Fission & Fallout

Does fallout from nuclear testing endanger the health of the world? Last week only Soviet tests were spraying deadly fission products into the atmosphere, but 20-odd shots had been fired in only seven weeks, and from Japan to Norway, from Canada to India, fears and protests formed an anxious chorus. Statesmen and housewives asked scientists for reassurance. They got little: the scientists were worried too.

> Norwegian scientists predicted that in a few days the radiation level in Norway would pass the worst period of 1958, when the U.S. and Britain were testing along with the Soviets.

> Japan reported that rain falling on Oct. 14 on Wakkanai, Hokkaido, was 3,000 times as radioactive as rain analyzed before the Russians resumed testing.

>The U.S. Public Health Service found that milk in New Orleans registered 530 micromicrocuries* of radioactive iodine per liter. In other Southern cities the level was nearly as high.

Deadly Balloon. No responsible authority believes that fallout from the Soviet tests is strong enough yet to damage health. But authorities point out that weather and other uncertain factors can concentrate fallout to high local levels. And the worst is still to come: most of the dangerous radioactive products of the Soviet tests are still floating high in the stratosphere. No one can predict how much harm they will do when they eventually come down.

When a bomb in the megaton range explodes within a few hundred feet of the ground, as some of the Soviet tests presumably did, it produces three kinds of fallout:

sbLOCAL FALLOUT. This is mostly the coarse, comparatively heavy material blasted out of the bomb crater. Although extremely dangerous, it spreads only a few hundred miles from the explosion. It does not get into the long-term circulation of the atmosphere, and it does not threaten the earth as a whole.

After the bomb's fireball has expanded to full size (1 1/2 miles diameter for a one-megaton bomb), the cloud of hot gas rises like a balloon, dragging with it a column of dust. Some of the dust falls to the ground within a few hours, becoming part of the local fallout. The rest climbs high in the atmosphere with the cooling, condensing cloud.

sbTROPOSPHERIC FALLOUT. In temperate latitudes, most of the dust, and part of the cloud, never gets higher than some 35,000 ft. This collection of nuclear fission products is the tropospheric fallout, and since the troposphere is the part of the earth's atmosphere that contains rain clouds, even its finest particles of radioactive material are likely to be carried back to the ground by falling snowflakes or water droplets.

So far, most of the fallout from the current Soviet tests has been tropospheric. One of the radioactive air masses from the Russian Arctic test site rose into an air current that carried it from eastern Canada down the U.S. East Coast to Florida. Then it veered westward, along the Gulf Coast, dropping radioactivity that coated local pastures and showed up in the milk of grazing cows. Wandering clouds of tropospheric fallout from the Soviet tests have been detected over most of the Northern Hemisphere. They all crossed Soviet territory, many of them veering south across thickly populated parts of Russia while still in the first flush of their radioactivity.

Whether the local radiation levels are dangerous is a matter of dispute. The U.S. Public Health Service suggests vaguely that 10 to 100 micromicrocuries of iodine 131 in milk is the "guidance" amount that small children should not exceed per day over a full year. Although the figures for milk in many Southern cities have been far above the top daily limit, the PHS is not alarmed because iodine 131 has a short half life* of eight days. When the Russians finish their current test series, most of the accumulated iodine 131--whether in milk, in air or on grass--will vanish in a few weeks. Other short-lived isotopes in the tropospheric fallout will die off also.

sb5TRATOSPHERIC FALLOUT. The third kind of fallout--stratospheric--will not disappear so quickly. Bombs of more than a megaton of power send a large part of their ballooning fireballs climbing high into the stratosphere where there are no falling raindrops or snowflakes. In the frigid stable stratosphere, extremely fine particles of radioactive matter from a big bomb may hang suspended for years--and the bigger the bomb, the more of its dangerous fallout goes into the stratosphere.

In one sense this is fortunate: while the deadly stuff is hanging many miles above the earth, its short-lived isotopes disintegrate and virtually disappear. But two of its most dangerous constituents, strontium 90 and cesium 137, would not fade into harmlessness even if they floated in the stratosphere for a century. And fact is that few fission products stay up nearly so long.

Around the equator, air from the troposphere passes upward into the stratosphere, then whirls toward the poles. When it reaches the Arctic, it sinks and re-enters the troposphere (see diagram). If the sinking air carries radioactive particles with it, they are quickly whipped around the earth by fast low-level winds and are eventually deposited on the surface in rain or snow.

Inexorable Climb. The stratospheric circulation has its greatest effect in late winter and early spring, and that is when the Public Health Service expects the Soviet strontium 90 and cesium 137 to fall most heavily. Most of the fallout will contaminate the temperate parts of the Northern Hemisphere, where the bulk of the world's population lives.

How much long-term damage will be done? Radiation health experts are still debating, but all agree about one thing: if Khrushchev explodes his threatened 50-megaton test as the climax of the Soviet series, radiation all over the earth will start an inexorable climb toward a much higher and more dangerous level than it has ever reached before.

*One micromicrocurie is roughly equivalent to one-millionth of one-millionth of the radioactivity of one gram of radium.

*Time during which the radioactivity is reduced by half.

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