|
A large qualitative change in the nature of nuclear weapons occurred four-and-a-half decades ago when nuclear fission (the splitting of atoms) and nuclear fusion (the fusing, or joining of atoms) were combined into 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. 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. 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.1 For instance, the destructive area per ton of TNT equivalent of the Hiroshima bomb was about 500 square meters (about 600 square yards), which is 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 (ECF)2 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 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.3 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).4 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.5 A fusion reaction between these two isotopes produces an alpha particle, which is a helium nucleus and a neutron (see diagram below).
 D + T --> 4He + neutron
The total energy released per D-T fusion reaction is 17.6 MeV, most of which is the kinetic energy of the neutron. While not achieving the levels of thermonuclear bombs, laboratory ECF facilities have achieved a significant number of fusion reactions (1012 to 1013 neutrons per shot). 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, light and heavy ion beams, chemical explosives, and 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 viable.
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 (see below for a definition of ignition and of nuclear 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:
Official US planning documents for the Stockpile Stewardship program demonstrate that the 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.6 We note that the DOE has denied that it intends to design pure fusion weapons. But the technical work DOE 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 on the merits of comparison with other approaches to solving energy problems, especially given the enormous expense of these devices and the very long time frame it is likely to take for this research to lead to fruition (several decades or more). There are far more promising approaches to dealing with energy issues than ECF schemes.7 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
CTBT negotiations involved extensive discussion of allowing some fission explosions. Initially, the US wanted the CTBT to allow for hydronuclear testing which would yield up to four pounds of nuclear explosive energy. 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 US 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 ("LMJ"- a fusion research facility 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. |
(Both operating and planned facilities) | |||
| Location | Driver | Operating parameters | Neutron production per shot |
| Sandia National Laboratory (USA) | PBFA-II (light ion beam) | 36 beams 100 TW (design) 10 TW (1988 actual) | unknown |
| Sandia National Laboratory (USA) | z-pinch | 2 megajoules (MJ) 290 terawatts (TW) 140 eV temp. | D-T target not used yet |
| Sandia National Laboratory (USA) | X-1 (successor to z-pinch) (conceptual design) | 16 MJ 1000 TW | projection unknown |
| Europe | Heavy Ion Design for Ignition Facility (HIDIF) (conceptual design) | 48 beams 1 MJ 27 TW | projection unknown |
| Lawrence Livermore Natnl. Laboratory (USA) | NOVA laser | 10 beams ~40-70 kilojoules (kJ) ~100 TW | 108 - 3.6 x 1013 |
| Lawrence Livermore Natnl. Laboratory (USA) | National Ignition Facility (NIF) | 192 beams 1.8 MJ ~360 TW | 1019 (projected under maximum 20 MJ yield scenario) |
| Osaka (Japan) | GEKKO-XII | 12 beams 15-30 kJ 0.1-10 nanoseconds | 1013 |
| Osaka (Japan) | Kongoh (under design) | 92 beams 300 kJ 100 TW | ? |
| Bordeaux (France) | Laser Megajoule | 1.8 MJ 120 TW | same range as NIF |
| VNIIEP (Russia) | Iskra-5 | 12 beams 15 kJ 0.25 nanoseconds | ? |
| Sources: Daniel Schirmann and Mike Tobin, 1996; André Gsponer and Jean Pierre Hurni, 1998; Guillermo Velarde, Yigal Ronen, and José M. Martínez-Val, eds , 1993; R. A. Lerche and M. D. Cable, 1996; Neal Singer, 1998. For complete reference information, see Dangerous Thermonuclear Quest, available from IEER. | |||
|
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, as is the case for all present nuclear weapons, are illegal. 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. 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.8 This is because all nuclear explosions of possible military consequence are expected to occur in well under one millisecond. Other physical criteria are also needed to define a nuclear explosion: Criticality: As we have noted above, the US 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: 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."9 In other words, 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. Ignition: Another criterion which is especially helpful in defining fusion explosions is ignition. It has been defined in two different ways:
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 US government agencies on nuclear weapons issues, would limit neutron production to 1014 neutrons/shot. This corresponds to an explosion of 0.1 gram of high explosives. Since this limit has already been approached by Magnetized Target Fusion experiments (1013 neutrons) and reportedly by Russian high explosive research (1014 neutrons), 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 LLNL senior weapons scientist and one of the pioneers of laser fusion research, would ban tritium use in systems driven directly or indirectly 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 due to the greater difficulty in achieving other fusion reactions, such as the D-D reaction, in sufficient numbers in a single shot.13 High-explosive-driven components will most likely be key to the miniaturization of pure fusion devices - a necessary step towards pure fusion weapons. This potential is the reason behind the proposed ban on tritium in combination with high explosives. 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 (see "Dear Arjun" column). 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 US 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 US 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 (or their equivalents in other countries) would likely exert considerable 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 knowledge 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 towards disarmament. Indeed, as noted elsewhere in this newsletter, 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:
|
Institute for Energy and Environmental Research