Atomic Tests: The Blast at Lop Nor

• Share

(2 of 3)

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."