Over the decades the information provided by nuclear tests has enabled the development of an enormous battery of techniques for the design of nuclear weapons, including theoretical methods and calculations, computer codes, and diverse kinds and sizes of laboratory experiments. While full-scale testing of nuclear weapons is the one way in which all the relevant characteristics of a new warhead design can be definitively determined, new weapons can be designed without full-scale tests. The degree of confidence in the functioning of a new warhead that has been designed without full-scale tests depends on (i) the range and sophistication of the techniques that are used in the design process, (ii) the complexity of the design, and (iii) the relation of the new design to the designs of warheads that have already been tested.

Techniques for Warhead Design

There are seven broad categories of techniques that can assist in the design of new warheads without full-scale testing:

1. Nuclear explosions ranging from a few tens of pounds to a few hundred tons of TNT equivalent or less that are not quite full-scale explosions, but which yield most of the crucial information about the functioning of the weapon, other than its exact explosive yield.
2. Small-scale nuclear explosions with a nuclear yield of a few tens of pounds or less (hydronuclear testing).
3. Tests of many of the properties of nuclear charges using materials that cannot sustain chain reactions (hydrodynamic testing).
4. Experiments in nuclear fusion to develop understanding of the thermonuclear component of weapons as well of the deuterium-tritium boosters that make the fission components of warheads more efficient.
5. Computer modeling.
6. Theoretical models and calculations (other than computer models).
7. Other related experiments, field tests, theoretical work, and modeling exercises, for instance using nuclear reactors, conventional explosives, etc. to determine the properties of various components and subassemblies of warheads. This includes work on basic science in various disciplines such as nuclear physics and radiochemistry.

Nuclear weapons have been successfully designed without full-scale tests. In fact, the design of the bomb dropped over Hiroshima was not tested prior to its wartime use. That is because Manhattan Project scientists and engineers were very confident that the relative simplicity of the "gun-type" design combined with the various theoretical, laboratory, and non-nuclear field tests they conducted were sufficient to guarantee success. In contrast, they were far less sure of the implosion design that was needed for the plutonium weapon. One reason was that the timing of the firing of the conventional explosives was so critical that they could not predict the performance of the weapon based on theoretical considerations and laboratory experiments and non-nuclear field tests alone.

Hydronuclear and hydrodynamic testing

Nuclear weapons designers have been using hydrodynamic testing as well as full-scale testing for designing new weapons, which includes ensuring their safety. Since full-scale testing would be ended by a comprehensive test ban, some scientists claim that testing at some level, such as hydronuclear testing, is essential for determining the safety of nuclear weapons. In particular, such testing can be important for helping to determine what is called "one-point safety" or "single-point safety" of warheads in the absence of full-scale testing. One point safety means ensuring a nuclear explosion will not result if any point on the conventional explosive that surrounds the fissile material were accidentally detonated. The purpose of determining one-point safety is to help prevent accidental detonations of nuclear weapons. The United States has used nuclear tests extensively to determine one-point safety since 1955. During the 1958-1961 moratorium, Los Alamos put together a program for hydronuclear testing for studying one-point safety. The risks of a failure to determine one-point safety prior to putting a warhead into production have therefore been recognized for well over three decades.

A recent report of the Natural Resources Defense Council set forth what can be learned about nuclear weapons design at various levels of nuclear explosive yield (expressed as equivalent weights of the conventional explosive TNT).(1)

• a yield of less than about one-tenth of a pound: information about one-point safety;
• a yield of less than about half-a-pound: information about the achievement of criticality (needed to initiate the nuclear explosion);
• a yield of under four pounds: criticality tests, measurements of temperature and pressure conditions as criticality is achieved, and other essential design information on the characteristics of the weapon at the very start of the nuclear explosion;
• a yield of a few pounds to several hundred pounds: data enabling estimation of the yield of the weapon, and all the other data from lower yield tests;
• a yield of a few tens of tons: development of advanced weapons consisting of fissile materials only (that is no thermonuclear component);
• a yield of a few hundred tons: most of the essential data about boosted fission as well as thermonuclear weapons.

The United States would carry out its hydronuclear testing program at the Nevada Test Site. This would be in addition to its extensive hydrodynamic testing program at Los Alamos and Lawrence Livermore National Laboratories. The main stated public official purpose of these facilities is to ensure "safety" and reliability" of the U.S. nuclear arsenal. The devices could also aid in designing new weapons. The U.S. is also building an advanced hydrodynamic testing facility called Dual-Axis Radiographic Hydrotest (DARHT) facility at Los Alamos. The term "dual axis" refers to two X-ray machines that would be built to photograph the interior of materials being compressed to represent a nuclear warhead pit. The materials tested in DARHT could be a dense non-radioactive element used to simulate the pit, depleted uranium, or perhaps even plutonium-242. (Photographs with X-rays are called radiographs.) The X-rays are generated by creating powerful electron beams in an accelerator and stopping the beam in a tungsten target.

DARHT is to be built in two stages, with one-X-ray machine being built by 1997 and a second to be added (if the first works well) by the year 2000. The two independent axes of observation will enable three-dimensional observation of the compression of materials simulating the pit of a warhead. This could possibly provide far more data for warhead design. Construction of DARHT began in 1994 without a full environmental impact statement. It was stopped in January 1995 by a court order pending completion of an environmental impact statement.

The utility of DARHT for its stated safety and reliability purposes is a matter of some dispute within the nuclear establishment. Los Alamos is, of course, convinced of the need for it. But a 1992 Sandia National Laboratory report stated that the aims of the first part of DARHT could be accomplished by an $8 million upgrade to the FXR machine at Lawrence Livermore National Laboratory (compared to the $85.6 million cost of the first part of DARHT) and that for reasons dealing with uncertainty of performance and other factors, the second arm of DARHT should be postponed.(2) The total estimated cost of DARHT is $123.8 million. The labs also want an even more advanced hydrotest facility (AHF), with four to six X-ray beams, currently estimated to cost $422 million.

Laser Fusion

Another new facility, called the National Ignition Facility (NIF) is proposed to be built at Lawrence Livermore National Laboratory. This has not yet obtained final approval, pending the outcome of a technical study. NIF is a larger version of a laser fusion machine that already exists at Livermore.

Laser fusion is a process in which powerful lasers are simultaneously focused on a minute pellet of tritium and deuterium, raising temperatures to levels comparable to those in the interior of the sun. This initiates a tiny thermonuclear reaction, which is essentially a very small scale version of a thermonuclear bomb. The process is also called inertial confinement fusion (ICF). The often-stated purpose of such experiments for over two decades has been to develop a device for generating electricity from fusion. But, while the scientific and commercial feasibility of this or any other method of generating electricity from fusion is decades away at best, the more immediate weapons applications of inertial confinement fusion have been officially acknowledged.

According to a Livermore document about NIF, the inertial confinement fusion program has, besides its potential application to laser fusion power generation, "an essential role in accessing physics regimes of interest to nuclear weapons design and to provide nuclear weapon related physics data, particularly in the area of secondary design." It would also "provide an aboveground simulation capacity for nuclear weapons effects on strategic, tactical, and space assets (including sensors and command and control)...."(3)

In sum, the new hydrotest facility, DARHT, and the new laser fusion machine, NIF, as well as various other preparations and existing facilities, will enable the United States to continue to design new nuclear weapons and to maintain the capacity to do so for the long term, despite its Non-Proliferation Treaty obligations to pursue nuclear disarmament in good faith and despite the end of the Cold War.

Institute for Energy and Environmental Research

Comments to Outreach Coordinator: ieer@ieer.org
Takoma Park, Maryland, USA

Last updated: August, 1996

1 Cochran, Thomas B., and Christopher E. Paine, The Role of Hydronuclear Tests and Other Low-Yield Nuclear Explosions and Their Status Under a Comprehensive Test Ban. Washington, D.C.: Natural Resources Defense Council, March 1995, pp. iv-v.

2 Ramirez, Juan J., T.F. Godlove, W. B. Herrmannsfeldt, D.J. Nagel, and P. Sprangle, "DARHT Feasibility Assessment Independent Consultants DFAIC Panel," Sandia Report SAN92-2060. UC-700. Albuquerque, New Mexico: Sandia National Laboratories, September 1992, p. 14. See also DARHT project October 1994 fact sheet from Los Alamos.

3 Lawrence Livermore National Laboratory 1994 document (UCRL-AR-110100-94) as quoted in Andrew Lichterman, Jacqueline Cabasso, and John Burroughs, "Comment of the Western States Legal Foundation on the Scope of the Proposed Proliferation Impact Review for the National Ignition Facility." Oakland California: Western States Legal Foundation, March 9, 1995. p. 7.