Plutonium grades and nuclear weapons

Useful paper by physicist Alan Roberts: 'Generating Electrical Power - And Atomic Bombs', EnergyScience Coalition Briefing Paper #17, http://www.energyscience.org.au/BP17%20DualUse.pdf



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

Jim Green
National nuclear campaigner - Friends of the Earth, Australia
jim.green@foe.org.au
Last updated: September 10, 2007.


Reactor grade plutonium can be used in nuclear weapons, albeit the case that weapons manufacture using reactor grade plutonium is more difficult and dangerous compared to weapon grade plutonium. In addition to the potential to use plutonium produced in a normal power reactor operating cycle, there is the option of using civil power or research reactors to irradiate uranium for a much shorter period of time to produce plutonium ideally suited to weapons manufacture.

A standard nuclear power reactor (1000 MWe LWR) produces about 290 kilograms of plutonium each year. Hundreds of tonnes of plutonium have been produced in power reactors (and to a lesser extent research reactors), hence the importance of the debate over the use of reactor grade plutonium in weapons.

Plutonium grades

For weapons manufacture, the ideal plutonium 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. (A sub-category - super grade plutonium - contains 2-3% plutonium-240 or less.)
* Fuel grade plutonium contains 7-18% plutonium-240
* Reactor grade plutonium 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 reactor grade plutonium and possibly some fuel grade plutonium).

According to the 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. Elsewhere, the Uranium Information Centre (2004) states that plutonium contained in spent fuel elements is typically about 60-70% Pu-239. Carlson et al. (1997) from the Australian Safeguards and Non-proliferation Office note that current commercial light- and heavy-water reactors contains around 50-65% Pu-239.

Weapon grade plutonium and fuel grade plutonium from power reactors

Nuclear power reactors can of course be operated on a much shorter than usual irradiation cycle in order to produce large quantities of weapon grade and/or fuel grade plutonium for use in weapons. 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.

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 reactor grade. 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 (eg 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.

Reactor grade plutonium

With the exception of a few contrarians (discussed below), there is general agreement that reactor grade plutonium 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).

The difficulties associated with the use of reactor grade plutonium 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 reactor grade plutonium) 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 reactor grade plutonium than separation from low burn-up spent fuel.

Weapons with reactor grade plutonium 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.

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".

A greater quantity of reactor grade plutonium may be required to produce a weapon of similar yield, or conversely there will be a lower yield for reactor grade plutonium compared to a similar amount of weapon grade plutonium.

Storage life would be adversely affected by the difficulties associated with reactor grade plutonium.

The majority view

A strong majority of informed opinion holds that reactor grade plutonium can indeed be used in nuclear weapons despite the above-mentioned problems.

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). At the other end of the spectrum, advanced nuclear weapon states such as the United States and Russia, using modern designs, could produce weapons from reactor-grade plutonium having reliable explosive yields, weight, and other characteristics generally comparable to those of weapons made from weapons-grade plutonium. ...
"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 concludes its 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 removed 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. And major weapon states like the United States and Russia could, if they chose to do so, make bombs with reactor-grade plutonium with yield, weight, and reliability characteristics similar to those made from weapon-grade plutonium. That they have not chosen to do so in the past has to do with convenience and a desire to avoid radiation doses to workers and military personnel, not the difficulty of accomplishing the job. 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. ... The main alternative to the once-through cycle involves the separation and recycling of the plutonium and uranium in the spent fuel. Not only is separation and recycle more expensive, it increases greatly the opportunities for theft and diversion of 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 current 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. it was fuel grade plutonium.

Hore-Lacey from the industry-funded Uranium Information Centre contends that the isotopic composition of the plutonium used in the 1962 test "has not been disclosed, but it was evidently about 90% Pu-239". However, there is no compelling evidence to judge whether the test used reactor grade plutonium or fuel grade plutonium.

Regardless of the debate over the quality of the plutonium used in the 1962 test, and the more general debate over the suitability of reactor grade plutonium for weapons, it is worth noting again that civil power and research reactors can certainly be used to produce weapon grade or fuel grade plutonium simply by limiting the irradiation time.

India Today reported in 1998 that one or more of the 1998 tests in India used reactor grade plutonium (Anon., 1998).

(In Lorna Arnold's 'official' history of the British bomb tests in Australia, titled "A very special relationship", and in other literature such as De Volpi (1996), it is stated that one of the two Totem nuclear tests at Emu Field in South Australia in 1953 used below-weapon-grade plutonium. However, measurements of Pu/Am ratios at the bomb sites by Australian nuclear physicists do not support the claim and the British have since stated that the plan to use below-weapon-grade plutonium was abandoned because it was not available in time for the test. The Pu/Am data is presented in P.A. Burns et al., Health Physics 67, 1994, pp.226-232.)

Contrary views

The industry-funded Uranium Information Centre (2002) notes that a significant proportion of Pu-240 would make a weapon "hazardous to the bomb makers, as well as unreliable and unpredictable", that plutonium for weapons is produced in dedicated production reactors usually fuelled with natural uranium, and that: "This, coupled with the application of international safeguards, effectively rules out the use of commercial nuclear power plants."

In the same paper, the Uranium Information Centre (2002) asserts that: "While of a different order of magnitude to the fission occurring within a nuclear reactor, Pu-240 has a relatively high rate of spontaneous fission with consequent neutron emissions. This makes reactor-grade plutonium entirely unsuitable for use in a bomb." The UIC refers to the Australian Safeguards and Non-Proliferation Office (1998-99) in support of that claim, though the ASNO material does not support such a strong claim.

According to Hore-Lacey (2003) from the UIC: "Due to spontaneous fission of Pu-240, only a very low level of it is tolerable in material for making weapons. Design and construction of nuclear explosives based on normal reactor-grade plutonium would be difficult and unreliable, and has not so far been done."

The UIC (2004) states: "The only use for "reactor grade" plutonium is as a nuclear fuel, after it is separated from the high-level wastes by reprocessing. It is not and has never been used for weapons, due to the relatively high rate of spontaneous fission and radiation from the heavier isotopes such as Pu-240 making any such attempted use fraught with great uncertainties."

Some of the above statements for the UIC imply that it is impossible to use reactor grade plutonium in weapons, but the available evidence does not support that argument. The assertion that reactor grade plutonium has never been used in weapons is, at best, questionable.

The Australian Safeguards and Non-proliferation Office (ASNO) also makes the dubious claim that there has been no "practical demonstration" of the use of reactor grade plutonium in nuclear weapons. (ASNO, 1998-99.)

Implications

The potentially catastrophic implications of nuclear weapons proliferation demands that a conservative approach be adopted to the question of reactor grade plutonium. In other words, for the purposes of public policy it should be assumed that reactor grade plutonium can be used to make nuclear weapons and that the difficulties and dangers of so doing would pose only a minimal deterrent. There are of course many related areas where the importance of a conservative position is accepted - in relation to the health effects of low-level radiation, for example.

Carlson et al. (1997), from the so-called Australian Safeguards and Non-Proliferation Office, state: "The situation which arose with the DPRK highlights the fact that production of separated weapons-grade material by a non-nuclear-weapon State should not be accepted as a normal activity. Even for nuclear-weapon States, the proposal for a convention on the cut-off of production of fissile material for weapons purposes has implications in this regard. A proscription on the production - or separation - of plutonium at or near weapons-grade would be an important confidence-building measure in support of the disarmament and non-proliferation regime."

Applying the conservative principle, ASNO's arguments ought to be extended to include reactor grade plutonium. Its production should be minimised (e.g. with a phase-out of nuclear power). Separation of any plutonium from irradiated materials ought to proscribed immediately.

References

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.

Australian Safeguards and Non-Proliferation Office, 1998-99, Annual Report, pp.55-59. <www.uic.com.au/nip18.htm>

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, <www.asno.dfat.gov.au/O_9705.html>

Carson Mark, J., 1993, "Explosive Properties of Reactor-Grade Plutonium", <ccnr.org/Findings_plute.html>.

De Volpi, Alex, October 1996, "A Cover-up of Nuclear-Test Information", APS Forum on Physics and Society, Vol. 25, No. 4.
<www.aps.org/units/fps/newsletters/1996/october/aoct96.cfm#a2>

Dolley, Steven, March 28, 1997, Using warhead plutonium as reactor fuel does not make it unusable in nuclear bombs, <www.nci.org/i/ib32897c.htm>.

Fetter, Steve, 1999, "Climate Change and the Transformation of World Energy Supply", Stanford University - Centre for International Security and Cooperation Report, <cisac.stanford.edu/publications/10228>.

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", <www.globalsecurity.org/wmd/library/report/1998/iran-fa.htm>. See Appendix A and references.

Hinton, J.P., October 1996, "Proliferation Vulnerability", Red Team Report. Sandia National Laboratories Publication, SAND 97-8203, <www.ccnr.org/plute_sandia.html>.

Hore-Lacey, Ian, 2003, Nuclear Electricity, Seventh Edition, Chapter 7, published by Uranium Information Centre Ltd and World Nuclear Association, <www.uic.com.au/ne.htm>.

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, <www.nci.org/k-m/mmi.htm>.

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, <www.uic.com.au/nip18.htm>.

Uranium Information Centre, October 2004, "Safeguards to Prevent Nuclear Proliferation", Nuclear Issues Briefing Paper 5, <www.uic.com.au/nip05.htm>. (Accessed May 1, 2005.)

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, <www.osti.gov/html/osti/opennet/document/press/pc29.html>. Also available at: <www.ccnr.org/plute_bomb.html>.

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. <www.ccnr.org/plute.html>.