Information about source points of anthropogenic radioactivity
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SECTION 7: PLUME PULSE PATHWAYSTable of Contents:
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1. Introduction |
The first atomic explosion at Alamogordo, New Mexico at 5:29 A.M., July 16, 1945, ushered in an era of the systemic release of biologically significant radionuclides from anthropogenic sources. Those who created these devices of destruction never imagined the silent efficiency or the hemispheric thoroughness of the biogeochemical cycling which now make these effluents available to all the inhabitants of the biosphere.
A proliferation of anthropogenic sources of nuclear contamination, including the development of nuclear weapons, followed this first test explosion in 1945. The most obvious sources of contamination were the many nuclear weapons tests (1945-1980), but equally significant release sources were the weapons production facilities and fuel reprocessing sites which evolved with the development of military nuclear capabilities (See RAD 11 for a summary of major nuclear waste source points). The creation of atomic power stations was the inexorable result of the exploitation of the fission process for military purposes and constitutes an unfortunate footnote to the Cold War. These nuclear generating facilities provide an additional opportunity for the release of low levels of radioactivity to the environment; whether there will be another accident at a nuclear power station as severe as the one that occurred at Chernobyl remains to be seen.
The nuclear effluents released from these anthropogenic
source points follow pathways, and create a baseline of nuclear contamination
which can and must be documented to allow evaluation of the environmental
impact of nuclear accidents such as Chernobyl as well as the future impact
of releases from thousands of other potential source points of radioactive
contamination.
PATHWAY MODELS: Nuclear weapons testing (1945-1978) resulted in local, tropospheric and stratospheric fallout patterns. Initially the low-yield "fat man" atomic weapons had only a modest input on stratospheric transport routes, but after the development of more powerful thermonuclear weapons in the mid-1950's (hydrogen bombs), stratospheric fallout became the principle mode of hemispheric transport of weapons tests fallout. Weapons testing stratospheric fallout occurred not only in a primary pulse in conjunction with a tropospheric component, but also as long-term fallout which continued in decreasing intensity over a period of decades, as documented by the Riso National Laboratories (Denmark) summary of cumulative fallout data in the next section of RADNET (RAD 8: Baseline Data).
In contrast to weapons testing pathways, Chernobyl contamination occurred primarily as a tropospheric injection of smoke and radionuclides which produced much higher than expected contamination in distant locations, as well as less than expected close-in fallout at the reactor accident site. The Chernobyl accident, which was hemispheric in its impact, serves as a model for the tropospheric dispersion of any major nuclear accident plume, given the caveat that weather conditions and reactor design help dissipate the local impact of the fallout pattern. Weapons testing fallout, Sellafield fuel reprocessing facility effluents, and later, the Chernobyl plume illustrate a fundamental reality about the biogeochemical pathways of effluents from a nuclear accident: radioactive contamination occurs not as one incident but as a series of pulses in time and space, impacting pathways to human consumption
primary pulse: direct deposition of anthropogenic nuclear effluents in the form of rapidly moving air-borne pulses of radioiodine and vaporized radionuclides (e.g. radiocesium) resulting from major nuclear accidents such as Chernobyl, with total global tropospheric transport times of as little as two weeks. Fallout from such events is associated with and maximized by rainfall (or snowfall) events which allow rapid transfer to human diet of radionuclides deposited directly in forage pathways (e.g. foliar deposition). Such transfer can occur within several days of the plume passage. Immersion, absorption and inhalation are other exposure pathways. See the EPA summary of pathway exposure in the previous section of RADNET, RAD 6.
secondary pulse: the slower movement of radioactive contamination in the abiotic environment including delayed particulate fallout, the mobilization and uptake of existing fallout, and its bioaccumulation in pathways to human consumption. Passage and uptake of the secondary (indirect) pulse of contamination from abiotic media to biological media can vary in time from weeks to years.
tertiary pulse: the delayed redistribution of wind-blown deposition, the remobilization of existing fallout, the transport of surface contamination by human activities (vehicles, foot traffic, train, marine, and air transport, on clothing, and in manufacturing processes, etc.), and the incorporation of multiple modes of pathway contamination into processed foods and consumer products which may be transferred to areas unaffected by the primary and secondary pulses of an accident plume (For an example of a tertiary pulse, see the Peak Pulse Analysis of Chernobyl Derived Radiocesium in Imported Foods in Section 9: Dietary Intake). Redistribution of wind-blown plutonium and other long-lived radionuclides from Chernobyl and military source points will continue for millenniums (239Pu 1/2T = 24,131 years).
Liquid releases from facilities such as Sellafield follow plume pathways involving a slower dispersion of the primary pulse with less obvious secondary and tertiary pulses of delayed contamination of pathways to human consumption.
Post-Chernobyl World Health Organization (WHO) Pathways Summary:
Following the Chernobyl accident, WHO issued this outline
of pathways exposure:
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Cloud Shine-Ground Shine:
Another angle from which to consider pathway exposure,
cloud shine and ground shine are airborne and deposited radioactivity characterizing
a nuclear accident. They provide pathways to external exposure (skin
irradiation and absorption). Cloud shine and ground shine assure the presence
of internal exposure pathways (inhalation, ingestion). These rapidly
moving pathway pulses, which have complex radionuclide composites, are
a formidable challenge to accurate biological monitoring, the prerequisite
of credible dose assessment.
2. Plume Pathway Model |
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Nuclear effluents are deposited in the abiotic environment (air, water, sediment or soil) and are soon transferred to biological media and follow one or more of the above pathways to human consumption. Radioactive contamination doesn't respect national or political boundaries; just because contamination is not reported by the media of a given country does not mean it is unable to cross national boundaries invisibly and impact widely separated and often isolated population groups.
3. Accident Plume Pathway Timetable |
Nuclear effluents move not only in space but also in time.
The rapid tropospheric transfer of radionuclides as volatile gaseous and
aerosol forms occurs much more quickly than the slower dispersion of stratospheric
fallout. Resuspension and remobilization of long-lived radionuclides occur
long after the shorter lived radionuclides have decayed, and their movement
through the biosphere can continue for thousands of years. In the first
few days of a nuclear accident, the presence of 131I and other
short-lived nuclides overshadows the presence of all other radionuclides.
As these nuclides decay, longer-lived isotopes such as 137Cs
emerge as the principle source of exposure. The surprising lesson of the
Chernobyl accident is that in between the overwhelming domination of the
radioiodine isotopes and in conjunction with the dispersion of radiocesium
(137Cs: 1/2T = 30.14 years), numerous other biologically significant
radionuclides such as ruthenium and tellurium also characterize an accident
plume pathway as it silently moves across national boundaries. The list
of indicator nuclides in the Plume Pathway Timetable, though incomplete,
helps denote the complexity and duration of nuclear accidents which then
can subject large population groups to low but biologically significant
exposure to long-lived radionuclides for generations. The indicator nuclides
listed in column one are present from the beginning of a release and provide
exposure even while masked by the more intense activity levels of the shorter-lived
nuclides. Long term exposure is a function of radioactive and biological
half-life as well as biological and mercantile availability. In the secondary
and tertiary stages of a plume pulse, exposure is primarily from inhalation
and ingestion of long-lived radionuclides. The total nuclide inventory
of any source term release in a major nuclear accident will vary widely
depending on the type of facility at which the accident occurs. The total
nuclide inventory listed below is within the same order of magnitude as
the Chernobyl source term.
Indicator nuclides | Total nuclide inventory | Exposure mode | Pathway distance | |
1 hour | short-lived | +/- 1x108 Ci | Inhalation, immersion | <50 miles |
1 day | 131I, 132Te, 99Mo, 239Nep | absorption | <1000 miles | |
1 week | 103Ru, 140Ba, 95Zr | Ingestion | 2,000-5,000 miles | |
1 month | 89Sr, 134Cs, 110mAg, 106Ru | secondary pulse | hemispheric | |
1 year | 154Eu, 154Ce, 90Sr, 137Cs, 241Pu | tertiary pulse: remobilized long-lived radionuclides | ||
10 years | 238,241Pu | |||
100 years | 241Am | |||
1000 years | 239Pu | |||
10,000 years | 99Te, 237Nep | |||
100,000 years | 129I |
4. Pathways: BIBLIOGRAPHY |
The following unannotated citations are basic information sources for understanding the fundamentals of the biogeochemical cycling of radioactive contamination in the environment. Other biological monitoring citations are found in RAD11: Anthropogenic Radioactivity: Nuclear Power Plants: Biological Monitoring.
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Belot, Y. (1986). Transfer of long-lived radionuclides through marine food chains: A review of transfer data. J. Environ. Radioactivity. 4. pg. 83-90.
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