Reliable evidence?

Nov 15th 2007
From The Economist print edition
America wants to ensure that its nuclear warheads would go bang rather than pop—but without letting them off to test them

OLD soldiers never die, they just fade away. Old weapons, on the other hand, hang around stubbornly. Those of the nuclear variety left over from the cold war are causing a bit of a nuisance. Thousands of them are ageing in silos. Ensuring that they do not deteriorate and would detonate if necessary is difficult. That is because of the Comprehensive Test Ban Treaty which, as its name suggests, forbids contracted parties from letting off nuclear explosives in peacetime.

Although America has yet to ratify this treaty, its policy is to act as though it had. It stopped the tests of real warheads (such as the one illustrated above) in 1992. That means scientists wishing to find out whether a particular batch is still potent cannot just pluck a warhead at random from the stockpile and try to explode it. One way to overcome this would be to replace the warheads with newer designs that, proponents argue, would not need to be live-tested in this way. The older warheads, the most elderly of which will reach the end of their 30-year design lifespan in 2008, could then be retired without compromising the country's nuclear shield.

The Reliable Replacement Warhead programme, as this scheme is known, has reached a crucial point. A full (and costed) design for the new warheads was supposed to be unveiled in December. However the National Nuclear Security Administration, the part of the Department of Energy responsible for the programme, said this week that the report is now expected by August 2008. Some people suspect it will be delayed yet further by the presidential election next year.

A year's delay will not matter much. But should the programme be cancelled (and funding for it, first authorised in 2005, was all but eliminated by Congress for this financial year), America risks finding itself without enough nuclear-weapons scientists to keep its arsenal in tip-top condition in the future. For the truth is that the Reliable Replacement Warhead programme is also a job-creation scheme, designed to persuade some of the country's best brains that it is worth trading a career in industry for one in national defence.

Testing without testing
Scientifically speaking, the programme's goals look possible. Earlier this year the National Nuclear Security Administration chose a design that it thinks could be developed without any further live tests. In September Jason, an elite group of independent scientists, published its evaluation of what technical information it could assess about this design. It concluded that, in principle, it would indeed be possible to develop a replacement warhead without conducting any new nuclear tests. It recommended, however, that the final design be scrutinised in an independent peer-reviewed process. That would be a first. Allowing outsiders to assess a design for its strengths and weaknesses is not something that the Department of Defence has done in the past—and it is not, at the moment, proposing to change its mind.

According to Bruce Goodwin, who is responsible for nuclear technologies at the Lawrence Livermore National Laboratory in California—which, with the Sandia National Laboratories in New Mexico, put forward the winning design—the proposed replacement warhead is based on a weapon that was tested but not deployed some 20 years ago. The richness of the existing test data is what gave this design its winning edge over an alternative proposed by the Los Alamos National Laboratory (also in New Mexico, and which also had Sandia as a partner). Those data help to give confidence that “virtual” tests, run inside a supercomputer, will produce results that correspond to what would happen if a warhead were tested for real.

The quantities of data involved in such simulated tests are phenomenal. Staff at Lawrence Livermore say it takes their best computers six weeks to simulate what happens inside a warhead when it is going off. Such detailed modelling has only recently become possible. The supercomputers used in the early 1990s, when nuclear testing stopped, would have taken 60,000 years to process the same data.

Part of the reason for this is that a nuclear explosion has three stages. First, a specially shaped charge of chemical explosives surrounding a plutonium pit goes off. That compresses the plutonium while it is simultaneously bombarded with neutrons from a trigger made of polonium, and thus begins the second stage. In response to the neutrons, some of the plutonium atoms split apart, releasing energy and more neutrons. These, in turn, split more plutonium.

This is the famous chain reaction that lies at the heart of nuclear warfare. It is not, however, the source of a modern bomb's main explosive power, for just as the chemical explosives trigger a fission explosion by compressing the plutonium, that fission explosion is used to ignite the third stage, a still-larger fusion explosion, by compressing and heating the main part of the bomb. This is composed of a mixture of deuterium (a rare isotope of hydrogen) and lithium (a light metal). These react to form helium, yet more neutrons and a whole lot more energy. The result, a thermonuclear explosion, is what destroys the target, but the entire three-stage process has to be mimicked if computers are to test weapon-designs reliably.

The models involved in the winning Livermore/Sandia bid are certainly good enough to recreate the results of earlier tests (a trick known as “hindcasting”). Whether they can accurately forecast things, no one knows for sure. But so-called subcritical tests are allowed by the test-ban treaty, and that may add confidence to the process.

Some of these tests involve smashing or shooting at small shards of plutonium. Blowing up little bits of the metal this way, without compressing them in a symmetrical manner, is allowed because it does not result in a chain reaction. And the chemical-explosive detonator can also be tested using “simulants” that are not fissile but mimic the behaviour of the plutonium pit in other ways. Scientists can thus find out whether the charge would have detonated, had it been made of plutonium.

The fusion stage can also be examined within the rules. An enormous—and enormously expensive—system of lasers called the National Ignition Facility is being built at Livermore. It is designed to cause thermonuclear fusion in tiny pellets of deuterium (so small that they would not be covered by the test-ban treaty) and is expected to be completed in 2009.

For weapons scientists this is all exciting stuff. Not quite as exciting, perhaps, as letting bombs off for real, but not a bad substitute. The question for the politicians is whether that excitement—and the personnel and new bombs that will result from it—are worth the money. And that, in turn, depends on just what sort of nuclear arsenal America thinks it really wants.