The tritium factor

The awesome power of modern nuclear arsenals conceals a deep vulnerability. Sophisticated nuclear weapons are dependent upon tritium, a rare radioactive gas that decays inexorably by 5 per cent per year. Unless their tritium is replaced, the destructive capacity of the weapons wastes away.

Tritium dependence is a serious problem for the United States, which has been unable to produce any since 1988. It may also become a troublesome issue for the UK, whose tritium is produced in ageing reactors at Chapelcross on the Solway Firth in southern Scotland. On the other hand, control over tritium could help enforce comprehensive nuclear disarmament. It offers a way of dealing with the fear that a nuclear state might circumvent world-wide disarmament by hiding away a few of its weapons.

Surprisingly little has been published on the role of tritium in nuclear weapons. Attention was drawn to the issue in an article published in New Scientist in 1984 by Tom Wilkie (now the Independent's science editor). Tritium decay means that "old age can kill the Bomb", he wrote.

The US Department of Energy has just produced a bulky environmental impact statement saying why it needs tritium. All US nuclear weapons are now tritium-dependent, it says. Currently, the US is managing to live off its tritium stockpile, recycling the tritium from warheads decommissioned as part of the arms-control process. However, this source will start to run dry early in the next century.

Tritium is a form (an isotope) of hydrogen, the simplest element. Hydrogen atoms usually consist of a single electron orbiting a single proton. Tritium's nucleus contains two neutral particles (neutrons) as well as the positively- charged proton. The different nucleus gives tritium a fatally attractive property. Under extreme temperatures and pressures, tritium atoms fuse with deuterium (another form of hydrogen, containing one neutron) to release both neutrons and large amounts of energy.

This fusion reaction is the key to the hydrogen bombs that are far more destructive even than the atomic bombs used at Hiroshima and Nagasaki. However, the Department of Energy needs tritium not so much for the explosive power of hydrogen bombs but for its role in "boosting" atomic explosions, which in turn are used to set off the fusion reaction which powers the hydrogen bomb. In atomic bombs, a shell of chemical high explosives is used to compress a spherical core of uranium or plutonium. Even in 1945, however, weapons scientists knew that the resultant nuclear chain reaction could be intensified greatly by injecting a mixture of tritium and deuterium into the core. The interest is not in creating a mini-hydrogen bomb but in the neutrons the fusion produces: this accelerates the chain reaction in the uranium or plutonium so making the fission bomb more efficient.

Boosting was first employed in US and Soviet nuclear tests in the early 1950s. Only with boosting is it possible to build small, powerful weapons that can be carried by cruise missiles or multi-warhead ballistic missiles. The first atomic bombs weighed around four tons, and a heavy bomber was needed to deliver each one to its target.

Military tritium has traditionally been made by irradiating capsules of lithium in a nuclear reactor, then extracting the tritium in a specialised separation plant. However, by the late 1980s safety problems had closed the last of the reactors at Savannah River, South Carolina, used to produce tritium. The US nuclear industry has proposed building a new military reactor, but there are fears that its price tag of $6,000m might escalate uncontrollably.

Instead, the Department of Energy is investigating two options. One is to convert an existing civil reactor (or complete one whose construction has been abandoned). Using a civil reactor to produce military tritium risks blurring the distinction between civil and military nuclear energy central to the Nuclear Non-Proliferation Treaty.

The alternative is an approach as yet untried on a large scale: to produce tritium by bombarding either lithium or one of the isotopes of helium in a massive, new, continuously-operating particle accelerator. This would be 3,940ft long, buried 40-50ft underground at Savannah River. It would take five years to build, cost around $3,000m, and draw up to 550 megawatts of electric power from the grid, enough for a medium-sized city. However, the accelerator lacks the reactor's potential for a catastrophic radiation- releasing accident, and would also produce much less radioactive waste.

If pressure for comprehensive nuclear disarmament were to grow, tritium's significance would be quite different. Rigorous control over tritium could make it significantly harder to cheat on a disarmament agreement. Tritium has civil uses in medical isotopes but the world's largest supplier, Amersham International, uses at most 0.05 grams of tritium a year. This is about one hundredth of the only published figure for military applications: 4g per warhead.

Any realistic agreement to abolish nuclear weapons is likely to be phased in over 20 or 30 years. Over that period, a hidden nuclear weapon would require extensive maintenance. Tritium decay would weaken boosting, reducing the weapon's destructive power. The unboosted yield of a modern American warhead is just 500 tons - less than a fortieth of the explosive power of the Nagasaki bomb. So a violator might feel compelled to hide away not just weapons, but also a stockpile of tritium. Because that stockpile would decay, the violator would also need to try to hide (or covertly construct) a facility for separating out the decay products, purifying the tritium, and sealing it into pressure vessels ready for use.

Less advanced nuclear states probably possess simple, unboosted weapons that do not use tritium. However, such weapons are larger and harder to deliver to their targets. They also have vulnerabilities. Most nuclear weapons programmes have begun with designs that use radioactive polonium to produce a sudden reaction. Polonium decays much faster than tritium, so stockpiling it for decades is impossible.

Tritium controls will never on their own make nuclear disarmament watertight. However, these controls are worth investigating as supplements to the more usual ways of enforcing disarmament. Tritium can be an instrument for peace as well as a tool of war.

The writer holds a personal chair in sociology at the University of Edinburgh. He is the author of `Knowing Machines: Essays on Technical Change' (MIT Press).