, with an annual capacity of 750 tonnes of low-burn-up fuel. From 1971 to 2001 more than 35,000 tonnes were reprocessed.

The “Spent Fuel Standard” — Plutonium Accessibility
Four primary factors affect the theoretical usefulness of civilian spent fuel as a potential weapon material: (1) intense radioactivity of fission products (which decay with time); (2) the need for chemical separation of plutonium from fuel (which must be done by remotely operated equipment); (3) the isotopic composition of the plutonium (reactor-grade being less desirable material than weapon-grade); and (4) the difficulty of acquiring plutonium if the party in question does not already have spent fuel in its possession. Overcoming these factors depends on resources of the state or group trying to do so.

A goal of US plutonium disposition is to achieve the “Spent Fuel Standard” — that is, to make excess weapons plutonium as inaccessible and unattractive as is the much larger quantity of plutonium in commercial spent fuel. This raises two obvious questions: How difficult is it to recover plutonium from commercial spent fuel and use it in weapons, and how do the various proposed forms for plutonium disposition compare in this respect?

The difficulty of recovering plutonium from spent fuel or other disposition depends on resources of the state or group seeking to recover it. A weapon state with large reprocessing plants available, for example, could use those facilities to recover plutonium from spent fuel with relatively little difficulty. They could theoretically fabricate weapons from that plutonium: Time, cost, visibility, and effectiveness would be the principal complications. At the other end of the capability spectrum, a subnational group would need to accomplish several tasks: steal spent fuel without being caught, build a facility for chemically separating plutonium from the spent fuel without being detected, carry out hazardous processing without being stymied by unexpected difficulties, and then produce a nuclear weapon without prior experience.

Official DOE releases2 have acknowledged increased complexity and cost in “designing, fabricating, and handling” nuclear-weapons made with reactor-grade plutonium. DOE statements also affirm that nations must be “willing to make large investments ... to acquire weapon-grade rather than reactor-grade plutonium.” However, lower-grade plutonium has long been available to many nations in large quantities for essentially no additional production cost. The DOE statement thus implicitly acknowledges that being “willing” has less to do than “ineffectiveness” of such hypothetical nuclear-explosive devices.

Although specific explosive yield data has been published for a great many nuclear-explosive tests, no such information has been provided on the 1962 test yield, despite the informative value it would have. Disregarding many calls for release of this clarifying data, its absence unavoidably hints that the test yield from the so-called reactor-grade explosion might have been disappointingly low.

In the absence of any nuclear-weapon state publicly declaring that it has made warheads with anything less than weapon-grade plutonium (and/or uranium), it must be concluded that reactor-grade plutonium is still not a viable choice for military-quality nuclear weapons.

Although reactor-grade plutonium requires safeguards and protection to the same standards as weapon-grade plutonium, disposing of excess plutonium through reactor burnup meets the “Spent Fuel Standard,” sufficient for non-proliferation goals. Burning surplus plutonium as MOX fuel in nuclear reactors is technically secure and economically advantageous.

Alexander DeVolpi, retired reactor physicist from Argonne National Laboratory, Fellow APS


1 E.g.: (a) Physics Today, January 2015, page 9, referring to an article published July 2014, page 24; Physics Today, November 2010; (b) oftentimes in the pages of Arms Control Today; (c) in the peer-reviewed pages of Ann. Rev. Nucl. Part. Sci., 36 83 (1986); (d) also, "Denaturing Fissile Materials," invited review paper, Progr. in Nucl. Energy 10, 161-219 (1982); and (e) in my 3-volume book Nuclear Insights: the Cold War Legacy, (2009).

2 “DOE Facts Additional Information Concerning Underground Nuclear Weapon Test of Reactor-grade Plutonium,” US DOE, Office of Public Affairs, Contact: Sam Grizzle. (Also, “Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage and Excess Plutonium Disposition Alternatives,” U.S. Department of Energy report DOE NN-0007 (Jan. 1977).

These contributions have not been peer-refereed. They represent solely the view(s) of the author(s) and not necessarily the view of APS.