For half of the 20th Century, nuclear physics has held the prime position as the scientific discipline which produced weapons of unchallenged power. As a branch of theoretical physics it has made great strides since the first development of atomic weapons. To what extent have those subsequent discoveries had a military utility, and what further military applications are in prospect for the future, now that the Cold war is over?

The story of the development of the atomic bomb is well known. The first experiments in radioactive decay took place at the end of the last century. Einstein's theoretical formulation of the equivalence of mass and energy predicted that, if mass could be converted into energy, the atom bomb was a theoretical possibility. Atomic theory brought new insights into the nature of the atom, and developed hand in hand with quantum theory to predict atomic behaviour. By 1933 enough pieces of the jigsaw were there for one scientist to predict that "a chain reaction might be set up if an element could be found that would emit two neutrons when it swallows one neutron" (1). The discovery that Uranium atoms did just this came in 1939, and from then on it was as much an engineering problem as a physics problem to produce the fission bomb.

Much greater mass change, and hence energy change was theoretically possible through the fusion of lighter elements into heavier ones, than through the fission of heavy to light. The difficulty in fusing atoms together was that very high temperatures were necessary to trigger the process. This was the mechanism which fuelled the stars. The development of the thermonuclear bomb in the early 1950s demonstrated that fusion was achievable by using a fission weapon to provide the detonator. This meant that the yield of any nuclear weapon could be increased virtually without limit. Less obvious at the time, but of greater significance subsequently, has been the ability to make nuclear weapons smaller in size and of lower yield. The nuclear artillery shell, the missile warhead and the multiple warhead all became possible.

Military needs pushed forward nuclear physics and nuclear engineering in the early days. The construction of early nuclear power stations stemmed from the need to produce fissile material. Since then the fields of theoretical nuclear physics have overlapped more and more with those of high energy physics which were considered in Chapter 5. Nuclear tests continued, and for a brief period in the 70s the prospect of an enhanced radiation weapon (the Neutron Bomb) excited popular interest. Nuclear physicists had not been idle in the intervening years. The destructive power of nuclear weapons depends on the combination of a number of different effects. Typically the energy released by a fission type explosion is made up of 50% blast, 35% thermal radiation, 5% prompt radiation and 10% residual radiation. If a pure fusion weapon were possible then the proportions might be 20% blast and thermal, with the major 80% of energy being released as prompt radiation, and very little residual radiation. However, the pure fusion weapon is not as yet possible, and developments aimed at altering the balance between the fission trigger and the fusion element of the bomb have only modified the radiation element by a small margin. The enhanced radiation 8 inch artillery shell was claimed to only produce 10% enhancement in prompt radiation (2). The advantages of a weapon which leaves less residual radiation, and has a more precise blast effect, was to make its use appear more credible to an enemy. This was seen as particularly necessary in the case of short range weapons where the effects on friendly forces are critical. Yet the difference between the enhanced radiation weapon and the normal atomic shell is only at the margins. Indeed this appears to be the difficulty in many of the possible improvements in nuclear capabilities. Warheads can now be made as small as necessary, with whatever yield is required, and the development of different nuclear mechanisms seems to have little advantage to offer. There was a time when the atomic bullet seemed a logical development of the smaller and smaller atomic devices, but now the strategic realities of the command and control of nuclear weapons makes this a route of little interest.

The enhanced radiation weapon was seen by some to offer a significant improvement in nuclear deterrent capability. The prompt radiation would disable troops even in tanks, without the penalty of collateral damage and long term radiation. One route to achieve this would be through a pure fusion weapon. Current thermonuclear weapons gain their energy from Lithium Deuteride producing Tritium after irradiation by the neutrons from the Uranium or Plutonium fission reaction; and subsequent fusing of the Tritium isotope of Hydrogen into heavier elements. The energy in the fission bomb provides the temperatures and pressures necessary for sufficient time to cause the fusion process, which then releases the high energy associated with thermonuclear devices. Much development work is in progress in the civil sector towards taming the fusion process. As considerably more energy is released in the fusion of atoms than is necessary to initiate the process, there appears to be the prospect of limitless cheap power. While success in this field would have very significant implications for power generation (see Chapter 5), it is unlikely to affect nuclear weapon design. The two fields, while overlapping, have different objectives: the military seeks instantaneous fusion power; the civil seeks controlled prolonged fusion power. Currently one area might serve both: Laser-initiated fusion. The concentration in energy possible with high power lasers seems to offer the prospect of containing sufficient energy to initiate fusion. The successful development of a civil power generator based on such a device might well offer possibilities for a pure fusion weapon. The other routes being followed by civil researchers follow the large scale engineering inappropriate to weapon development.

While a pure fusion weapon could offer advantages in cost, size, lack of residual radiation, and a move away from dependence on Uranium and Plutonium, there are a number of areas for research into improvements in the fission and fission-fusion weapons of today. The enhanced radiation weapon has demonstrated that it is possible to optimise the weapon for a particular form of damage (although the other forms of damage are not eliminated. All nuclear weapons have similar effects (3).The balance of damage caused by heat, blast and radiation can be changed by changing the yield or by altering the height of the detonation between a ground burst and a height at which the fireball no longer touches the ground. Yet in every case the energy is radiated out as an expanding sphere until it meets resistance. A bullet is highly directional. Anti-tank shells have shaped charges to penetrate in the direction which will give most lethality to a tank. Mines are arranged to fire their charges up into the vulnerable parts of vehicles. Yet nuclear weapons remain like the cartoon anarchist's bomb: equally destructive in all directions. This suggests that energy is wasted and collateral damage is more difficult to control. One can visualise targets in which an asymmetry of nuclear effect would be advantageous. A nuclear weapon designed to destroy enemy missile silos would be improved if the nuclear effects could be channelled forward in a cylinder with dimensions related to the accuracy of delivery. A battlefield nuclear weapon would be improved if all its effects could be concentrated in the enemy's direction without danger to friendly forces. There seems no reason why development of such asymmetries in nuclear effects could not be developed along the lines of the shaped charges of conventional explosives. The question is whether the advantages derived are sufficient to outweigh the costs, both political and financial, of such development.

The next area to consider for nuclear weapon development is whether the primary and secondary effects of a nuclear explosion are amenable to change, and if so what advantages can be gained. Looking at the possibilities for the future, one writer (4) , in the last days of the Cold War, identified 15 different areas where selective enhancement of nuclear effects could improve capabilities. By changing the average molecular weight of the materials in the weapon, the surface area and the energy distribution, the device could be optimised for the production of X -rays. Such a weapon detonated in space could be used to damage ballistic missiles during their exo-atmospheric phase and against satellites. Work had also been in progress on the X-ray laser which would be excited by the nuclear explosion. This would allow much greater concentration of X-ray energy flux, assuming that it could be directed against the target in space. X-rays are absorbed by the atmosphere, although generating an Electromagnetic Pulse (EMP) in the process. This EMP can effect electronic devices, and radio communications. The weapon might instead be optimised for Gamma ray, microwave, infra-red, radio frequency or visible light emission. Such devices exploded in space or in the atmosphere will have different characteristics. If the radiation is focused or directed, it can be more damaging but requires accurate control. The EMP effect can be optimised in a number of ways. The type of fall-out could be tailored to the require effect. The bomb debris could be transformed from the minor part it plays currently, into a directed projectile. Were such a development possible, a nuclear projectile (one in which the motive force is provided by a nuclear explosion) might be able to shoot down vehicles in space and the upper atmosphere.

There is therefore the possibility of a large number of avenues for the development of nuclear weapons, and most would require exclusively military funding. For Western nations, given the political realities of the use on nuclear weapons, there seems little point in devoting great resources to fields where, whatever the military potential, the weapons could not be used. Yet neither the end of the Cold War nor the 1995 Non Proliferation Treaty conference have brought a worldwide diminution of interest in nuclear weapons. France continued nuclear tests in the South Pacific, despite worldwide condemnation. It has however said that it will accede to a Complete Test Ban Treaty. China continues nuclear weapon tests, and indicates that it will need to carry out peaceful nuclear explosion in any event. Beyond the five declared nuclear states of the United States, Russia, United Kingdom, France and China, there are a number of other states with an interest in nuclear capability. One study (6) identifies Israel, India and Pakistan as de facto nuclear states. Argentina, Belgium, Brazil, Germany, Iran, Iraq, Japan, Netherlands, North Korea, South Africa, Sweden and Switzerland are all assessed as capable of becoming nuclear weapon states relatively easily. Ukraine, Kazakhstan and Belarus still have access to Soviet nuclear systems. There are also concerns about the future use of nuclear material by terrorists. Nuclear weapons therefore remain an important area of concern for security policy, and nuclear physics will continue to contribute to the development of defensive and offensive measures.

In any analysis of the most productive areas for research in military uses of nuclear physics, the political dimension must be considered. The Partial Test Ban Treaty of 1963 inhibited the development of weapons where their interaction with the atmosphere is of a crucial importance. The Outer Space Treaty of 1967 has had a similar effect on those nuclear weapons which capitalise on positioning beyond the atmosphere. The Nuclear Non-Proliferation Treaty (NPT) was extended indefinitely on 11 May 1995. An important element of the agreement was the requirement for the signing of a Complete Test Ban Treaty (CTBT) by 1996. Nevertheless computer modelling, using the data from recent underground nuclear tests can produce results for some novel nuclear weapon development. In addition, nuclear weapons can be improved, within the constraints of a CTBT in exactly analogous ways to conventional weapons. They can be delivered with greater accuracy over greater ranges in shorter response times: the technologies necessary are all non-nuclear. What nuclear physics can provide is a weapon which is optimised for particular targets, improvements in ease of production, cost and maintenance of the weapons, and of particular importance improved weapon safety.

Within the area of target optimisation, it is likely that an intense area for research will be the generation of electromagnetic energy for offensive purposes. As dependence on electrical and electronic systems increases, the vulnerability to enemy induced failures of these systems also increases. The EMP associated with a nuclear explosion was a cause of considerable concern during the Cold War era of mutual deterrence, and great expenditure was incurred in taking protective measures. The EMP effect is greatest in exo-atmospheric or high altitude explosions. There are two different EMP generators. The plasma passing through the earth's magnetic field causes a broad band surge of electromagnetic radiation. In addition the ionising of the atmosphere by gamma and X-rays produces a current from the flow of electrons, which in turn produces electromagnetic radiation. It is however conceivable that a weapon could be designed to produce much more of its energy as a narrower band of electromagnetic radiation. If the wavelength is selected so that propagation in the atmosphere is unimpeded, then protection becomes much more difficult, and the yield of weapons can be much reduced for a given damage level. Microwave radiation in the centimetre wavelength band seems particularly well suited to this role. Even a relatively small proportion of a nuclear explosion that could be channelled into microwave radiation could wreak havoc to computer systems, microprocessor control systems, power sources, communication systems, and all things electrical. This would have considerable implications for command and control arrangements. Lack of practical testing following a CTBT may mean that research effort in this field will be redirected towards non-nuclear EMP generation.

Nuclear physics may have significant implications for nuclear weapons is in the manufacturing processes. The relatively slow proliferation of nuclear states has to a considerable extent been a function of the difficulties in producing weapon grade nuclear material, and the engineering problems in producing a reliable weapon. The processing to separate different isotopes of Uranium has been a complex and expensive undertaking involving a highly specialised engineering and chemical facilities. Research into simpler methods for normal industrial use will inevitably make the technology available to a wider number of nations. The break up of the Soviet Union has led to considerable concern about the potential for smuggling of weapon grade fissile material to prospective nuclear states. In addition the civil research into fusion power previously discussed may lead to simpler forms of nuclear weapon, which can be more easily manufactured. Such developments might have significant implications for the availability of nuclear weapons. Recognising these dangers in a world which has seen continuing conflict in the 1990's, The Bulletin of Atomic Scientists advanced its Doomsday Clock 3 minutes closer to midnight in December 1995. It had stood been put back to 17 minutes to midnight in 1991 reflecting the end of the Cold War. (7)

While there is some force in the argument that deterrence continues to operate between states with nuclear weapons, the same is not true for terrorist groups. It is clear that individual members of some groups have no concern about their own or their colleagues demise, and that they have no centre of value which can be held at risk for deterrence purposes. What role has research to play in countering the threat of nuclear terrorism? First, it is worth examining the type of nuclear threat that may be made. While the production, or theft, of an operable nuclear weapon is not to be discounted (8), it is highly improbable, given basic security measures that are in all States' interests. Nor would the highly technical production methods, or the highly defended nuclear armouries appeal to terrorist planners. Far more likely is the use of nuclear material, in however small quantities, as a contaminant for normal explosives. The targeting by terrorists of nuclear sites, both civil and military, with conventional explosives is also a possibility. Counters to all these possibilities are more in the realm of conventional security measures than the product of nuclear physics research.

Turning away from the weapons aspects of nuclear physics, there are other influences which this branch of research can have on security interests. Nuclear power has had a somewhat chequered career in the civil sector, and the problems of safety and waste disposal continue to cause disquiet, and hence slow the rate of development. On the military side the cost and constraints of nuclear power systems has restricted their use to where the operational need is over-riding, as in nuclear submarines, or where the power required can be produced economically, as in aircraft carriers. The developments in controlled fusion power already discussed may change the balance of advantage both in the civil and military sectors. This however is in the longer term, and currently the size of potential fusion power generators gives little hope for them being integrated into mobile systems. In any event, the research being undertaken in the civil sector will allow the military possibilities to be anticipated.

One aspect of nuclear power which does have strategic implications is the use of civil nuclear power stations as targets. The destruction of such a facility by conventional weapons would allow an enemy to generate many of the effects of a nuclear strike, without using nuclear weapons. The long term and widespread effects of the fire at the Chernobyl plant in 1986 indicated what a major disaster could be caused by a damaged nuclear power station. Two connected areas of research can thus be useful: one offensive and one defensive. Custom made conventional weapons designed to cause a nuclear power plant to run out of control would be complex, and would depend on the design of the various safety features in the target. The destruction of the protective containment vessel would stop power production and cause local contamination. For a Chernobyl-scale effect, the safety features must be countered. This then leads to consideration of defences against such weapon developments by a potential enemy. There are passive measures such as the siting of power stations in isolated locations, and taking regard of prevailing winds. These are sensible precautions in any event. What is also necessary at the design stage of any nuclear power station is consideration of the redundancy and hardening of all safety features against a conventional weapon attack. In this regard over-insurance is to be encouraged, as modifications at a later stage will be very difficult. The use of nuclear power stations as weapons which bridge the nuclear threshold may have received insufficient attention in the past.

In summary, despite the important contribution that nuclear physics has made to weapon development in the past, it looks as though its future military application is much more limited. If the hopes of the NPT are fulfilled, there will be a continuing decline in the utility of nuclear weapons.

NOTES

1. Leo Szilard quoted in From Crossbow to H-Bomb by B & F M Brodie.p236

2. 'Enhanced-Radiation Weapons' by F M Kaplan in Scientific American May 1978 pp44-51.

3. For a technical description see 'The Effects of Nuclear War' by the Office

of Technical Assessment.

4. 'Third Generation Nuclear Weapons ' by T B Taylor in Scientific American

April 1987 pp22-31

5. Even the potential of laser fusion seems to require enormous supporting equipment. See 'Progress in Laser Fusion' by R.S.Craxton, R.L.McCrory & J.M.Soures, in Scientific American, August 1986 pp 60-71. This paper also gives a good description of the nuclear processes

6.Nuclear Wastelands by A.Makhijani, H.Hu & K.Yih (The MIT Press 1995) pp14-15.

7. For a discussion on the detailed reasoning behind the decision see The Bulletin of the Atomic Scientists March/April 1996 pp 17-23.

8. The feasibility of nuclear weapon production by small groups is discussed in The Nuclear Dimension by A.Loehmer in Technology and Terrorism by P.Wilkinson (Frank Cass, London 1993) pp 48- 59.