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Can 'reactor grade' plutonium be used in nuclear weapons?

Nuclear Monitor Issue: 

Author: Jim Green − Nuclear Monitor editor

Nuclear apologists often argue that the 'reactor-grade' plutonium (RGPu) routinely produced in power reactors cannot be used in nuclear weapons. Thus the purported links between nuclear power and weapons have no basis in truth, they argue.

The premise is false − RGPu can be used in weapons. Moreover, the links between nuclear power (and civil nuclear programs more generally) and weapons proliferation go well beyond the use of RGPu in weapons. Ostensibly civil nuclear materials and facilities can be used in support of weapons programs in many ways:

* Production of plutonium in power or research reactors followed by separation of plutonium from irradiated material in reprocessing facilities (or smaller facilities, sometimes called hot cells).

* Production of radionuclides other than plutonium for use in weapons, e.g. tritium, which is used to initiate or boost nuclear weapons.

* Diversion of fresh highly enriched uranium (HEU) research reactor fuel or extraction of HEU from spent fuel.

* Nuclear weapons-related research.

* Development of expertise for parallel or later use in a weapons program.

* Nuclear power programs justifying the acquisition of other facilities capable of being used in support of a nuclear weapons program, such as enrichment or reprocessing facilities.

A nuclear power reactor (1000 MWe LWR) typically produces 250−300 kilograms of plutonium each year, sufficient for 25−30 weapons. Total global production of plutonium in power reactors is about 70 tonnes per year. Over 2,000 tonnes of plutonium have been produced in power reactors around the world, hence the importance of the debate over the use of RGPu in weapons.

The problem is exacerbated by the separation and stockpiling of plutonium produced in power reactors, such that it can be used directly in weapons. Stockpiles of separated civil plutonium amount to around 270 tonnes and are continuing to grow − that is arguably the most dangerous and asinine of all the dangerous and asinine practices of the nuclear power industry.

Plutonium grades

For weapons manufacture, plutonium ideally contains a very high proportion of plutonium-239. As neutron irradiation of uranium-238 proceeds, the greater the quantity of isotopes such as plutonium-240, plutonium-241, plutonium-242 and americium-241, and the greater the quantity of plutonium-238 formed (indirectly) from uranium-235. These unwanted isotopes make it more difficult and dangerous to produce nuclear weapons.

Definitions of plutonium usually refer to the level of the unwanted plutonium-240 isotope:

* Weapon grade plutonium contains less than 7% plutonium-240.

* Fuel grade plutonium contains 7-18% plutonium-240

* RGPu contains over 18% plutonium-240.

Although somewhat imprecise, it is also useful to distinguish low burn-up plutonium (high in plutonium-239, including weapon grade plutonium and some or all fuel grade plutonium) from high burn-up plutonium (including RGPu and possibly some fuel grade plutonium).

According to Australia's Uranium Information Centre (2002), plutonium in spent fuel removed from a commercial power reactor (burn-up of 42 GWd/t) consists of about 55% Pu-239, 23% Pu-240, 12% Pu-241 and lesser quantities of the other isotopes, including 2% of Pu-238 which is the main source of heat and radioactivity.

The scientific consensus regarding RGPu

With the exception of a few contrarians (mostly from within the nuclear industry or funded by it), there is general agreement that RGPu can be used to produce weapons, though the process is more difficult and dangerous than the use of weapon grade plutonium (see Gorwitz, 1998 for discussion and references).

A report from the US Department of Energy (1997) puts the following view:
"Virtually any combination of plutonium isotopes − the different forms of an element having different numbers of neutrons in their nuclei − can be used to make a nuclear weapon. ...
The only isotopic mix of plutonium which cannot realistically be used for nuclear weapons is nearly pure plutonium-238, which generates so much heat that the weapon would not be stable. ...
At the lowest level of sophistication, a potential proliferating state or subnational group using designs and technologies no more sophisticated than those used in first-generation nuclear weapons could build a nuclear weapon from reactor-grade plutonium that would have an assured, reliable yield of one or a few kilotons (and a probable yield significantly higher than that). ...
Proliferating states using designs of intermediate sophistication could produce weapons with assured yields substantially higher than the kiloton-range possible with a simple, first-generation nuclear device. ...
The disadvantage of reactor-grade plutonium is not so much in the effectiveness of the nuclear weapons that can be made from it as in the increased complexity in designing, fabricating, and handling them. The possibility that either a state or a sub-national group would choose to use reactor-grade plutonium, should sufficient stocks of weapon-grade plutonium not be readily available, cannot be discounted. In short, reactor-grade plutonium is weapons-usable, whether by unsophisticated proliferators or by advanced nuclear weapon states."

According to Hans Blix, then IAEA Director General: "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 the matter in the Agency's Department of Safeguards." (Blix, 1990; see also Anon., 1990).

The IAEA Department of Safeguards has stated that "even highly burned reactor-grade plutonium can be used for the manufacture of nuclear weapons capable of very substantial explosive yields." (Shea and Chitumbo, 1993.)

With the exception of plutonium comprising 80% or more of the isotope plutonium-238, all plutonium is defined by the IAEA as a "direct use" material, that is, "nuclear material that can be used for the manufacture of nuclear explosives components without transmutation or further enrichment", and is subject to equal levels of safeguards.

An expert committee drawn from the major US nuclear laboratories concluded a report by noting: "Although weapons-grade plutonium is preferable for the development and fabrication of nuclear weapons and nuclear explosive devices, reactor grade plutonium can be used." (Hinton et al., 1996.)

According to Robert Seldon (1976) of the Lawrence Livermore Laboratory: "All plutonium can be used directly in nuclear explosives. The concept of ... plutonium which is not suitable for explosives is fallacious. A high content of the plutonium 240 isotope (reactor-grade plutonium) is a complication, but not a preventative."

According to J. Carson Mark (1993), former director of the Theoretical Division at 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."

According to Matthew Bunn (1997), chair of the US National Academy of Sciences' analysis of options for the disposal of plutonium from nuclear weapons: "For an unsophisticated proliferator, making a crude bomb with a reliable, assured yield of a kiloton or more − and hence a destructive radius about one-third to one-half that of the Hiroshima bomb − from reactor-grade plutonium would require no more sophistication than making a bomb from weapon-grade plutonium. ... Indeed, one Russian weapon-designer who has focused on this issue in detail criticized the information declassified by the US Department of Energy for failing to point out that in some respects if would actually be easier for an unsophisticated proliferator to make a bomb from reactor-grade plutonium (as no neutron generator would be required)."

According to Prof. Marvin Miller, from the MIT Defense and Arms Control Studies Program: "[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." (Quoted in Dolley, 1997.)

According to the Office of Arms Control and Nonproliferation, US Department of Energy: "There is clear scientific evidence behind the assertion that nuclear weapons can be made from weapons-grade and reactor-grade plutonium." (Quoted in Dolley, 1997.)

According to Steve Fetter (1999) from Stanford University's Centre for International Security and Cooperation: "All nuclear fuel cycles involve fuels that contain weapon-usable materials that can be obtained through a relatively straightforward chemical separation process. ... In fact, any group that could make a nuclear explosive with weapon-grade plutonium would be able to make an effective device with reactor-grade plutonium."

Nuclear tests using below weapon grade plutonium

The US government has acknowledged that a successful test using 'reactor grade' plutonium was carried out at the Nevada Test Site in 1962 (US Department of Energy, 1994). The information was declassified in July 1977. The yield of the blast was less than 20 kilotons.

The US Department of Energy (1994) states: "The test confirmed that reactor-grade plutonium could be used to make a nuclear explosive. ... The United States maintains an extensive nuclear test data base and predictive capabilities. This information, combined with the results of this low yield test, reveals that weapons can be constructed with reactor-grade plutonium."

The US Department of Energy (1994) makes the connection to debates over reprocessing, stating that: "The release of additional information was deemed important to enhance public awareness of nuclear proliferation issues associated with reactor-grade plutonium that can be separated during reprocessing of spent commercial reactor fuel."

The exact isotopic composition of the plutonium used in the 1962 test remains classified. It has been suggested (e.g. by Carlson et al., 1997) that because of changing classification systems, the plutonium used in the 1962 test may have been fuel grade plutonium using current classifications. De Volpi (1996) is sceptical that the plutonium used in 1962 the test would be classed as reactor grade using current classifications, but states that it was below weapon grade, i.e. he believes it was fuel grade plutonium.

India Today reported that one or more of the 1998 tests in India used RGPu (Anon., 1998) and the UK and North Korea may have tested bombs using RGPu or fuel grade plutonium (Jackson, 2009).

Limitations of RGPu

The difficulties associated with the use of RGPu in weapons are as follows.

If the starting point is spent reactor fuel, the hazards of managing that spent fuel must be addressed and there must be the capacity to separate plutonium from spent fuel. Spent fuel from power reactors running on a normal operating cycle will be considerably more radioactive and much hotter than low burn-up spent fuel. Thus the high burn-up spent fuel (and the separated RGPu) are more hazardous − though it is not difficult to envisage scenarios whereby proliferators place little emphasis on worker safety. It may also be more time-consuming and expensive to separate plutonium from high burn-up spent fuel than from low burn-up spent fuel.

Weapons with RGPu are likely to be inferior in relation to reliability and yield when compared to weapon grade plutonium. Emission of fission neutrons from plutonium-240 may begin the chain reaction too early to achieve full explosive yield. However, devastating nuclear weapons could still be produced. Radiation and heat levels could diminish reliability through their effects on weapons components such as high explosives and electronics.

Nuclear researcher, regulator and adviser Ian Jackson (2009) states: "As well as poor efficiency, reactor grade plutonium does have some practical drawbacks from heating effects which can damage weapon components. Reactor-grade plutonium contains plutonium-238, which is self-heating. (In fact, plutonium-238 heat sources are used to power satellites and deep-space probes.) An 8 kilogram plutonium weapon core would generate about the same heat output as a 100W light bulb. This, of course, would be in close contact with the HE lenses surrounding the core, and might melt or distort their shape. Self-heating might also cause metallurgical phase changes in the granular structure of the plutonium core, damaging its perfectly spherical geometry. Because of these heating difficulties, once assembled, a reactor-grade plutonium bomb would need to be continuously cooled. The design of the weapon might also need to be modified to incorporate heat shunts that would help mitigate self-heating problems caused by plutonium-238."

According to Leventhal and Dolley (1999), the high rate of neutron generation from plutonium-240 can be turned to advantage as it "eliminates the need to include a neutron initiator in the weapon, considerably simplifying the task of designing and producing such a weapon".

Weapon grade plutonium and fuel grade plutonium from power reactors

In addition to the potential to use plutonium produced in a normal power reactor operating cycle in weapons, there is the option of using power or research reactors to irradiate uranium for a much shorter period of time to produce plutonium ideally suited to weapons manufacture − weapon grade plutonium. It is sometimes argued that short irradiation times would adversely effect the commercial operation of a power reactor, but that would probably be of minimal concern to a would-be proliferator.

Gilinsky, Miller and Hubbard (2004) note that the debate over the potential use of RGPu in weapons diverts attention from the potential to use power reactors to produce large quantities of weapon grade and near-weapon grade plutonium from partially irradiated spent fuel. They write: "For example, if the operator of a newly operating LWR unloaded the entire core after 8 months or so the contained plutonium would be weapons-grade with a plutonium-239 content of about 90 percent. The amount of plutonium produced would be about 2 kilograms per ton of uranium, or about 150 kilograms per 8 month cycle. This comes to about 30 bombs' worth. Does a would-be nuclear weapon state need more? If the short refueling cycles were continued the annual output of weapons-grade plutonium would be about 200 kilograms (allowing for refueling time), but this would require a large amount of fresh fuel."

Mian and Ramana (2006) state that a typical 220-megawatt pressurized heavy-water reactor could produce 150−200 kilograms per year of weapon grade plutonium when operated at 60-80%.

During a normal reactor operating cycle (in which fuel typically remains in the reactor for 3-4 years), a large majority of the plutonium formed is RGPu. However, the grade of the plutonium varies depending on the position of the particular fuel elements in the reactor. Carlson et al. (1997) note that: "Even though fuel assemblies are moved around during refuelling, some parts of fuel rods will have a plutonium isotope composition closer to that of [weapon grade plutonium]."

Weapon grade plutonium can be inadvertently produced in power reactors. Carlson et al. (1997) cite the example of leaking fuel rods in a reactor in the US in the 1970s, leading the utility to discharge the entire initial reactor core containing a few hundred kilograms of plutonium with 89-95% Pu-239.

Fuel grade plutonium is produced in some nuclear reactors. It is often produced in tritium production reactors, and can also be produced in power reactors in initial core loads and in damaged fuel discharged from the reactor earlier than normal (Carlson et al., 1997).

Carlson et al. (1997) note the normal operation of on-load refuelling reactors (e.g. certain gas-graphite and heavy water reactors) can result in some low burn-up plutonium.

The development of fast breeder technology has the potential to result in large-scale production of weapon grade plutonium (Carlson et al., 1997).

Carlson et al. (1997) note that at least five tonnes of civil plutonium under IAEA safeguards is in the upper range of fuel grade plutonium or weapon grade plutonium.

Anon., November 12, 1990, "Blix Says IAEA Does Not Dispute Utility of Reactor-Grade Pu for Weapons", Nuclear Fuel, p.8.

Anon., October 10, 1998, "The H-Bomb", India Today.

Blix, H., November 1, 1990, Letter to the Nuclear Control Institute, Washington DC.

Bunn, M., June 1997, paper presented at International Atomic Energy Agency Conference, Vienna.

Carlson, J., J. Bardsley, V. Bragin and J. Hill (Australian Safeguards and Non-Proliferation Office), "Plutonium isotopics − non-proliferation and safeguards issues", Paper presented to the IAEA Symposium on International Safeguards, Vienna, Austria, 13-17 October, 1997,

Carson Mark, J., 1993, "Explosive Properties of Reactor-Grade Plutonium",

De Volpi, Alex, October 1996, "A Cover-up of Nuclear-Test Information", APS Forum on Physics and Society, Vol. 25, No. 4,

Dolley, Steven, March 28, 1997, Using warhead plutonium as reactor fuel does not make it unusable in nuclear bombs,

Fetter, Steve, 1999, "Climate Change and the Transformation of World Energy Supply", Stanford University - Centre for International Security and Cooperation Report,

Gilinsky, Victor with Marvin Miller and Harmon Hubbard, 22 Oct 2004, 'A Fresh Examination of the Proliferation Dangers of Light Water Reactors',

Gorwitz, Mark, 1996, "The Plutonium Special Isotope Separation Program: An Open Literature Analysis".

Gorwitz, Mark, 1998, "Foreign Assistance to Iran's Nuclear and Missile Programs", See Appendix A and references.

Hinton, J.P., October 1996, "Proliferation Vulnerability", Red Team Report. Sandia National Laboratories Publication, SAND 97-8203,

Jackson, Ian, 2009, 'Nuclear energy and proliferation risks: myths and realities in the Persian Gulf', International Affairs 85:6, pp.1157–1172,

Leventhal, Paul, and Steven Dolley, (Nuclear Control Institute), 1999, "Understanding Japan's Nuclear Transports: The Plutonium Context", Presented to the Conference on Carriage of Ultrahazardous Radioactive Cargo by Sea: Implications and Responses,

Mian, Zia and M. V. Ramana, Jan/Feb 2006, 'Wrong Ends, Means, and Needs: Behind the U.S. Nuclear Deal With India', Arms Control Today,

Selden, R. W., 1976, "Reactor Plutonium and Nuclear Explosives", Lawrence Livermore Laboratory, California.

Shea, T.E. and K. Chitumbo, "Safeguarding Sensitive Nuclear Materials: Reinforced Approaches", IAEA Bulletin, #3, 1993, p.23,

Uranium Information Centre, 2002, "Plutonium", Nuclear Issues Briefing Paper 18,

US Department Energy, June 1994, Office of the Press Secretary, "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium", DOE Facts (1994) 186-7. Reproduced on the US Office of Scientific and Technical Information website, Also available at:

US Department of Energy, 1997, Office of Arms Control and Nonproliferation, January, "Final Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage and Excess Plutonium Disposition Alternatives", Washington, DC: DOE, DOE/NN-0007, pp.37-39,

From WISE/NIRS Nuclear Monitor #787, 6 June 2014

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