August 26, 1998 Reactor-Grade Plutonium Can be Used to Make Powerful and Reliable Nuclear Weapons: Separated plutonium in the fuel cycle must be protected as if it were nuclear weapons. by Richard L. Garwin(1) Senior Fellow for Science and Technology Council on Foreign Relations, New York Draft of August 26, 1998 FAX: (914) 945-4419; Email: rlg2 at watson.ibm.com As access to technology advances throughout the world, the barrier to the acquisition of nuclear weapons by terrorists or nations is more and more the barrier to weapon-usable fissionable material-- traditionally high-enriched uranium or "weapon-grade" plutonium. Even a modest nuclear weapon delivered by aircraft, missile, ship, or truck can threaten the lives of 100,000 people. Therefore it is important to understand whether reactor-grade plutonium from the nuclear fuel cycle-- typically 65% fissile (by thermal neutrons) compared with 93% fissile for weapon-grade material-- can readily be used to create nuclear weapons. Unfortunately, the answer is that it can be so used. The conclusion, therefore, is that separated reactor-grade plutonium must be guarded in just the same way as if it were weapon-grade plutonium if it is not to contribute greatly to the spread and possible use of nuclear weaponry. The facts required to judge the utility of reactor-grade plutonium (R-Pu) for use in nuclear weapons were first made widely available in 1993 by J. Carson Mark.(2) The isotopic composition of reactor-grade plutonium as compared with weapon-grade Pu (W-Pu), results in four differences between R-Pu and W-Pu: 1. The "bare sphere" critical mass for R-Pu is about 13 kg, vs. 10 kg for W-Pu (both alpha-phase metal of density 19.6 g/cc). As regards the usability of R-Pu to make nuclear weapons, the larger critical mass for R-Pu means that about 30% more R-Pu metal is needed than W-Pu to build a weapon. 2. The alpha-particle radioactivity of R-Pu contributes 10.5 watts of heat per kg R-Pu, vs. 2.3 W/kg for W-Pu. The greater heat evolution (68 watts for half a bare-sphere critical mass of R-Pu, vs. 11 watts for half a bare-sphere critical mass of W-Pu) means that the thick high-explosive that surrounds the plutonium and any additional metal shells in a simple implosion weapon will overheat if R-Pu is substituted for W-Pu. Mark estimates the amount of aluminum heat conductor that would suffice to cool the R-Pu. So-called In-Flight Insertion devices that were used in early nuclear weapons would allow adequate cooling of the plutonium until it is inserted into the high explosive a few minutes before detonation. 3. The continuing neutron emission from spontaneous fission of Pu-240 contributes 360 neutrons per second per g of R-Pu, vs. about 66 neutrons per second per g of W-Pu. According to Mark, as the fissionable material is being compressed so that it becomes critical, a neutron injected at the worst possible time would cause the earliest model of implosion weapon to have an explosive yield between 1 and 2 kilotons (that is, between 1000 tons and 2000 tons of high explosive such as TNT) rather than the full yield of some 20 kilotons when neutron injection is optimally timed to occur near the time of maximum criticality. In contrast, in 1972 the U.S. Government officially revealed that the U.S. possessed more advanced nuclear weapons whose yield would not be diminished by the injection of a neutron at no matter what instant of time. With this type of design, the spontaneous neutrons from R-Pu would in no way diminish the reliability or the expected yield. 4. A mass of R-Pu provides greater radiation exposure to a person than does W-Pu. At a distance of 1 meter from an unshielded 6-kg mass of each, the radiation field is 30 millirem per hour for R-Pu, vs. 5 millirem per hour for W-Pu. The greater external radiation from a weapon component of R-Pu compared with that of W-Pu means that the dose of 5 rem long deemed acceptable for a radiation worker would be received in 160 hours one meter from a bare core of R-Pu, vs. 1000 hours for a core of W-Pu. These facts are interpreted by various bodies as follows: Mark 1993: "The difficulties of developing an effective design of the most straightforward type are not appreciably greater with reactor-grade plutonium than those that have to be met for the use of weapons-grade plutonium." CISAC(3) 1994: "In short, it would be quite possible for a potential proliferator to make a nuclear explosive from reactor-grade plutonium using a simple design that would be assured of having a yield in the range of one to a few kilotons, and more using an advanced design. Theft of separated plutonium whether weapons-grade or reactor-grade, would pose a grave security risk." American Nuclear Society Special Panel Report(4) 1995: "We are aware that a number of well-qualified scientists in countries that have not developed nuclear weapons question the weapons-usability of reactor-grade plutonium. While recognizing that explosives have been produced from this material, many believe that this is a feat that can be accomplished only by an advanced nuclear- weapon state such as the United States. This is not the case. Any nation or group capable of making a nuclear explosive from weapons- grade plutonium must be considered capable of making one from reactor- grade plutonium." U.S. Department of Energy(5) 1997: "Proliferating states using designs of intermediate sophistication could produce weapons with assured yields substantially higher than the kiloton-range made possible with a simple, first- generation nuclear device." and "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. Theft of separated plutonium, whether weapons-grade or reactor-grade, would pose a grave security risk." As an author of the 1994 CISAC report, I helped formulate the statement that I quote above. What should the reader believe? Individuals are often skeptical of official statements, and it is often said "Those who know, don't speak; and those who speak, don't know." But that is not the case with the members of CISAC, all of whom endorsed this statement; they both know and speak. It is particularly to be noted that among the Committee are the following physicists who are knowledgeable about nuclear weapons and who reviewed a secret study done for CISAC by the Los Alamos National Laboratory and the Lawrence Livermore National Laboratory-- the United States' two nuclear weapon design laboratories. Besides myself, these include John P. Holdren, Michael M. May, and W.K.H. Panofsky. May is a former director of the Lawrence Livermore National Laboratory. WHY IS THERE SKEPTICISM ABOUT THE WEAPON UTILITY OF REACTOR-GRADE PLUTONIUM? The United States has long opposed the spread of nuclear weapons and nuclear weapons technology to additional states, and especially to terrorists. This position is that adopted by almost all of the nations of the world, including Japan, as embodied in the Non- Proliferation Treaty (NPT), which entered into force in 1970. Now 185 nations have signed the NPT, which makes it illegal to transfer nuclear weapons technology from a nuclear weapons state, and also illegal for a non-nuclear weapons state to acquire nuclear weapons. At the same time, the NPT encourages the transfer of nuclear technology for civil uses, and thus the technology of nuclear reactors and fuel fabrication and reprocessing can be communicated to any state that is a member of the NPT, whether a nuclear weapon state or a non- nuclear weapon state, provided that safeguards are in place inhibiting the diversion of weapons-useable materials. According to the NPT, it is illegal for the United States to explain to a non-nuclear weapon state how to make a nuclear weapon, and that is why details of how to fabricate a nuclear weapon from reactor-grade plutonium cannot be published here or communicated to any non-nuclear weapon state. In 1976 the United States decided that releasing some additional information about nuclear weapons would actually aid in preventing their spread-- the purpose of the NPT. The result was a 1976 briefing(6) by the Department of Energy to nations with active nuclear power programs, so that they should understand the utility of reactor-grade plutonium in the fabrication of nuclear weapons, and thus adopt measures to protect and account for plutonium in spent fuel downloaded from nuclear power reactors. This was the information published more fully in the 1993 article by Mark. The nations signing the NPT, and the nuclear power industry worldwide, would be delighted if plutonium produced by nuclear reactors that operate to generate electrical energy were not usable to make nuclear weapons, but the facts are otherwise, as explained in the previous paragraphs. Nevertheless, some interpret their own wishes as the facts; and beyond those who are confused in this fashion there are advocates and publicists (either without the ability to form their own judgment or who do not recognize the responsibility to do so) who repeat arguments that-- if true-- would cut one possible link between nuclear power and nuclear weapons. My colleague, Ambassador Imai was an International Member of the American Nuclear Society Special Panel, which reported as I have quoted above. The ANS panel included Harold M. Agnew, who was present in December 1942 at criticality of the first Fermi reactor, worked at Los Alamos to build the atomic bomb, and was director of the Los Alamos National Laboratory for 10 years. But Ambassador Imai has more recently expressed doubts about the utility of reactor-grade plutonium for making nuclear weapons. As I read closely his remarks(7) I see that he suggests that the four disadvantages of reactor-grade plutonium discussed above mean that nuclear explosives made from this material could not be reliable, might be "toy weapons", and that any nation (as distinguished from terrorists and rogue states, which could not arm themselves with nuclear weapons) wanting nuclear weapons would "probably equip themselves with modern weapons, mainly thermonuclear bombs" instead of "unreliable bombs with reactor grade plutonium." But the impact of the authoritative comments that I have quoted (and my own view)(8) is that a nation could indeed make reliable fission weapons (and hence the "primaries" for thermonuclear weapons) by the use of reactor-grade plutonium. The Summary of a February 1998 report(9) of the Royal Society of Britain, chaired by Sir Ronald Mason, states of the U.K. activity in reprocessing fuel from its nuclear reactors and from those of foreign customers including Japan: "The existence of plutonium stocks, in whatever form, is of concern on two counts: radiotoxicity and proliferation risk. Whilst not underestimating radiotoxicity risks, the chance that the stocks of plutonium might, at some stage, be accessed for illicit weapons production is of extreme concern. The current stockpiling policy should not be maintained without careful study of alternatives." More directly, it observes: "The surest anti-proliferation measure is to stop reprocessing spent fuel and to reduce the quantity of separated plutonium in store." However I was troubled by the report's statement (p. 6): "The critical mass of fissile plutonium (Pu-239 and Pu-241) needed to sustain a chain reaction in reactor-grade plutonium may be <an order of magnitude> greater than for weapons-grade plutonium. The reliability and yields of weapons constructed from reactor-grade plutonium might also be reduced. However, an experienced weapons designer could have confidence in a weapons system based on reactor grade plutonium <with 85% fissile content>. Reactor grade plutonium, of known isotopic composition, must therefore be regarded as a plausible target for determined terrorist groups or states wishing to make nuclear weapons." As should be clear from my own analysis, the portions I have surrounded by angular brackets "<...>" are incorrect. We have noted that the bare-sphere critical mass of reactor-grade plutonium extracted from highly irradiated spent fuel from a normal pressurized water reactor or boiling water reactor operating at 43,000 megawatt-days per kg fuel is 13 kg-- only 30% (not the 100 kg which would be a factor 10 or "an order of magnitude") greater than the 10-kg critical mass of weapon-grade plutonium. And this is reactor-grade plutonium with 66% "fissile content". The point is that "non-fissile" Pu-240 is fissionable with the fast neutrons that carry the chain reaction in plutonium metal; in fact, even pure Pu-240 has a critical mass of 40 kg-- smaller than pure U-235-- for use in a nuclear weapon. In a clarifying letter,(10) Sir Ronald Mason states that the "order of magnitude greater" critical mass refers to plutonium oxide, as compared with plutonium metal; he writes also that he agrees the data I provide above on critical mass, and notes further that the yield will depend somewhat on the precise isotopic composition. WHO IS CAPABLE OF USING REACTOR-GRADE PLUTONIUM TO MAKE NUCLEAR WEAPONS? None of the five nuclear weapon states (U.S., Russia, Britain, France, and China) is believed to have in its stockpile nuclear weapons made from reactor-grade plutonium. In part this is due to their light- water power reactors coming later than their nuclear weapons programs. But per unit of heat removed generated in the reactor (which is a limiting characteristic and cost of a plutonium production program), plutonium is best obtained by reprocessing at low burnup, and hence while it is still "weapon grade". And at higher burnup, much of the Pu- 239 generated in the reactor is fissioned and thus lost. As Carson Mark made clear, the difficulties in making a nuclear weapon with reactor-grade plutonium are not different in kind than those involved in the use of weapon-grade material. Made of reactor- grade plutonium, a simple fission weapon a fraction of the time may have an explosion yield of 1000 to 2000 tons of high explosive-- the equivalent of 1000 truck bombs going off simultaneously at one point, plus the effects of nuclear radiation; but it would never have a lower yield, and a fraction of the time it would have full design yield of 20 or 40 kilotons. As for the "more sophisticated" designer, it is my own judgment that not only the five nuclear weapon states, but also the nuclear weapon establishments of India, Pakistan, and Israel are capable of converting reactor-grade plutonium into nuclear weapons that have similar yield and reliability to those made with weapon- grade plutonium. (This paragraph was drafted before the nuclear weapon test explosions by India and Pakistan in May, 1998). In conclusion, separated plutonium-- whether weapon grade or reactor grade-- poses a similar danger of misuse in nuclear weapons and must be provided similar physical protection, control, and accountancy. This has been recognized by the International Atomic Energy Agency (IAEA) from its beginning-- all plutonium (except Pu-238 of isotopic purity greater than 80%) is regarded as equally hazardous from the point of view of diversion to nuclear weaponry. WHAT SHOULD BE DONE ABOUT NUCLEAR WEAPONS AND ABOUT EXCESS NUCLEAR WEAPON MATERIALS? I am a member of the Committee on International Security and Arms Control (CISAC) of the National Academy of Sciences, which in July 1997 published a report(11) that strongly urges the U.S. and Russia to reduce their nuclear weapons to a level of 2000 total weapons, in contrast to the present 10,000 to 20,000 they now possess. If the weapons cannot be dismantled and disposed of immediately, then they should be demilitarized-- rendered incapable of being detonated before the plutonium or uranium can be removed. And the excess weapons should immediately be subject to bilateral and as soon as possible to international (IAEA) accounting. It was not the task of the CISAC to evaluate the risk of separated civil plutonium being used in nuclear weaponry, but it is clear that the CISAC analysis considers separated reactor-grade plutonium, when it has been extracted from spent fuel, as representing the same degree of hazard as does weapon plutonium and that it should be subject to the same measures of physical protection and accountancy. We urge that the weapon-usable plutonium and high-enriched uranium from dismantled weapons must be protected according to the "stored nuclear weapons standard" and it is important that the separated weapon materials be converted as soon as possible to meet the "spent fuel standard". The nuclear weapon materials in that form are then no more attractive a source of weapon-usable material than the much larger amount of Pu present in the unprocessed spent fuel from the 400 power reactors in the world today. Pakistan's nuclear explosions are an urgent reminder that the excess high-enriched uranium from dismantled nuclear weapons in Russia and the United States is an ideal material from which to make fission weapons, and that more must be done to provide the conditions and resources to dilute this material to the status of low-enriched uranium (less than 20% U-235) so that it cannot be directly used to make a fission weapon. An up-to-date and thorough presentation of the current status of physical protection, the implication of the stored weapons standard, and what steps could be taken to strengthen global standards, is now available on the web and is forthcoming in book form.(12) I agree with Ambassador Imai(13) that Japan and the other non-nuclear states of the NPT should play a more active role in urging the U.S. and Russia to more rapid reductions in their nuclear weaponry and to detailed consideration of the elimination of nuclear weapons. The CISAC report considers elimination (or, rather, prohibition) of nuclear weapons as worthy of discussion, but argues that until agreement on prohibition can be reached, it is both practical and essential to make massive reductions in all nuclear weapons. ---------------- 1 The author consulted for the Los Alamos National Laboratory from 1950 to 1993, and since then for the Sandia National Laboratories. Most of his work at Los Alamos was involved with nuclear weapons design, manufacture and testing. In recent years he has reviewed for the Department of Energy matters related to nuclear weaponry and particularly the Stockpile Stewardship program, primarily as a member of the JASON group of consultants to the U.S. Government. Some of the JASON reports are available at http://www.fas.org/rlg together with other recent papers by the author. He is a member of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. In 1997 he received the Enrico Fermi Award from President Clinton and the Department of Energy, "for a lifetime of achievement in the field of nuclear energy." He is a member of the National Academy of Sciences' Committee on International Security and Arms Control, which in 1994 and 1995 published two reports "The Management and Disposition of Excess Weapons Plutonium". He served on the 9-person Commission to Assess the Ballistic Missile Threat to the United States, established by the U.S. Congress, that issued its report in July 1998. 2 J. Carson Mark, "Explosive Properties of Reactor-Grade Plutonium," Science and Global Security, 4, 111-128, ______________________________ 1993. Mark headed the Theoretical Division at the Los Alamos National Laboratory for decades; he died in 1997. T-Division played a major role in nuclear weapons design, and Mark was intimately involved in the design of both fission weapons and thermonuclear weapon. Mark had already in August 1990 prepared a report for the Nuclear Control Institute (http://www.nci.org) titled "Reactor-Grade Plutonium's Explosive Properties". 3 "The Management and Disposition of Excess Weapons Plutonium," Committee on International Security and Arms Control (CISAC) of the National Academy of Sciences, National Academy Press, Washington, DC (1994). The full text is available at http://www.nap.edu/readingroom/enter2.cgi?0309050421.html. See pages 32-33 for discussion of nuclear weapons from reactor-grade plutonium. 4 "Protection and Management of Plutonium", American Nuclear Society, Special Panel Report (August 1995). See page 25. 5 "Final Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage And Excess Plutonium Disposition Alternatives", U.S. Department of Energy, pp. 66-68, 13 January 1997. (Available at http://twilight.saic.com/md/docs/ as "finalnew.pdf". 6 At a meeting of the Atomic Energy Forum/American Nuclear Society. 7 Plutonium, 19, 3-5, Autumn 1997. __________ 8 Correspondence between Ambassador Imai and myself is reproduced in Plutonium, 22, Summer 1998. __________ 9 "Management of Separated Plutonium", The Royal Society (The UK Academy of Science), London (ISBN: 0 85403 514 1). Summary at http://www.royalsoc.ac.uk/st_pol24.htm. 10 Sir Ronald Mason, letter to R.L. Garwin, 2 June 1998. 11 "The Future of U.S. Nuclear Weapons Policy," report of Committee on International Security and Arms Control, National Academy of Sciences, Washington, D.C., National Academy Press, June 1997. Text available at http://www.nap.edu/readingroom/enter2.cgi?0309063671.html. 12 Full text at: www.ksg.harvard.edu/bcsia (click on "Publications"). Matthew Bunn, "Security for Weapons-Usable Nuclear Materials: Expanding International Cooperation, Strengthening International Standards," in Comparative Analysis of Approaches to Protection of Fissile Materials: Proceedings of a Workshop at Stanford California, July 28-30, 1997. Livermore, CA: Lawrence Livermore National Laboratory, Document Conf.-97-0721, 1998 13 "Japan Should Initiate Creation of International Committee to have Specific Plan for the Elimination of N-Weapons," Plutonium 21, 2-6 (Spring 1998). _________
WHAT JAPAN CAN DO TO HELP ITS ECONOMY AND DEPLOY ADDITIONAL NUCLEAR POWER. o Safe and affordable energy is essential to a modern, developed society, and electrical power from nuclear reactors can be both safe and affordable. Japan should continue to deploy light-water reactors as energy demand requires, with a government role in ensuring safety of operation. o The normal operation of the uranium supply and enrichment market is adequate for powering Japanese reactors for several decades. However, a particular opportunity is available to buy from Russia about 10,000 tons of low-enriched uranium that would be produced by blending excess high-enriched uranium from Russian nuclear weapons; this would be delivered as 4% U-235 and would suffice to power all existing Japanese reactors for ten years. o For the long run, fuel for all of the world's reactors could be supplied for centuries and even thousands of years by uranium from seawater-- a field in which Japan has played a leading role. Recent Japanese work (May 1998) projects a cost of $100 per kilogram of uranium from seawater, in comparison with something like $20/kg of uranium from ore. But the seawater resource in enough to operate 10,000 power reactors for 1000 years (without breeding). Even at $200/kg, uranium from seawater would be cheaper than reprocessing spent fuel and recycling plutonium and uranium. Uranium at $200/kg would increase the cost of nuclear energy by about 0.4 cents per kWh. o As of December 1996, there was in Japan about five tons of civil unirradiated plutonium, and about 15 tons of civil unirradiated Japanese plutonium in foreign countries. In addition, in spent civil reactor fuel in Japan there was almost 50 tons of plutonium. We note that ten tons of civil plutonium would suffice to make more than 1000 nuclear weapons. As with civil and military plutonium anywhere in the world, these Japanese stocks must be protected and safeguarded if they are not to contribute to the acquisition of nuclear weapons by other nations and sub-national groups. o Finally, the interests of the Japanese consumer of electrical energy and of the producer of electrical energy would be well served by the availability of a mined geologic repository, whether it is used for the vitrified fission products from the reprocessing plants at La Hague, France, or at Sellafield, Britain, or from reprocessing in Japan. Furthermore, the repository could equally well hold spent fuel in appropriate disposal casks, as is planned in the United States. It seems to me highly desirable to have competitive, commercial, mined geologic repositories in various countries of the world, with the repositories and the waste forms (including spent fuel) regulated by the International Atomic Energy Authority. Many areas of the world, as well as the nuclear energy industry itself, would benefit from the availability of such repositories, which might be built in China, in the United States, in Australia and Africa.