Images de page
PDF
ePub

CHAPTER IV Marine Radioactivity

For the past 25 years, oceanographers have obtained considerable information about marine radionuclides and about the oceanic processes-physical, chemical, biological, and geological-that influence them. This information can be applied to the issue of radioactive waste disposal. This chapter summarizes our present understanding of the sources and fate of marine radionuclides and the mechanisms for human exposure to oceanic radioactivity. In addition, this chapter identifies some other areas requiring more research and greater understanding.

Sources of Oceanic Radioactivity

The Periodic Table of Chemical Elements provides a framework for relating radionuclides to each other and to their chemical properties. Figure IV-1 (a modified version of the Periodic Table) identifies some of the important radionuclides in the marine environment and their sources: primordial radionuclides generated from decay of primordial isotopes; radionuclides produced by cosmic rays; or radionuclides produced by humans (anthropogenic). Elements in the table that indicate an atomic mass are radioactive, and those with only a chemical symbol are essentially stable. For the known 104 chemical elements, about 1,400 radioactive nuclides have been identified, although many of these nuclides have very short half-lives (ie., less than an hour).' Because only those with moderate to long half-lives are important in natural systems, about 60 radionuclides are significant. About half of these occur naturally; the other half are anthropogenically produced.

It is useful to consider radioactive waste disposal in the context of the existing natural marine radioactivity. Primordial radioactive elements were present during the Earth's formation; these that are still with us must have half-lives that are comparable to the age of the Earth or the universe (5 or 9 billion years). Upon decay, these primordial elements generate radioactive daughter products that exhibit a wide range of chemical and radioactive characteristics. In addition, about 10 important radionuclides, which are generated by the interaction of cosmic rays with gas molecules in

the Earth's upper atmosphere, are transported to the oceans, the biosphere, and land.

Primordial and cosmic ray-induced radionuclides have been sources of marine radioactivity throughout biological evolution. To these natural sources, however, humans have introduced a variety of additional sources of radioactivity called anthropogenic radioactivity.

The Primordial Elements and Radioactivity

Chemical elements were formed through a series of thermonuclear reactions that generated a wide range of stable and unstable nuclides. A stable atom has a certain proportion of protons and neutrons in its nucleus. For the lower mass elements, this proportion tends to be about an equal number, for the heavier nuclides the number of neutrons generally exceeds the number of protons. Without this favorable proportion, an element will be radioactive and will emit radiation to become stable. This process is known as radioactive decay. An element undergoes this process at a characteristic rate-known as its half-life-which characterizes the time of decay for half of the atoms of a given radionuclide. (A brief description of this process is found in Appendix A.)

Early in the history of the universe, there were undoubtedly a large number of primordial radioactive elements. But after billions of years, many of those radionuclides have virtually all decayed. Today we are left with only those that have very long half-lives and with their decay products. On human time-scales, the Earth's quantity of the remaining nuclides is essentially constant. There is no present source to renew them, and their decrease in the future will be determined by their half-lives, which are very long on human time scales. Thus, the radioactivity they produce will not diminish significantly.

It is useful to separate the primordial radionuclides into two groups: 1) those that decay directly to a stable product; and 2) those that generate a series of daughter products spanning a range of chemical elements each with a distinctive behavior in the marine environment. Table IV-1 lists the first group. The radioactivity

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

The nuclides with their atomic mass shown are radioactive. For elements with more than one principal nuclide, the one with the longest
half-life is shown. The letters beneath each nuclide indicate their sources: p = primordial; c = cosmic ray produced; and
a = anthropogenic.

Figure IV-1.-Periodic Table of Elements.

Source: Kester, Dana R. 1983. Graduate School of Oceanography, University of Rhode Island, Kingston, Rhode Island.

[blocks in formation]

nuclides listed in Table IV-1, potassium-40 and rubidium-87 are the most important contributors of natural radioactivity to the ocean.

The second group of primordial radionuclides consists of three nuclides (thorium-232, uranium-235, and uranium-238), each of which initiates a complex series of decay products ending with stable isotopes of lead. Table IV-2 lists primary components of these three series.

The chemical elements in these series have a wide range of marine geochemistries. The concentration of uranium is relatively uniform in the ocean and the element does not adsorb strongly onto particles. Thorium, protactinium, polonium, and lead tend to adsorb onto particles and become enriched in marine sediment. Radium is relatively soluble in seawater. After radium is generated from the decay of thorium in sediment, it enters an aqueous phase and diffuses out into seawater. Radon, produced from the decay of radium, is an inert gas that can be transferred to the atmosphere.

Cosmic Ray-Produced Radionuclides

Cosmic rays are high-energy atomic nuclei of galactic origin, consisting primarily of protons (86%) and helium nuclei (13%), with small amounts of heavier

Table IV-2.-Decay Series Initiated by Three Primordial Radionuclides Showing the Daughter Products with Half-lives Greater than One Day

Nuclide

Uranium-238 series

238U (uranium)

234Th (thorium)

234U (uranium) 230Th (thorium) 226 Ra (radium).. 222Rn (radon) 210Pb (lead).

210Bi (bismuth) 210Po (polonium)..

206 Pb (lead)..........

Uranium-235 series

235U (uranium)

231Th (thorium)

231 Pa (protactinum).

227 Ac (actinium)

227Th (thorium)

223 Ra (radium)..

207 Pb (lead)......

Thorium-232 series

232Th (thorium)

228 Ra (radium)...

228Th (thorium)

224 Ra (radium).

208 Pb (lead).

[blocks in formation]

Half-life Type of Decay

'H (hydrogen; tritium)

12.3 years

Beta

"Be (beryllium)

53.3 days

EC'

1.6 x 10 years

Beta

[blocks in formation]

5,730 years

Beta

[blocks in formation]

2.60 years

Beta, EC

7.4 x 10' years

Beta

[blocks in formation]

650 years

Beta

[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]
[blocks in formation]

stable

[blocks in formation]
[blocks in formation]

36C1 (chlorine) 37 Ar (argon)

"Ar (argon)

EC-electron capture mode of decay.

Source: Friedlander, G., J.W. Kennedy, and J.M. Miller. 1964. Nuclear and Radiochemistry. John Wiley & Sons, New York, 585 p.

Figure IV-2 shows the concentrations of the major radionuclides in seawater. Potassium-40 accounts for more than 90 percent of the radioactivity in seawater. The relative abundance of radionuclides in seawater and in marine sediments is illustrated by comparing Figure IV-3 with Figure IV-2. Potassium-40 is important in sediments as well as in seawater, but there are also a large number of other radionuclides that contribute to the total radioactivity of sediments.

Table IV-4 lists the characteristics of significant anthropogenic radionuclides. Of these, the transu

RADIOACTIVITY IN SEAWATER (Cufies/Kilogram)

nuclei-all of which are accelerated to very high energy by interstellar magnetic fields. Solar flare activity also contributes to the cosmic ray flux received by the Earth.

When these rays collide with gas molecules in the upper atmosphere they produce neutrons, protons, and alpha particles. These secondary particles then initiate nuclear reactions that produce radioactive nuclei, notably hydrogen-3 (tritium) and carbon-14. (Table IV-3 lists the cosmic ray produced radionuclides and their half-lives.) Other sources of radioactive nuclides are the spallation of heavy atmospheric constituents (e.g., argon), and entry to the atmosphere of interplanetary dust (e.g., aluminum-26). Because radionuclides are being produced continuously in the upper atmosphere, their concentration at the Earth's surface approaches a steady state between the rate of production and the rates of removal and decay. Their halflives are short compared to the age of the Earth. They have been extremely useful in radioactive age-dating, particularly carbon-14 which becomes incorporated into living organisms and thus provides a means of dating organic remains over the range of hundreds to tens of thousands of years.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small]
[merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

ranic nuclides are noteworthy, because each of the lower transuranic elements (neptunium, plutonium, americium, curium, berkelium, and californium) have one or more nuclides with half-lives ranging from one hundred to ten million years. Thus, they are capable of persisting in the marine environment for a long time.

The Fate of Radionuclides in the Ocean

A variety of oceanic processes determine the fate of radioactive substances: physical dispersion and advection, isotopic dilution with stable nuclides of the same element or with isotopic analogs of chemically similar elements, marine chemical cycles, adsorption onto suspended particles, and uptake and transfer through the marine food web-with possible transfer to humans. Marine sediments also play a very important role in the fate of radionuclides, because they become a repository for some elements and a site of release to the ocean for other elements.

Dispersion and Advection

Oceanic dispersion and advection dilute concentrations of substances away from their sources and transport material throughout the world's ocean. These processes range in scale from tens of meters to thousands of kilometers. An important oceanic feature in the physical distribution of material is the main thermocline, where density increases downward at depths ranging from about 200 to 1,500 meters, with depth and gradient depending primarily on location, season, and storms. This thermocline is present in nearly all regions of the ocean except high latitudes, where extensive cooling by the atmosphere leads to mixing of ocean waters to depths of several thousand meters.

To simplify the concept of ocean transport processes, the ocean can be featured as four boxes (Figure IV-4): a surface mixed layer, a deep mixed layer, a thermocline layer of restricted mixing, and a high latitude layer that is vertically mixed throughout its entire depth range. This simplified model accounts for many aspects of ocean transport processes over periods of years, because seasonal effects which vary latitudinally

« PrécédentContinuer »