Pure Fusion Weapons?

By Hisham Zerriffi And Arjun Makhijani | 1999-01-01 12:00:00


Nuclear weapons changed in 1953, when nuclear fission (the splitting of atoms) and nuclear fusion (the fusing, or joining of atoms) were combined, creating thermonuclear weapons, known more generally as "hydrogen bombs." So far, only a fission explosion has generated the high temperatures and pressures necessary to trigger the thermonuclear explosion in a hydrogen bomb. For this reason, all current generation thermonuclear weapons have a fission "primary" that sets off a fusion explosion in the "secondary." However, pure fusion weapons, that is, weapons that would not need a fission trigger, have long been thought of as "desirable" by nuclear weapons designers, in part because they would not produce fission-product fallout.

The scientific feasibility of pure fusion weapons has not yet been demonstrated, but if the technical hurdles are overcome, the use of nuclear weapons as instruments of war could be fundamentally transformed, introducing new proliferation dangers and radically reducing the chances of getting complete and enduring nuclear disarmament.

Thermonuclear explosions, unlike explosions caused by chain reactions in fissile materials like plutonium, do not require a minimum critical mass. Thus, pure fusion weapons could be made with very low yields, and would not produce fallout, blurring the distinction between conventional explosives and nuclear explosives. Yet, the lethality of the weapons, due to neutron radiation and explosive force, would still be great.1 In fact, the radius of lethality of small pure fusion weapons per unit of explosive power would be far greater than that of large fission weapons.2 For instance, the destructive area per ton of TNT equivalent of the Hiroshima bomb was a hundred times smaller than the estimated lethal radius of a one-ton TNT equivalent pure fusion bomb. The adverse implications of this military arithmetic for nuclear nonproliferation and disarmament would be profound.

Explosive Confinement Fusion 3

Fusion reactions release energy when two light nuclei combine. (Fission, on the other hand, releases energy through the splitting of heavy nuclei.) The underlying reason for the energy release is the same as that for fission - that is, the nuclei that are present initially are heavier than the products of the nuclear reaction; the difference in mass shows up as energy. Pure fusion weapons (as well as controlled fusion energy) have been unattainable so far because it is very difficult to create the conditions that enable a large enough number of nuclear fusion reactions to occur and generate a net output of energy without using a fission trigger. At close range, positively-charged nuclei exert repulsive (opposing) electrical forces on each other. These forces must be overcome if the nuclei are to be brought close enough together to sufficiently increase the probability of fusion reactions occurring. This is done by heating the fuel to extremely high temperatures (hence the term "thermonuclear") comparable to or higher than temperatures in the interior of the sun. This allows the kinetic energy (the energy of motion) of the nuclei to be large enough to overcome the repulsive force. 4

The most common man-made fusion reaction, and the one responsible for most of the fusion energy release in thermonuclear explosions, involves two isotopes of hydrogen: deuterium (D) and tritium (T)5.While not achieving the levels of thermonuclear bombs, laboratory Explosive Confinement Fusion (ECF) facilities have achieved a significant number of fusion reactions.

All ECF schemes have two basic components: the fuel pellet and the driver. The fuel pellet contains the fuel, typically a mixture of deuterium and tritium, as well as other components. The driver provides the energy to the pellet to compress it to the high densities and temperatures needed to initiate the fusion reaction. Types of drivers that have been considered include lasers and other electromagnetic energy sources.

The ratio between the fusion energy output and the driver energy output is called gain. A gain of one is required to prove the scientific feasibility of any fusion scheme. When the gain is less than one, there is a net energy loss and the fusion scheme is not feasible.

There are two essential scientific and technical accomplishments that are needed to make pure fusion weapons. First, their scientific feasibility must be established. Second, they must be made small enough to be deliverable weapons. The National Ignition Facility (NIF), under construction in California, and a similar one under construction near Bordeaux in France (Laser Mégajoule, or "LMJ") are designed to establish the scientific feasibility of pure fusion explosions. While the laser beams they use cannot be miniaturized into weapons, the goal of the devices is to achieve a gain greater than one. The ignition of the fuel pellet would result in small fusion explosions. The lessons learned from these laser fusion experiments could be used in experiments using other drivers with a potential for miniaturization into weapons. For example, experiments on NIF could be used to design optimal targets for experiments using high-energy capacitors or drivers using combinations of chemicals and electromagnetic energy that can be made compact enough for weapons. Experiments with these types of devices are being conducted at Los Alamos National Laboratory and Sandia National Laboratory in New Mexico, the former in collaboration with Russia. One result of these combined efforts could be significant advances towards the design of pure fusion weapons.

Disarmament and Non-Proliferation Implications

Though scientific feasibility has yet to be proven, the research on pure fusion explosions itself raises serious questions. At the very least, it sends a dangerous signal about the intent of the nuclear weapons powers to continue to develop and enhance their arsenals. The effects on disarmament and nonproliferation efforts are already grave. India's refusal to sign the Comprehensive Test Ban Treaty (CTBT) was, in part, a reaction to this type of research by the nuclear weapons states. In turn, its subsequent decision to conduct underground nuclear tests was partly related to its conclusion that the CTBT had changed from a non-discriminatory instrument designed to promote both non-proliferation and disarmament into a tool for non-proliferation alone. Furthermore, some fusion research appears to violate the CTBT, as we discuss below.

Other potential problems include:

the possibility that pure fusion weapons, a long-time goal of the nuclear weapons designers, will be achieved;

the development by the United States (and possibly other nuclear weapons states) of new fission-fusion thermonuclear weapons designs;

the possibility that the United States will withdraw from the CTBT under the "Supreme National Interest" clause to test either new generations of weapons or modifications to existing designs of thermonuclear weapons;

the spread of information and computer codes on the physics of thermonuclear explosives, since there are non-weapons research aspects to most of these facilities.6

Official U.S. planning documents for the Stockpile Stewardship program demonstrate that the Department of Energy (DOE) plans to maintain and exercise the ability to design new nuclear weapons. It is quite conceivable that DOE weapons scientists would conduct at least preliminary design investigations of pure fusion weapons once the necessary data were available. According to the DOE's rationale, it is not only necessary to have advanced facilities to interest and retain scientists, it is also necessary to allow them the opportunity to practice their design skills.7 The DOE has denied that it intends to design pure fusion weapons, but the technical work it is doing could lead to such weapons nonetheless because it is compatible with pure fusion weapons research and development.

Potential energy applications have been claimed for the various explosive fusion programs. However, energy devices should be justified by comparison with other approaches to solving energy problems, especially given the enormous expense of these devices and the very long time frame (several decades or more) this research is likely to take to lead to fruition. There are far more promising approaches to dealing with energy issues than ECF schemes.8

Does Fusion Research Violate the CTBT?

The legality of fusion research under the Comprehensive Test Ban Treaty is a complicated and as yet unresolved question. There are two key issues involved: interpretation of the treaty language, and the precise definition of a "nuclear explosion."

Language of the CTBT

Article I of the Comprehensive Test Ban Treaty states that:

1. Each State Party undertakes not to carry out any nuclear weapon test explosion or any other nuclear explosion, and to prohibit and prevent any such nuclear explosion at any place under its jurisdiction or control.

2. Each State Party undertakes, furthermore, to refrain from causing, encouraging, or in any way participating in the carrying out of any nuclear weapon test explosion or any other nuclear explosion.

The United States government, both in previous statements and in its submission of the treaty to the U.S. Senate for ratification, has stated that ECF experiments are not covered by the treaty. The U.S. position has been based on an interpretation of the Nuclear Non-proliferation Treaty, which bans the use of "nuclear explosive devices" by non-nuclear weapons states. However, the CTBT goes further, banning any "nuclear explosion," including "peaceful nuclear explosions" by any state, and is intended to constrain weapons development by all states.

CTBT negotiations involved extensive discussion of allowing some fission explosions. Initially, the U.S. wanted the CTBT to allow for hydronuclear (i.e. fusion) testing which would yield nuclear explosive energy equivalent to up to four pounds of TNT. However, it changed this position in 1995 and argued for a "zero-yield" treaty, which was the version of the treaty that was adopted. Unfortunately, zero-yield was not defined, though the negotiating record for hydronuclear explosions clearly indicates that this should be well under four pounds of TNT equivalent. As a result, the parties to the CTBT are not permitted to conduct hydronuclear experiments. However, the U.S. and Russia believe that they are permitted under the treaty to continue "sub-critical" experiments involving both plutonium and conventional explosives, because the plutonium would not reach criticality.

Our research indicates that NIF, the Laser Mégajoule project in France roughly equivalent to NIF, and all other facilities designed to create thermonuclear explosions of even a few pounds of TNT equivalent are illegal under the CTBT. Even their construction is illegal since the CTBT requires the prevention as well as the prohibition of explosions. Parties are also enjoined from "causing, encouraging, or in any way participating in" any nuclear explosions. The intent of these facilities is to cause nuclear explosions. Only a legally binding, permanent, and verifiable commitment under the CTBT not to use tritium fuel in these machines would render their construction legal. However, in that case the machines would be useless since their entire purpose is to achieve ignition.

Defining a "nuclear explosion"

The clarification of Article I of the CTBT requires that a nuclear explosion be defined. It is clear that nuclear yields that derive from super-critical explosions, however small, are illegal, as is the case for all present nuclear weapons. But this does not allow us to set a numerical limit for what explosive force deriving from nuclear reactions of other kinds, for instance, sub-critical reactions, would be illegal. Hence, finding a precise definition is quite complex.

An explosion is an interplay between the total amount of energy released, energy density, and the time in which the energy is released. The time factor is perhaps the easiest to define because all nuclear explosions of possible military consequence are expected to occur in well under one millisecond.9 Other physical criteria are also needed to define a nuclear explosion.

Criticality: As noted above, the U.S. has used the threshold of criticality to define nuclear explosions of fissile materials. Under this definition, the sub-critical experiments involving high explosives and fissile materials conducted at the Nevada Test Site are deemed to be allowable under the CTBT.

Specific Energy Release: The release of nuclear energy in an explosive fashion is not really an explosion unless the energy released is greater than the energy used to initiate the explosion.10

Ignition: Another criterion which is especially helpful in defining fusion explosions is ignition. It has been defined in two different ways:

1.The creation of a self-propagating burn wave in the fuel pellet. This is a concept somewhat analogous to the concept of criticality in fission explosions.11

2. A gain of one. In other words, the fusion energy output of the fuel pellet is equal to or greater than the driver energy output.

We propose that the definition of explosions as those achieved in ECF systems with a gain of one is a minimally satisfactory definition for the purposes of CTBT compliance. The advantage of this proposal is that it is not limited to any particular technology or an arbitrary yield, but rather is based on a comparison of energy use and energy production. To be in compliance, the fusion reactions would have to have an energy release that is less than the driver energy input into the fuel pellet. In that case, the conditions for establishing scientific feasibility of pure fusion explosions would not be achieved.

Any definition of a fusion nuclear explosion geared to ignition would still allow a considerable loophole for pure fusion weapon development even though it would meet the letter of the CTBT. This is because a great deal of research on weapons applications can be conducted at gains just under one - that is, just below the ignition threshold. Therefore, it would be helpful to set other limits to constrain the development of new weapons. The following two limitations have been proposed by experts with experience in nuclear weapons issues:

The Garwin limit: This proposal, by Richard Garwin, a long-time consultant to various U.S. government agencies on nuclear weapons issues, would limit neutron production per shot to the equivalent of an explosion of 0.1 gram of high explosives. This would effectively freeze the program until such time as a review of fusion experiments has been completed.12 Similarly, experiments at facilities such as NIF would be limited, but not prohibited, by this proposal.

The Kidder limit: A proposal by Ray Kidder, a retired senior weapons scientist and a pioneer of laser fusion research, would ban tritium use in systems driven by high explosives. Facilities designed to achieve ignition or burn in D-T fuel pellets would be unlikely to accomplish these goals in fuel pellets without tritium. High explosive-driven components will most likely be key to the miniaturization of pure fusion devices - a necessary step towards pure fusion weapons - so a ban is proposed on tritium in combination with high explosives. 14

While each of these limitations by itself leaves significant loopholes, collectively they could provide reasonable protection against development of fusion weapons while allowing some fusion research to continue. This would allow for the continuation of all research on non-explosive magnetic confinement fusion, as well as most experiments at existing laser facilities, such as the NOVA laser at Livermore Laboratory. However, many new or planned facilities would be illegal.


While our technical review of the record indicates that facilities such as NIF and Laser Mégajoule are illegal under the CTBT there is as yet no official interpretation of the CTBT in regard to fusion explosions. Hence, the U.S. and other countries are proceeding as if their plans are legal under the CTBT. An official opinion by the CTBT review conference, which defines an explosion for the purposes of the treaty and sets limitations on research based upon that definition, is needed. This should take into account the facts set forth above as well as the clear intent of the CTBT to constrain new weapons development. The present U.S. interpretation, shared by several other states, is clearly unacceptable. It deems explosions in NIF and Laser Mégajoule to be legal. If this is accepted, there would be no upper limit to pure fusion explosions under the CTBT, which would severely undermine it in the long term and possibly render it meaningless.

Facilities and experiments such as NIF and Magnetized Target Fusion devices pose threats to both the CTBT and the disarmament process. If ignition is demonstrated in the laboratory, the weapons labs and the DOE would likely exert pressure to continue investigations and to engage in preliminary design activities for new generation weapons (even if the goal is simply to keep the designers interested and occupied). Ignition would also boost political support and make large-scale funding of such activities more likely.

Even without the construction of actual weapons, these activities could put the CTBT in serious jeopardy from forces both internal and external to the nuclear weapons states pursuing this research. Internally, the same pressures that could lead to the resumption of testing of the current generation of weapons could also lead to the testing of new weapons (to replace older, and supposedly less safe or reliable weapons). Externally, the know- ledge that the nuclear weapons states are engaging in new fusion weapons design activities could lead other states to view this as a reversal of momentum toward disarmament. Indeed, this scenario has already occurred with the Indian and Pakistani nuclear tests.


The following recommendations, taken together, are aimed at preventing the development of pure fusion weapons:

Ignition of the fusion fuel should be used as the definition of a fusion nuclear explosion for purposes of CTBT compliance. This would prohibit all ignition experiments as well as planning or construction of all facilities designed to achieve ignition. This appears to be the minimum necessary to meet the letter of the CTBT. Construction of NIF and LMJ should be stopped.

The total fusion energy output should be limited.15 This would prevent attempts to gain weapons-related information by increasing the energy of the driver and fusion energy output while staying below ignition.

The use of tritium should be banned in all systems that use high explosives (as proposed by Ray Kidder).

Hisham Zerriffi is Project Scientist and Arjun Makhijani is President of the Institute for Energy and Environmental Research, Takoma Park, MD. Email: <hisham@ieer.org>

1. For instance, the lethal area of a pure fusion weapon with an explosive force of one ton of TNT equivalent would be on the order of a hundred times larger than a conventional bomb with the same explosive force. This is because most of the lethality of pure fusion weapons would derive from the intense neutron radiation rather than the explosion.

2. As nuclear weapons get larger, the destructive area per unit of explosive power declines.

3. In this article, we designate all devices that could activate pure fusion explosions by various confinement schemes under the rubric of "explosive confinement fusion" or ECF.

4. This is a simplified description of thermonuclear fusion. For example, the physics of plasmas, indeed the definition of a plasma, is significantly more complex and precise than what is presented here. However, this explanation of fusion is sufficient in order to understand the issue. A more detailed description can be found in the report, Dangerous Thermonuclear Quest, available on the ieer web site: <www.ieer.org>

5. Deuterium is a non-radioactive isotope, with one proton and one neutron in the nucleus. Tritium, which has one proton and two neutrons in its nucleus, is highly radioactive. A fusion reaction between these two isotopes produces an alpha particle, which is a helium nucleus, and a neutron.

Here we use the chemical symbols for elements to represent their nuclei, since at the temperatures involved in thermonuclear fusion, all atoms are converted into free electrons and nuclei - i.e., into plasmas.

6. For example, astrophysics experiments would be conducted at the National Ignition Facility, and experiments at entirely unclassified facilities are carried out in non-nuclear weapon states such as Germany and Japan.

7. See H. Zerriffi and A. Makhijani, The Nuclear Safety Smokescreen, (IEER, May 1996).

8. See Science for Democratic Action, Vol. 6 No. 3 for articles on energy options for reducing greenhouse gas emissions. See also A. Makhijani and S. Saleska, The Nuclear Power Deception, (IEER, 1996), chapter 9. (Soon to be published in book form by Apex Press.)

9. While there is no exact definition of reaction time for an explosion, we use one millisecond as a reasonable value to distinguish a steady-state regime from an explosive regime.

10. Thus a 1987 Los Alamos report on the testing moratorium of 1958-1961 states that "a nuclear explosion has never been defined officially, but we consider a reasonable definition to be a specific fission energy release that is comparable to or greater than that of high explosive itself, about one kilocalorie per gram."

11. John Lindl, "Development of the Indirect-Drive Approach to Inertial Confinement Fusion and the Target Physics Basis for Ignition and Gain," (Lawrence Livermore National Laboratory preprint, publication numbers UCRL-JC-119015 and L-19821-1, November 1995), p. 6. Published in Physics of Plasmas, Vol. 2, No. 11, (November 1995), pp. 3933-4023.

12. National Research Council, Commission on Physical Sciences, Mathematics, and Applications, Committee for the Review of the Department of Energy's Inertial Confinement Fusion Program, Review of the Department of Energy's Inertial Confinement Fusion Program: The National Ignition Facility, (Washington: National Academy Press, 1997), pp. 10-11.

13. This limit has already been approached by Magnetized Target Fusion experiments and reportedly by Russian high explosive research. See Suzanne L. Jones and Frank N. von Hippel, "The Question of Pure Fusion Explosions Under the CTBT," Science and Global Security, Vol. 7, 1998, pp. 5-6.

14. However, such a ban would not impose any limits on laser-driven or ion-beam driven research or even the Sandia wire-array z-pinch - all potential contributors to the development of pure fusion weapons. The wire-array z-pinch also has some potential to be reduced in size so as to be usable as a weapon.

15. To 1014 neutrons/shot, as proposed by Richard Garwin.

Pure fusion weapons could radically reduce the chances of getting nuclear disarmament.

Even the construction of the National Ignition Facility is illegal since the CTBT requires the prevention as well as the prohibition of explosions.

Peace Magazine Jan-Feb 1999

Peace Magazine Jan-Feb 1999, page 24. Some rights reserved.

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