The moratorium ends 1 July, and this week President Clinton must decide whether to allow testing to resume.
Once again the President is under pressure. If he gives the go- ahead, he will disappoint a block of Democratic senators in the Congress and most members of the House of Representatives. To resume testing, they argue, would be to send the wrong signal about the administration's intentions to work towards a worldwide ban on tests and to stop the spread of nuclear weapons. The Nuclear Weapons Non-Proliferation Treaty expires in 1995.
If he says no, he risks upsetting the generals in the Pentagon and America's special relationship with Britain. Britain's generals, like their counterparts in the Pentagon, say they need to test to keep Britain's nuclear deterrent safe and reliable.
The Congressional moratorium provides for testing to be resumed after 1 July, only if it is 'in the interests of the United States'. Even the British testing has to be seen as being in America's best interests. Mr Clinton's national security advisers have told him that testing is necessary, and they have recommended a three-stage plan.
The first stage is for three tests of the new US W80 nuclear warhead for an air-launched cruise missile. The aim is to improve the weapon's resistance to accidental launch through fire. The second three explosions would test the reliability of the W86 and W87 warheads for the Trident submarine-launched missile and the W87 on the intercontinental MX missile. Critics of the programme say the tests are not needed, and the improvements would not be fitted to the missiles in any case because of lack of funds.
The last stage would test Britain's ageing WE177 air-dropped bomb, which lacks some of the basic safety features of the American weapons, such as the latest type of non-sensitive high explosive. At least one of the three British explosions would test the British-designed warhead for its new Trident submarine missiles.
The nine-test plan is supported by the US Department of State, which is seeking to keep good relations with Britain; the Pentagon, which is recommending the tests but does not want to pay for them from its reduced coffers; and, of course, the US nuclear weapons laboratories, which face severe cutbacks if they do not create some work for themselves.
The leading US government department against the tests is the Arms Control and Disarmament Agency, which believes the reasons given by the generals and the weapons labs for testing are fraudulent. The Department of Energy, which oversees the production of nuclear weapons, does not consider the cost to be justified. They are concerned that the nine-test plan would give the wrong signal to the other nuclear weapons states - Russia, France and China - and to those who are not yet nuclear weapons powers but have the capability, such as India, Israel and Pakistan.
There have not been any nuclear tests anywhere since last summer when the US conducted an explosion shortly before the Congressional moratorium went into effect on 1 October. China held a nuclear test several months earlier - believed to have been one of the biggest ever. Britain postponed its test.
A measure of how difficult it is to move the supporters of testing beyond the Cold War mentality and towards a comprehensive ban is that the US Army still maintains a squad of nuclear weapons experts ready to test again in the atmosphere. Such tests have been banned since the Limited Test Ban Treaty was announced by President Kennedy 30 years ago, but the US has been spending an estimated dollars 20m each year to keep the so-called 'Safeguard C' project going.
Crews have been on standby at the Nevada test site, in Hawaii and on a remote Pacific atoll where bombs used to be exploded before they were forced underground by the 1963 treaty. Congressional aides say it has probably cost more than dollars 1bn, but because of the secrecy surrounding the nuclear weapons programme the project has never had an official budget or an audit. 'Safeguard C' was an oddity of the American nuclear deterrent, an insurance policy against Soviet blackmail. It was part of the package the US generals insisted on, in return for endorsing the limited test ban in 1963. The 'Safeguard C' crews are 'firemen who sit in the firehouse and hope there will never be a fire', said Roger Ray, a physicist who helped to run the project for two decades.
From 1946 to 1962, the US conducted 200 atmospheric tests in the Pacific and Nevada, causing serious radiation problems for the Pacific islanders, and for the Americans who lived downwind from the test site, mostly in Utah. Britain exploded 21 bombs in the atmosphere from the Pacific islands before it joined the US underground testing programme in Nevada. Since 1963, it has exploded 21 weapons underground in Nevada under the special 1958 US-UK agreement. Most of the tests for safety and reliability are executed in vertical shafts 1,000- 2,000ft deep and 12ft wide. In some cases, so-called 'weapons effects' tests used horizontal tunnels to gauge the effects of nuclear blasts on other weapons. The tests cost Britain between dollars 30- dollars 40m each, which, as they say in Nevada, is a very expensive hole in the ground.