THE TRIGGER

When there is enough fissile material for a bomb, a trigger for explode it has to be devised.

Last March British agents seized some electronic components destined for Iraq at London's Heathrow airport. The press variously identified them as capacitors, krytrons or initiators, which are three very different things; but the only important thing appears to be that they were called "nuclear triggers" in Iraqi telephone conversations intercepted by British intelligence, for otherwise they are quite harmless components that can be used for quite different purposes. This lot was apparently meant for ensur-ing a simultaneous explosion that triggers the A-bomb.

A small amount of uranium 235 or plutonium 239 will not ex-plode, because the chain reaction is dependent on the neutrons released by split atoms hitting further atoms, which will split and liberate more neutrons. If too many neutrons escape via the sur-face of the uranium, not enough of them will be left to keep the chain reaction going. Only when several subcritical masses are joined into a more than critical mass will the surface-to-volume ratio become small enough to prevent most neutrons from escap-ing and thus keeping up the chain reaction.

Another way of accomplishing this is to compress the fissile material of an originally subcritical mass so that fewer neutrons are likely to escape from it: a bullet is less likely to escape from a forest without hitting a tree when the forest is denser. This is done by explosives, which not only compress the fissile material, but keep it together in the first few microseconds of the explosion, so that the fissile mass does not blow itself apart without being used up. Timing is of the essence here, and this is what the special (non-in-ductive) capacitors were needed for. The actual mechanism is said to consist of a heart-shaped explosive with a slow detonation rate enclosing a cylinder with the fissile material. The top indentation touches the cylinder, and the rest of the heart is coated with a high-rate explosive. When the latter is detonated at the bottom (point) of the heart, it sets off a fast shock wave that will reach the top of the cylinder at the same time as the slow shock wave will reach the bottom (which is the reason for the heart shape); the result is a uniform and powerful implosion of the cylinder, compressing the fissile material and letting the chain reaction proceed (usually started by a neutron-emitting initiator in the center).

The real point here is not the timing electronics intercepted at Heathrow, but the special explosives that will produce the im-plosion of the cylinder. Saddam has facilities for producing both. What initially gave his bomb program away was the vast explosion that blew up the plant making the slow ("HMX" or high melting point explosive") in August 1989. A journalist for a British publica-tion went to see what was going on, was caught and hanged over the protests of much of the civilized world (but not Bush's gentler kinder White House). The plant was feverishly rebuilt and produc-tion of this and the fast explosive (''RDX,'' rapid detonation ex-plosive) continued until it was presumably taken out in January as part of the "nuclear facilities."

The stockpile of weapons, if any, cannot be very large and if it exists, it is untested. Delivery by the unreliable and imprecise Scuds would be prohibitively wasteful, but as a defense against ground forces the use of a small U 235 bomb is conceivable, especially if detonated "dirtily"' on the ground or under the surface, so to spread comparatively long-lasting radioactivity.

A study of the technology of proliferation should convince anyone that the only lasting solution is political. The problem and its ineffective solutions are akin to gun control and the ludicrous idea of stopping or even curtailing crime by banning guns.

Similarly, terrorist nuclear bombs are technically possible and ever more feasible, with technical fixes and safeguards quite inef-fective; the way to stop them is an international community that will not tolerate terrorism. As of now it does not exist. The US, under Reagan, took no action when its military attache to East Germany was deliberately prevented from receiving first aid and left to bleed to death as his murderers in army uniforms looked on. Britain, under Thatcher, took no meaningful action when a policewoman was shot dead from the windows of the Libyan em-bassy. The French pay thinly veiled ransom for hostages, and the Germans use only slightly thicker veils. All of them treat terrorist thugs like Arafat or Mendela as if they were civilized. Such an in-ternational community of clownish wimps will not only do nothing against terrorism, including rogue nuclear bombs, but is the ideal breeding ground for it.