Dangerous Thermonuclear Quest

A great qualitative change in the nature of nuclear weapons occurred four-and-a-half decades ago when nuclear fission and nuclear fusion were combined into thermonuclear weapons, popularly called "hydrogen bombs." Even so present-day generation nuclear weapons produce considerable levels of fallout in the form of highly radioactive fission and this has been one of the main factors limiting their military application.

Pure fusion weapons -- that is, weapons that would not need a fission trigger -- have long been thought of as desirable by the nuclear weapons establishment because they would not produce fission product fallout. Yet, the lethality of the weapons due to neutron radiation as well as explosive force would still be great. Moreover, thermonuclear reactions do not require a minimum critical mass to be assembled. Hence pure fusion weapons could range in size from a few kilograms to multi-megatons of TNT equivalent.

Pure fusion weapons 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 in a small enough space and short enough time. Fusion reactions involve the fusion of light nuclei, which results in a release of nuclear energy. (Fission, on the other hand, releases energy through the splitting of heavy nuclei.) Positively-charged nuclei exert repulsive (opposing) electrical forces on each other, which are very strong at close range. These forces must be overcome if the nuclei are to be brought close enough together so that the probability of fusion reactions may be high. This is done by heating the fuel to extremely high thermal temperatures (hence the term "thermonuclear") -- comparable to or higher than temperatures in the interior of the sun. This allows the kinetic energy of the nuclei to be large enough to overcome the repulsive force. The even greater attractive force between nuclear particles, which operates to a significant degree only at very close range, is then able to overcome the repulsive electrical force and produce nuclear fusion.

Keeping together a dense enough mass of nuclear material for long enough to produce a large number of nuclear reactions, but short enough to produce an explosion, has proved possible by only one means. That has been to create the extremely high temperatures and pressures by a fission explosion. The barriers for producing pure fusion weapons -- that is, weapons that would not require this "primary" fission explosion (called the trigger of the thermonuclear warhead) -- have so far proved insuperable. However, work in the last decade or so has resulted in a great deal of technical progress, to the point that it is possible to conceive of pure fusion explosives that could be compact enough to be used as weapons. If such weapons were to be developed, it would represent a fundamental transformation in the potential employment of nuclear weapons as instruments of war and have a substantial adverse effect on non-proliferation and nuclear disarmament.

Summary of Findings

1. Pure fusion weapons can, in theory be made in varying sizes from small to huge. By reducing the sizes of possible nuclear explosions, pure fusion weapons could lower the threshold of nuclear weapons use. This would tend to erode the norm against use of these weapons of mass destruction that has been built up for over half a century.

2. The scientific feasibility of pure fusion weapons has not yet been established. Until recently, there were no devices that could establish such feasibility.

3. Major advances in the last decade in plasma physics and in various technologies to achieve fusion reactions at high rates in laboratory setting have opened up new possibilities for pure fusion weapons.

4. Three major technologies could contribute to the establishment of the scientific feasibility of pure fusion weapons, and other weapons that do not require fission triggers: (i) inertial confinement fusion programs designed to achieve ignition (such as the National Ignition Facility being built at Livermore), (ii) capacitor or high explosive driven electromagnetic devices such as the Magnetized Target Fusion program at Los Alamos and Arzamas-16, and (iii) other non-fission methods of generating intense x-rays, such as the wire-array z-pinch program at Sandia Lab. These programs reinforce each other in the development of fusion technology.

5. In some respects it will be less difficult to make pure fusion weapons than to generate commercial power from pure fusion, once thermonuclear ignition has been achieved in a laboratory setting.

6. One major roadblock to the development of pure fusion weapons is the achievement of ignition in the plasma. This requires sufficient compression and heating of the fuel pellet in a precise way. The second problem is to get large enough non-fission energy sources ("drivers") to perform the work of compression and heating of the fuel pellet to thermonuclear conditions within practical limits of size and weight.

7. Several technologies could help overcome the technical hurdles facing pure fusion weapons. They include development of advanced materials manufacturing technology, which could lead to reduced driver size and help make better ICF targets.

8. Once ignition has been demonstrated at a laboratory level, it will be difficult to contain the development of pure fusion weapons. Fusion weapon proliferation controls will be far more difficult than with fission weapons because the materials are not currently under the same level of international control and because more of the relevant literature is non-classified. In fact, pure fusion weapons would by-pass most of the system of international non-proliferation safeguards. Political pressures to develop such weapons would also be likely to intensify.

9. The main mechanism to prevent a radically new and dangerous nuclear weapons situation from developing in the world is to ban the construction of machines that could achieve ignition of thermonuclear plasma without a fission trigger. Devices that use high explosives either directly or indirectly as part of the driver pose special dangers because they could be converted to practical weapons with less difficulty once feasibility is established. Other less restrictive mechanisms could include a ban on the use of tritium in devices involving the use of high explosives.

10. The NPT allows for a broad range of ICF research. But the CTBT is much more stringent. For instance, while the NPT allows peaceful nuclear explosions, but Article I of the CTBT bans nuclear explosions altogether. Unfortunately, it does no define them. However, there is no technical basis on which laboratory thermonuclear explosions can be excluded from this ban.

11. Our review of the CTBT indicates that experiments, and hence facilities, designed to achieve thermonuclear explosive ignition are illegal. This includes large laser facilities such as the NIF in the US and the Laser Mégajoule project in France, as well as the planned wire-array z-pinch facility called X-1. However, this question is still a matter of international debate and controversy because there is no official public negotiating record on this issue in the specific context of the CTBT negotiations that led up to Article I which bans all nuclear explosions.

12. There is as yet no official interpretation of the CTBT in regard to experimental fusion explosions and the facilities designed to achieve them. Hence, the US and other countries are proceeding as if their plans for facilities like NIF and LMJ are legal under the CTBT. An official opinion on this issue is urgently needed. In this context, it is important to note that our research shows that neither facility is essential to maintaining the safety of existing nuclear stockpiles or to their reliability for deterring nuclear attacks.

Recommendations

It is essential to prevent the development of pure fusion weapons. Such weapons would greatly complicate the already difficult task of achieving enduring non-proliferation and complete nuclear disarmament, as required by Article VI of the NPT. A set of policies that restrict explosive confinement research is needed to accomplish this goal:

1. Ignition of the fusion fuel should be used as the definition of a fusion nuclear explosion for purposes of CTBT compliance, by analogy with hydronuclear fission experiments. This would prohibit all explosive ignition experiments. Therefore construction of the National Ignition Facility at Livermore, California, the Laser Mégajoule project in France and planning of all other explosive research facilities designed to achieve thermonuclear ignition should be stopped.

2. The combination of high explosive drivers and the use of tritium in fusion research should be banned.

3. The total fusion energy output in explosive fusion research should be limited to 1014 neutrons per shot. This would prevent attempts to gain useful information by increasing the energy of the driver and fusion energy output while staying below ignition. Explosive fusion research should be distinguished from other fusion research by defining explosions in this context as events occurring in less than one millisecond.

4. The next CTBT conference, which may be held as early as September 1999, should issue a formal opinion explicitly including laboratory thermonuclear explosions within the prohibition of nuclear explosions in Article I of the CTBT.

5. Magnetized Target Fusion experiments that would achieve ignition should be stopped.

6. ICF research and other research not designed to achieve ignition should be reevaluated for its potential to contribute to pure fusion weapons development and that of nuclear weapons that do not require a fission trigger. In the meantime it may be allowed to continue since it is not prohibited by any treaty.

7. Stockpile stewardship programs, under which ECF research is conducted, should reflect the spirit of the CTBT and exclude weapons design aspects.

8. The nuclear weapons states should declare formally that they are not going to design new nuclear weapons or upgrade old weapons. As part of this declaration, they should explicitly renounce the development of pure fusion weapons and all other weapons that do not require fission triggers.

9. A widespread public debate on the disarmament and non-proliferation consequences of pure fusion weapons is needed to forestall the emergence of serious new problems.

10. The CTBT should be ratified by all countries without conditions. In other words, ratification should not be conditional on projects such as NIF and LMJ.