Part 4

When a nuclear device is detonated, the release of energy is due to the fission of uranium or plutonium atoms or to the fusion of hydrogen atoms. At the instant of fission, some 75 radionuclides, or fission products, are created.

From these primary fission products, about 100 other radionuclides may be formed, some existing only for microseconds and others for thousands of years. The radionuclides of significance to biologists are those that exist long enough—no matter how brief the time—to have an impact on a biological system.

Factors of biological transport and concentration of long-lived radionuclides make efforts to assess possible environmental effects particularly difficult. It has been asserted, for example, that probably every living cell formed since the early 1950s contains some of the radionuclides produced by nuclear testing. No one knows the significance of such a condition, if it indeed exists. It is certain only that some of the long-lived radionuclides already placed in the environment will be detectable there for hundreds of years and hence will continue to provide material for biological studies.

Examining Environments

When radioactivity is injected randomly into the atmosphere by a nuclear detonation, biological disposition begins in many ways, each related to the character of the explosion and the environment in which it occurs. Fallout studies thus involve the tracing of mixed fission products in the biosphere and the collection and analysis of thousands of samples of plant and animal tissue, and usually of water and soils, at many successive times. The radiobiologist then attempts to interpret the accumulated evidence of uptake of radionuclides. Some fallout studies may require sampling over large areas of the earth. Other investigations of fallout or of radioisotopes introduced deliberately into controlled field plots may require years of patient observation in small and circumscribed areas.

Studies of ocean fallout, for example, have ranged over hundreds of thousands of square miles of open water. The 1955 United States survey of the Western Pacific was conducted by a scientific team aboard a Coast Guard vessel, the Roger B. Taney, in a program called Operation Troll. In 7 weeks the team cruised 17,500 miles, making collections of water and marine organisms at 86 ocean stations on a route extending from the Marshall Islands through the Caroline Islands and the Mariana Islands to the Philippines and finally to Tokyo. The expedition took samples of plankton at depths down to 200 meters and water from the surface down to depths of 600, 800, 1000, and 1200 meters.

Environmental studies at nuclear test sites or in controlled ecosystems involve not only long-term, periodic sampling of plants and animals but also years of detailed examination of soils, meteorological conditions, and other factors.

TERRESTRIAL ECOLOGY RESEARCH

In programs of such scope and duration, the problems of interpretation are great. Broadly, environmental studies give consideration to:

1. The amounts and kinds of radioactivity released to the environment.

2. The rates of uptake by the biological system.

3. The amounts and kinds of radioactivity within the system.

4. The rates of metabolic transfer or elimination.

5. The amounts and kinds of radioactivity concentrated in tissue and acting internally.

6. The time required for biological processes to be completed and for any biological effects to develop.

Biological Inventories

Familiarity with the biological components of an ecosystem is essential to meaningful radiobiological assessment.

Inventories of natural components were not made in the early nuclear test programs because of inadequate realization of the biological potential. Later, they could be made only after radionuclides already had been introduced into the environments.

The survey of the mid-Pacific region before Operation Crossroads represented the earliest effort to examine an environment in detail before a nuclear detonation, but was designed so that it had only inferential value for other long-range biological research. The test surveys were useful, however, in expanding knowledge of specific environments. In addition, it was standard practice to make comparative collections of organisms in regions removed from the test sites to establish base lines, or “controls”, against which to measure radiobiological developments.

The most extensive inventory of an environment—an inventory designed specifically in relation to an anticipated nuclear detonation—was that made between 1959 and 1962, as a preliminary phase of Project Chariot, in the Cape Thompson area of Northwest Alaska. Chariot was a part of the AEC Plowshare Program in which it was proposed to excavate a harbor at the mouth of the Ogotoruk Creek, which empties into the Chukchi Sea. Although the excavation project actually never was undertaken, the “predetonation” environmental investigations involved 3 years of coordinated research into the climatic, marine, coastal, and terrestrial aspects of the region, and detailed studies of the history and the radiological and ecological situations of the human population.

The program was an effort to make a model environmental inventory. Its significance was both in its assessment of the base for determining the “biological cost” of the proposed operation and in the thoroughness of its documentation of the environmental features of a part of the world that previously had been virtually unexplored. It was a prototype for future studies.

Measurements and Interpretations

Determination of the amounts and kinds of radioactivity in a biological sample is a process wholly dependent on instruments, since radiation usually cannot be detected by the senses.

A biological sample is any material of measurable biological significance. A sample of tissue or similar organic material usually is dried or reduced to ash in a muffle furnace before it is examined with a radiation counting device.

Improved instruments now permit the counting of radioactivity at levels so low as to have been imperceptible a few years ago. The samples, placed in lead chambers for maximum shielding from background radiations, are examined by multichannel analyzers capable of recording radiation emissions continuously over long periods of time.

Data-processing techniques have been employed in the handling and interpretation of information from long-range biological sampling and analysis programs. Analog computers have been used experimentally for theoretical projections of results.

Scientists at the AEC’s Oak Ridge National Laboratory, for example, have developed experiments in which an analog computer is programmed to keep a running balance of the net changes—simultaneous gains and losses—of radioactivity in the various compartments of a representative ecosystem. The computer becomes an electronic image of the biosphere, using known or assumed rates of energy transfer and photosynthesis to predict probable radiological results of tracer experiments of environmental contamination.

ENVIRONMENTS—SINGULAR, YET PARTS OF A WHOLE