A chilling wind blows in from the Gobi desert: Tests must halt if we are to stop the spread of the bomb, says John Hassard

LAST week the Chinese government carried out its 38th nuclear weapons test - the country's 16th underground nuclear explosion. The bomb had an explosive power equivalent to about 20,000 tons of TNT - approximately the same size as the one that destroyed Hiroshima.

Last week too, President George Bush signed a bill imposing an immediate nine- month moratorium on US nuclear testing - a moratorium that effectively includes Britain, because the UK depends on the use of American facilities in the Nevada desert to carry out tests of nuclear weapons. France and Russia have already declared unilateral moratoriums on nuclear tests, so four out of the five nuclear powers have stopped testing.

For the first time since the nuclear age dawned with an explosion at Alamogordo in the New Mexico desert on 16 June 1945, there is the possibility that all nuclear testing might be ended - a move that would revive efforts to stop the spread of nuclear weapons to other countries and could limit the manufacture of new and ever more sophisticated weapons by the acknowledged nuclear nations.

But the moratoriums are only temporary, and the signs are that the Chinese, far from ending their tests, intend to carry out another one soon. The issue is urgent, because the main international bulwark against the further spread of the bomb, the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), comes up for renewal in three years' time.

The site of the Chinese explosion was a bleak granite ridge rising to 8,800ft (2,700m) in Xinjiang province, one of a series of ridges pushing east into Kazakhstan and north into Mongolia. They rise like islands above the dunes and saltpans of the Gobi desert.

China's nuclear technicians have dug a long, gently downward-sloping tunnel into the ridge's side, just below its crest. They have dumped the excavated rock in screes down the valley. Giant pockmarks scar the landscape; here and there the sand has turned to glass, and monstrous blemishes have scorched the rock.

On 25 September, just one second before 4pm Nanking time, the ridge shook and a low rumble was heard above the wind. Ripples of sound spread out round and through the Earth to arrays of listening seismographs thousands of miles away. Computers studied their shapes and types, and homed in on the end of the tunnel in that barren ridge.

Six countries have openly tested nuclear weapons: the United States, the Soviet Union, Britain, China, France and India (although India claims it was a 'peaceful nuclear device'). Other countries have weapons but know they do not even need to test them, unless it be for some political reason. Others have tested all the non-nuclear components of nuclear weapons, including non-nuclear detonations. All those countries with open or covert programmes have the necessary delivery system, whether it be intercontinental ballistic missile, jet aircraft or converted tugboat. Some obtained nuclear weapons because they wished to continue eating at the diplomatic top table, others because they wished to be invited to it.

The NPT, which came into force in 1970, is the most visible international stricture on the spread of nuclear weapons. Each of the parties to the treaty undertakes to pursue effective negotiations 'in good faith' on measures to stop the nuclear arms race at an early date and to bring about nuclear disarmament. Members of the NPT are also supposed to work towards 'a treaty on general and complete disarmament under strict and effective international control'.

Until the Start treaty in July 1991 there had been no real sign of any members of the nuclear weapons 'club' even noticing this last provision. And there is one issue that really galls the nuclear 'threshold' nations: that of nuclear testing. Short of dropping a nuclear weapon on somebody else's city, testing is the most potent way of showing you have the bomb and that you are trying to improve it.

Testing is seized upon by countries without nuclear weapons as a sign of the nuclear powers' hypocrisy. And threshold countries such as India regard the NPT as nuclear apartheid, designed to maintain the asymmetric status quo and suppress the honest aspirations of developing countries.

What further annoys the nuclear have- nots is the fact that, while the nuclear haves have been trumpeting the virtues of the NPT, they have secretly been allowing the export of nuclear weapons technology to those willing to pay enough. In recent years, the league table of guilty parties has been topped by Germany, Britain, the Soviet Union, the US and France. Even the revelations about the Department of Trade and Industry's complicity in dealing with Iraq appear not to have dampened our ardour in selling what we can where we can.

China, since 1984, has maintained a 'discreet and responsible attitude so as to ensure that (nuclear) co-operation is solely for peaceful purposes . . .' according to Jiang XinXing, head of China's nuclear industry. But it was not always so. China's past history of nuclear trade with Pakistan, only parts of which have been uncovered so far, is particularly interesting.

Until President Bush signed the American test ban, those who urged nuclear restraint had little credibility because Britain, the US and China continued to violate the spirit of the NPT by testing weapons.

A Comprehensive Test Ban Treaty (CTBT) would help, but could it be verified? Using only commercial satellite imagery, Vipin Gupta, of Imperial College, London, published a prediction a month ago that China was about to conduct a test, and a week ago Imperial 'heard' its echo, using seismic data. This was a very small test, far smaller than the last Chinese test on 21 May. And China made no particular attempt to hide the test - why should it, since it was breaking no agreement? Militarily significant nuclear tests can be detected by present technology, but given a sensible upgrade of present systems the ability could be vastly enhanced. (Each test costs some dollars 70m, so the savings from a CTBT could finance the verification hardware and personnel needed).

This is not just an academic exercise: we are running out of time. If the NPT is to have any chance of surviving and being made workable beyond 1995, the international community needs to solve the problems shown up by Iraq. Establishing a CTBT is not the whole solution, but it is an essential first step. Rapid growth in the number of nuclear haves is frighteningly close. After that, the spread into sub-

national groups will turn inevitably from nightmare into reality.

The writer is a nuclear physicist at Imperial College, London, and a consultant to the Verification Technology Information Centre.

(Photograph omitted)