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Environmental radioactivity.

Radiation. Hardly any other scientific phenomenon generates more anxiety. Yet life itself evolved in an environment of low radiation levels as the Earth was created by nuclear processes. The Earth is old enough, however, that much of the original radioactivity has disappeared; the radioactivity still remaining, together with the products resulting from the bombardment of the planet by cosmic rays originating in deep space, constitute the present day natural background radiation level to which all life is constantly exposed. Man-made increments to the natural levels of background radiation commenced about 50 years ago with the discovery of nuclear fission. Largely, it is this radiation arising from the testing of nuclear weapons and the exploitation of nuclear power which raises major concerns in the minds of most citizens.

Sources of Radioactivity and Their Relative

Significance

A basic quantity used in the assessment of the effects of radiation is the radiation dose which, besides other factors, is a function of radioactivity levels and types ([alpha], [beta], [gamma]). Three main contributors to natural radiation doses delivered to the human body are cosmic rays, terrestial gamma-radiation (from radioactivity in the earth's crust, air and building materials) and the naturally occurring radionuclides taken into the human body (eg. [sup.40.K). These natural sources provide about 78% of the overall radiation dose to the public and are obviously the most significant. Medical applications of nuclear techniques, on the other hand, account for about another 21%. Nuclear fallout contributes about 0.4%, whereas regular discharges from nuclear facilities account for less than 0.1% of the overall radiation exposure. The balance of the dose results from occupational exposure and miscellaneous sources such as consumer goods.

Catastrophic events such as nuclear power plant accidents (eg. Windscale, Three Mile Island and Chernobyl) involving releases of substantial amounts of radioactivity can significantly alter these relative contributions (and hence estimates of risk) on a regional or global scale. For example, it has been estimated that, besides the direct effects on immediate area residents, which are considerably higher as a result of the extensive local contamination, the radiation exposure of an average European from the radioactivity released during the Chernobyl nuclear power plant will increase by 1-1.5% over a 30-year period. (The corresponding increases in Canada and Central America are among the lowest and are estimated to be about 0.01%). It is clear that natural radiation doses can vary considerably throughout the world depending on such factors as the construction materials used for housing, the altitude of residential communities, environmental surroundings and personal habits and occupations (uranium miner, pilot, etc.).

Disposal of Radioactive Wastes

The safe disposal of radioactive wastes generated during nuclear fuel cycle activities is one key problem of our time. In Canada, over 110-million tonnes of low-level waste containing natural radionuclides have been generated by the uranium mining and nuclear fuel manufacturing industry. Numerous methods for the stabilization or management of such wastes have been suggested. Since the radiation dose to the general public from this waste is considered to be very low, no decision has yet been made on the preferred disposal method.

Stockpiles of high-level radioactive waste -- spent nuclear fuel from nuclear reactors -- continue to grow around the world. As yet, no country has proposed a site for the ultimate disposal of such wastes. Since radioactive decay is accompanied by the release of large amounts of energy, such wastes are temporarily stored in large steel pools having a plastic lining and filled with water. The amount of radioactivity stored at such sites far exceeds that supplied by weapons' testing fallout or that discharged by nuclear facilities to the environment (see Figure 1 for [90] Sr, a major fission product with a half-life of 29 years, which is being stored at the US and Canadian facilities in the Great Lakes basin). About 10000 tonnes of high-level waste from Canadian nuclear power reactor operations is an interim storage at various production sites.

Numerous methods for the disposal of high-level radioactive waste have been proposed. The preferred solution seems to be to deposit canisters of nuclear waste in deep rock formations in a geologically stable region. Several countries, including Canada, are evaluating such sites to establish that the ground water circulating in the joints and fractures of most of these rocks will not transport such wastes to the biosphere at a later time.

Environmental Distribution

Naturally-occurring sources of radioactivity are not homogeneously distributed all over the surface of the earth; the distributions which are observed today are controlled by weathering, recycling and various physical, chemical and biological processess. The 'routine' radioactive emissions from site-specific nuclear facilities can also migrate to distant locations thus resulting in inhomogeneous distribution in the environment. Similarly, varying amounts of fallout radionuclides from nuclear weapons testing have been detected in all environmental matrices from around the world, including polar locations. Table 1 gives some estimates of the amounts of major man-made ([sup.90.Sr], [sup.137.Cs]) and natural ([sup.40.K], [sup.226.Ra], [sup.238.U] and [sup.232.Th]) radionuclides in some environmental compartments.

Biogeochemical Cycling of Radionuclides

Environmental behaviour of radionuclides is governed by a highly complex set of natural processes and circumstances. Radionuclides which are introduced into the environment are transported within the various geosphere components such as air, soil, water, and sediment. They are subsequently introduced into the biosphere and transferred to man.

Studies using fallout and natural radionuclides have shown that radionuclides injected into the stratosphere have a residence time of about one to five years. On the other hand, the mean residence time in the dust particle-rich lower troposphere is of the order of four to 40 days. Debris released as a result of the Chernobyl accident in 1986 had a mean residence time of about 14 days in the Canadian atmosphere. Radionuclides so delivered can be readily transferred from plants to animals from which they finally reach humans. The grass -- cow -- milk and lichen -- reindeer -- meat pathways are well-established for the transfer of [131] I (half-life eight days) and other radionuclides to human populations. The ability of forests to scavenge and retail certain fallout radionuclides is well-known; however, it has not been determined how long different radionuclides are retained by the forests before being removed to the water basins by the run-off.

In the aquatic environment, those radionuclides in soluble form and chemically analogous to nutrient elements will tend to follow econological pathways in a similar fashion to their stable analogues. Thus [sup.90.Sr] and [sup.226.Ra] will behave like calcium (and hence the term 'bone seekers' for such radionuclides) while [sup.40.K] and [sup.137.Cs will follow the movement of potassium and sodium. Physico-chemical and biolohival interactions play a very important role in the aquatic transport of radiunuclides. Following the Chernobyl accident, it was observed that biological parties carried fallout radionucldes from the Mediterranean Sea surface waters to depths below 100 meters much more rapidly than had been expected from previous studies on weapons' debris.

Both soils and sediments act as reservoirs of many man-made radionuclides from where they may either be leached or translocated. Thus one reason why [sup.90.Sr] and [sup.137.Cs] are still present in measurable amounts in the Great Lakes (despite cessation of significant fallout inputs since about 1963) is that both radionuclides are also being leached from the surrounding soils to the lake waters.

Environmental Radionuclides as Tracers

The presence of both natural and man-made radionuclides in the environment provides convenient tracers to evaluate natural processes. Airborne [sup.222.Rn] (half-life 3.8 days) and its daughters have been extensively used to study aerosol residence times in the troposphere and cosmic ray-produced radionuclides, as well as [sup.210.Pb] half-line 22.3 years) have been employed to study precipitation processes. Such information is of use in 'acid rain' studies. Uranium isotapes, [sup.129.I] (half-life 1.6 x 10 [7] years) and other relatively shorter lived radionuclides such as [sup.3.H] and [sup.14.C] are used for dating groundwater or for tracing its movement. Recently, [sup.137.Cs] and other particle-reactive radionuclides have been extensively used to estimate soil erosiol rates.

Ultimately the oceans provide a sink for transported radionuclides whether they arrive by way of the hydrologic cycle or by direct exchange. As can be seen in Table 1, their concentrations in seawater, biota and the marine sediments are relatively low. Concentrations of these nuclides can very significantly with depth in sediments depending upon many factors such as the paleoclimate and biological, chemical, physical and geochemical processes. In a number of instances, the net effect of these factors, whether separately or combined, has presented oceanographers with techniques for the study of marine processes, including the chronology of sediment deposition. One such example depends on the differing behaviours of uranium and thorium isotopes. Uranium exhibits a residence time in the ocenas considerably in excess of thorium isotopes which are rapidly removed from the sea by scavenging particles. Thorium-230, a member of the [sup.238U] decay chain, is hus separated from its parent isotope [sup.237.U] and consequently decays in the sediment column wuth its half-life of 8 x [10.sub.4] years, until such time as reaches radioactive equilibruam once again. Thus a time scale of up to approximately 400000 years can be constructed from the variability of [sup.230.Th] with depth in sediment cores.

Radiation and Ecosystem Health

As noted earlier, we are aware that the nuclear fallout and power industry emissions have made only small additional contributions to the naural radiation. On the other hand, we are also aware of the serious consequences of nuclear war or a very severe nuclear accident. Following such a catastrophic episode, the effects will be both direct (instant casualties and loss of infrastructure) and indirect (destruction of complete ecosystems).

From extensive experiments conducted during the past three decades or so, we know that external gamma-irradiation of terrestrial ecosystems can cause significant damage to all components. Almost all such studies have been on populations and communities, with the major emphasis being based on acute exposure to high-radiation levels. Much less is known about genetic effects or those arising from chronic, low-level exposure eg. from open sources such as uranium mine wastes. In the aquatic ecosystems, a similar level of understanding exists.

It is generally accepted that man is the most sensitive target of ionising radiation. The idea is that if he is protected, then other forms of life would not be subject to the deleterious effects of radiation to a greater degree. However, this hypothesis has not been rigorously evaluated. Evidence is constantly emerging that natural processes concentrate radionuclides in various biota to levels significantly higher than in the universal media, air and water; little is known about the long-term consequences of such processes and the effects of enhanced radionuclide concentrations on biota themselves.

Radiation and Human Health

The effects of radiation on human health have been studied by expert bodies for over half a century and more is known about radiation is risks than about those from practically any other contaminant in our environment. The effects of large doses of radiation on humans are well-understood, and such doses are clearly hazardous. Relatively high doses arose in local areas during the nuclear devastation of Hiroshima and Nagasaki and as a result of the Chernobyl nuclear accident. On other occasions, lower doses of radiation have also resulted in observable effects on humans. Such effects can, however, be avoided. The risks arising from low-level exposure are not precisely known; they must be inferred (extrapolated) from the better-established effects arising from high-level exposure. Some researchers postulate a threshold below which the risk is effectively zero; others tend to follow the more conservative approach of the linear model where the risk is directly proportional to the radiation dose received. Although evidence exists to support other theories, attempts to prove conclusively that any one theory is correct are frustrated by the experimental difficulties associated with unambiguous identification of the causes of health effects.

One problem is that, due to the virtually insurmountable difficulties imposed by the long-term nature of such experiments (at least three generations of millions of people living under controlled conditions), little statistically valid work is available. The second problem is that, since low-level radiation does not produce a unique set of health effects, it is almost impossible to establish whether the minor changes arising in illness patters are attributable to low-level radiation exposure or to some other causes.

Conclusions

Radiation's power and mystery exacerbate a deep cultural anxiety. This is probably due to the fact that the secrecy shrouding the discovery of controlled nuclear fission, which culminated in its first application as an atomic bomb, has left suspicion and anger in the public mind. So long as people worry about nuclear destruction, radiation will cause anxiety no matter how safe the nuclear fuel cycle activities may become. Yet radiation, coupled with human ingenuity, performs many beneficial tasks also. This means that for good or ill, radiation will remain an integral part of our lives. To live with radiation is indeed to constantly weigh its risks against its benefits.

From an environmental point of view, no entirely satisfactory solution exists for the disposal of radioactive wastes at the present time. Constructive uses have yet to be found for the large quantities of heat released in vicinity of nuclear power plants. Detailed studies of the factors influencing interactions and migration of radionuclides in the environment have contributed much to several fields of knowledge. However, much remains to be done, particularly where our knowlege is incomplete (eg. biogeochemical cycling of radionuclides) or where the results and opinions of research are contradictory (eg. effects of low-level radiation) or inconclusive.
COPYRIGHT 1991 Chemical Institute of Canada
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Joshi, S.R.; Walton, Alan
Publication:Canadian Chemical News
Date:Aug 1, 1991
Words:2292
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