Atomic Tests: The Blast at Lop Nor

The bleak land around Lop Nor, a salt lake in the Takla Makan Desert of Red China’s Sinkiang province, is one of the most remote and unpleasant places on earth. But last week Lop Nor was suddenly familiar to all the world when President Johnson pinpointed it as the place where the Chinese had conducted their first atomic test.

U.S. authorities have not yet told all they know about the Chinese test, presumably because disclosure would divulge too much about their detection methods, which are extremely efficient. They predicted the blast weeks in advance, reported it almost as soon as it happened, named its fissionable material, estimated its energy and followed the spreading cloud of radioactivity as it circled the Northern Hemisphere on the fast westerly winds that prevail at high altitudes.

Slim Tower. The Chinese test was in the atmosphere; the nuclear device was probably perched on a slim tower several hundred feet high to keep the fireball out of contact with the ground. This type of test, outlawed by the U.S.. Russia and Britain by the 1963 test ban treaty, has much to recommend it to the novice nuclear power. The explosion’s position is known precisely, and it can be watched by hundreds of instruments, some of them so close that they are vaporized a few microseconds after they send their data.

Underground tests, such as the one that the U.S. conducted last week in a salt dome near Baxterville, Miss., are much more expensive and not as convenient to observe. They are also harder to detect and might well be carried out in secret.

Near-surface explosions can never be secret. They proclaim themselves loudly in many different ways. The shock wave smacks the ground hard, starting characteristic earth waves that may be detected by seismographs thousands of miles away. In the air the shock wave turns into a sound wave that weakens as it travels until it dwindles into a brief rise of barometric pressure. In its last weak form, the wave can cover thousands of miles before it becomes too faint for microbarographs to distinguish it from natural variations of atmospheric pressure. The U.S. undoubtedly had many seismographs and microbarographs stationed around China to be on the alert for its maiden test.

Prattling Particles. Radios and radars were also alert. Any nuclear explosion sets off a great variety of electromagnetic waves, some of which are in the radio segment of the spectrum. They travel great distances, guided around the curve of the earth by ionized layers in the upper atmosphere, and they are not difficult to detect. The explosion-born pulse of radio waves disappears quickly, but another radio effect lingers on. As the mushroom cloud climbs into the stratosphere, its radioactivity releases a vast number of electrons that ionize a mass of air and turn it into a radio wave reflector. This air mass shows up on long-distance radars, and it may distort radio waves coming from beyond it. A combination of all these long-distance methods of measurement can pinpoint the explosion accurately and give a good idea of its strength.

The AEC classed the Chinese explosion as “weak,” meaning its energy was equal to about 20 kilotons of chemical explosive. But only the testers themselves can now be sure whether the low power was intentional, to save precious fissionable material, or a result of poor design and construction. Radioactive particles collected by high-flying airplanes may soon provide an answer, however, for the particles prattle all sorts of secrets: whether the fissionable material used was plutonium or U-235; how much of it was wasted; whether an attempt was made to get fusion (hydrogen bomb) action.

Clicking Counters. Except for describing the bomb as weak, U.S. authorities at first released no figures, and the Weather Bureau, which traced the radioactive cloud, reported its directional progress only, making no comment on its intensity except to say that it was not strong enough to be at all dangerous. But in bomb-bitten Japan, where radiation watching is something of a national hobby, rooftop Geiger counters started clicking ominously. Scientists caught rain water to measure its activity, and jets brought samples down from the sky. About 30 hours after the explosion the radiation count at Niigata, 180 miles north of Tokyo, rose from zero to 30,000 micromicro-curies per square meter of ground. The level at Tokyo’s Institute of Meteorological Research rose from the normal 100 micromicrocuries per square meter to 120,000. This level is the highest since the big Russian test of 1962. but it is not considered dangerous to humans.

At first, U.S. avithorities seemed to agree that the Chinese must have used plutonium as their fissionable material. The process of separating U-235 from natural uranium requires enormous amounts of electric power, and China is power poor. Plutonium, on the other hand, is made in nuclear reactors, which require little external power. China is known to have reactors, and both air surveyance and ground spying have reported a large reactor complex near Paotow in Inner Mongolia. Japanese students of Chinese activities also agreed that China must have used plutonium because it lacked the electricity needed for the production of U-235.

But the neat theory was destroyed when the AEC announced a preliminary analysis. That report indicated that the Chinese test used “a fission device employing U-235.” Unless the Russians in friendlier years got the Lop Nor bomb work started with a goodly amount of U-235, the Chinese must somehow have scraped up the electricity to make the stuff, or less likely, invented a new and better process.

Implosion. Another nugget of information in the AEC report was word that the Chinese depended on an implosion (inward-striking detonation) of chemicals to compress their U-235 and make it fission. Such a device is more effective than shooting two chunks of fissionable material toward each other in an apparatus like a gun barrel, as was done in the U.S. bomb exploded over Hiroshima. The U.S. also used the implosion method in its earliest nuclear weapons. Although a surprising number of commentators assumed that use of implosion showed advanced skill by the Chinese, the AEC did not agree. “The low yield of the test,” it said, “coupled with other information obtained from the radioactive debris indicates that the technology of the device is that which we would associate with an early nuclear test.”

Spotting the actual test site should not have been hard. Since the Russians stopped supplying them with the latest Soviet missiles and interceptors, the Chinese have been almost helpless against photographic flights by U-2s and other high-flying airplanes. Deep in the desert, the site in Sinkiang requires conspicuous roads, transport vehicles, housing, supply dumps. Its burst of activity before the test must have been plainly visible to U-2s and perhaps to reconnaissance satellites orbiting overhead. If such activity still continues in the hostile Takla Makan, the Chinese are likely as Secretary Rusk announced last week, to shoot a second test soon.