What is a nuclear test? Nuclear physicists find it necessary to make many kinds of tests, most of them quietly in laboratories. But the phrase "nuclear test" has come to mean an experiment that involves a nuclear explosion. Since all such tests are dangerous, the U.S. fires its small ones on the Nevada desert, its big ones on remote Pacific islands. Most of the 169 U.S. tests were exploded on tall towers so that their expanding fireballs would not hit the ground. A few nuclear devices were dropped from airplanes or suspended from captive balloons.
What are the dangers? All near surface tests are classed as "atmospheric" because their radioactive byproducts mix with the atmosphere and may be carried around the earth. Tests exploded under the sea are of much the same nature. Underground shots are considerably safer. The U.S. has fired about half a dozen small tests in tunnels dug into Nevada mountains. Their radioactivity was well confined, and so far there have been no reports of contaminated ground water, but large underground tests could conceivably poison the water supply of an entire state. For relatively small nuclear devices the U.S. is likely to continue underground tests, but the more powerful sod busters of the future will have to be tested in the open because the earth's crust cannot hold them. Former Atomic Energy Commissioner Willard F. Libby estimates that a ten-megaton test would need to be exploded 18 miles below the surface. At this depth the rock is probably so plastic that digging a test hole would be impossible. Another possibility is to fire tests above the earth's atmosphere, as the U.S. did on a small scale in Project Argus (August-September 1958). But a big, rocket-borne test involves intricate problems in technology; countless things could go wrong. A premature explosion on earth, or too early an explosion aloft, could contaminate the atmosphere with radioactive products. All tests in the atmosphere, including last week's Soviet test, will surely raise the level of the earth's radioactivity. The dirtiest tests in the past were fission-fusion-fission bombs, the first of which, exploded by the U.S. in 1954, killed a Japanese fisherman by its fallout and seriously injured many people in the nearby Marshall Islands. When the Russians fol lowed with similar dirty tests, radiation increased all over the world. Especially frightening was the fallout of strontium 90, a deadly fission product that settles in bones and may cause cancer. The new Soviet test series will not necessarily scatter much-dreaded strontium 90, but radioactive products of some sort are sure to, result.