Radioactive Fallout

I

Introduction

Radioactive Fallout, deposition on the surface of the Earth of radioactive particles, released into the atmosphere either by nuclear weapons or by discharge from nuclear energy installations. Public interest has centred particularly on the effects of fallout from the period of large-scale atmospheric nuclear-weapons testing in the 1950s and early 1960s. Various allegations of resulting ill effects were made for many years, but only in 1984 was a landmark legal decision reached when, in the United States, a federal judge in Utah ruled that 10 people had suffered from cancer because of government negligence concerning public exposure to fallout in that state. Another decision was reached in 1985 by the Pensions Appeal Tribunals of England and Wales with respect to a veteran of British atomic tests on Kiritimati (Christmas Island) in the 1950s. Since the dissolution of the Soviet Union in 1991, information has emerged on the effects of fallout on large inhabited areas of the country during atmospheric tests.

Since the signing of the limited test ban treaty in 1963—one of the fruits of the international arms control—fallout levels have waned worldwide. Some fallout was produced by the USSR's Chernobyl nuclear accident.

II

Mechanism of Fallout

Radioactive fallout material is produced through nuclear fission and the activation of soil, air, water, and other materials in the vicinity of the detonation (see Atom).

Individual radioactive particles are invisible and so light that they might drift around the world endlessly without settling to earth. This condition could be achieved, however, only if a nuclear bomb were detonated at a considerable distance beyond the Earth's atmosphere. When a nuclear weapon is exploded close to the Earth's surface, the violence of the detonation pulverizes vast quantities of surface material, much of which is drawn into the fireball and subsequently sucked into the hot mass that rises to form the characteristic mushroom cloud. Inside the fireball and stem of the bomb cloud the radioactive particles become attached to heavier particles. These heavier particles then act as ballast.

The more massive bits of matter fall back to earth within a matter of minutes, forming an extremely localized fallout, which might be called fallback. Less massive but easily visible particles, borne downwind by the bomb cloud, fall within several hours, and are designated local fallout. The nature and extent of local fallout depend on the type and size of the explosion, the altitude of detonation, and the strength and direction of the winds.

Microscopic particles stay aloft for longer periods of time. If the bomb explosion is of small or medium power, the bomb cloud may not penetrate the tropopause, that is, the atmospheric layer between the troposphere and the stratosphere. In this case, known as tropospheric fallout, the bomb fragments are swept around the world in a zone at the latitude of the explosion and are brought to earth when rain and other forms of precipitation cleanse the foreign material from the atmosphere.

If the force of the detonation is sufficient to inject the bomb debris above the tropopause, many of the small particles remain in the stratosphere to be acted on by stratospheric winds. This is known as stratospheric, or global, fallout. Because no precipitation occurs in the stratosphere, these particles may remain there for considerable periods of time. They are scattered horizontally, so that some of the particles, after having made a number of revolutions about the Earth, are found throughout the stratosphere. Vertical mixing, especially in the polar regions during winter and early spring, brings the material into the troposphere, where it behaves as tropospheric fallout.

III

Persistence of Bomb Fallout

The fission fragments produced by the splitting of uranium atoms or plutonium atoms and neutron-activated materials make up approximately 300 different radioactive isotopes. Each radionuclide is characterized by its own half-life, that is, by the time required for half of the radioactive substance to undergo spontaneous decay. Within the first hour after the explosion, most of the extremely short-lived substances—that is, those with half-lives that are measured in seconds and minutes—decay, and the total radioactivity from the bomb decreases more than a hundredfold.

After the first hour the remaining radioactivity dissipates at a constantly slower pace. The longer-lived fission products account for the bulk of the residual radioactivity. A few fission products are extremely long-lived; for example, the radionuclide strontium-90 (symbol ⁹⁰Sr), also known as radiostrontium, has a half-life of 28 years. These long-lived species constitute the long-term radiation hazard.

IV

Biological Effects of Global Fallout

The long retention of bomb debris in the stratosphere allows time for some of the short-lived fission products to be dissipated in the atmosphere. In the case of tropospheric fallout, some radioactive decay occurs in the atmosphere, thereby reducing somewhat the radiation dosage to those exposed on the Earth's surface.

Long-lived radionuclides, such as ⁹⁰Sr, do not decay much during the time spent in the stratosphere, however, and therefore they may exist for many years as a potential hazard, primarily through contamination of the foods that are consumed by humans.