Influence of Radioactive Isotopes Upon Environment

INFLUENCE OF RADIOACTIVE ISOTOPES UPON ENVIRONMENT

INFLUENCE OF RADIOACTIVE ISOTOPES UPON ENVIRONMENT

Anisoara Preda, Felicia Vasut, Nicolae Bidica, Cosmin Dragoiu

National R & D Institute for Cryogenics and Isotopic Technologies – ICSI Rm. Valcea

ABSTRACT. The main objection against nuclear power is the risk of “radioactivity” (radioactive isotopes) to the environment where it may cause humans health effects. We are concerned with the chemical aspects of the sources of releases and the migration of the radionuclides in the environment.

Their chemical properties, together with hydrology, determine how fast they will move from their point of entry into the groundwater to water resources used by man.

Most of the radionuclides produced in nuclear tests, accidents and the normal fuel cycle are short lived. Such releases occur from mining and milling operations, particularly of uranium ores, the nuclear fuel fabrication processes, normal operation of nuclear reactors, reprocessing of spent nuclear fuel, nuclear weapons production and recovery, transportation of nuclear material, testing of nuclear weapons and accidents, and from storage of nuclear wastes.

The types of radionuclides, in the atmosphere are dependent on their source, their mechanisms of production and the nature of the particular environment. While some species are gaseous, others are associated to particles with properties and half-life that are strongly dependent from the particle size and density. In the environment where released large amounts of nuclear fission products and actinides elements from nuclear weapons testing and from accidental and intention discharged from nuclear operations and fuel processing. The paper presents the influence of radionuclides upon the groundwater, air and soil and also the influence of this about the human health.

1. Introduction

The discovery of radioactivity a century ago opened up a new field in science, that of the atomic nucleus, which culminated 40 years later in the discovery of fission, and its practical consequences in the form of nuclear weapons and nuclear power reactors.

The three naturally occurring radioactive decay series are known as the thorium series, the uranium series and the actinium series. A fourth series, which originates in the synthetic element neptunium, is neptunium series.

Nuclear radiation is all around us in the environment. Low-level radiation is found in the oceans and waterways, the rocks and soils, the plant materials and in the atmosphere surrounding the planet. The radiation that we are exposed to can be said to come from two sources, that which occur naturally and that which is due to the activities of man.

2.NaturalRadiation

Terrestrial
Radioactivity in nature comes from two main sources, terrestrial and cosmic. Terrestrial radioisotopes are found on the earth that came into existence with the creation of the planet. Although some are long gone, some radioisotopes take a long time to decay and become non-radioactive (on the order of hundreds of millions of years) and are still around today.

Radioactive elements found in rock, soil, water, air, and in food from the earth make there way in our bodies when we drink water, breath air or eat foods which contain them. These naturally occurring radioisotopes such as carbon-14, potassium-40, thorium-223, uranium-238, polonium-218, and tritium(hydrogen-3) expose us to radiation from within our bodies.

By far, the largest contributor to our daily exposure of radiation is the natural world, and the major form of natural radiation is radon gas. Radon-222 is a naturally occuring decay product of uranium-238 which is commonly found in soils and rocks. Radon-222 is a gas which is odorless, colorless, tasteless and chemically nonreactive. As it escapes from the soils and rocks of which it is trapped, it enters the water we drink and the air we breath.

Since distribution of uranium in the earth's crust varies from place to place, so does the prevalence of radon gas. In areas where surface rocks contain a high concentration of uranium, radon gas could enter a home through a crack in the foundation. A concern for homeowners is the possibility that radon gas could accumulate to dangerous levels. This is especially a problem during the winter months when windows and doors are tightly shut.

Radon diffuses out of thorium and uranium minerals, and adds radioactivity to the ground water and to the atmosphere both by its own presence. Since Ra and Rn are among the most radio-toxic substances axisting, causing bone and lung cancer at relatively low concentrations, special attention must be devoted to their appearance in nature. In many places water from hot mineral wells is considered beneficial to health both for bathing and drinking (“spas” or hot springs). The water may be warm due to radiogenic heating at the source (minerals rich in U or Th) and have a high content of dissolved radium and radon.

The average exhalation rate of radon from the ground is 5 – 50 mBq/m2s, leading to a near ground level radon concentration of 1 – 10 Bq/m3, but varies widely with ground conditions. The concentration in air above ground depends also on temperature and wind conditions.

Many ores contain small amounts of uranium. During processing, uranium and/or its daughters may enter the product, causing a radioactive contamination problem. For example, when apatite is used to produce phosphoric acid, the gypsum by-product contains all the radium originally present, producing a -ray and inhalation hazard from Rn-daughters, making it unsuitable for building material.

Radon concentration in indoor air may be quite high, depending on site and building material.

Cosmic

Another source of natural radiation comes from the interaction of cosmic rays with the earth's upper atmophere. Cosmic rays permeate all of space and are composed of highly energized, positively charged particles as well as high energy photons. Approaching the earth at near the speed of light, most cosmic rays are blocked by the earth's protective atmosphere and magnetic field. As a byproduct of the interaction between cosmic rays (i.e. particles) and the atmosphere, many radioactive isotopes are formed such as carbon-14.

It is reasonable to assume that the production of 14C in the atmosphere has been constant for at least a million years, which means that equilibrium exists between the rates of formation and decay of the 14C in the atmosphere. Moreover, the half-life of 14C is sufficient to allow equilibrium between the 14C in the atmosphere, the oceans (including precipitations to ocean bottoms), and exchangeable carbon in natural materials. Thus from measurement of the specific radioactivity of carbon, it should be possible to determine when the sample became isolated from its natural environmental compartment.

Cosmic rays are also composed of high energy photons, and not all are prevented from reaching the earth's surface. It makes sense that the higher you are in altitude, the more you are exposed to cosmic radiation.

3.Nuclear Radiation from Human Activities

Although radioisotopes occur naturally in the environment, activities of humans have brought this radiation closer to us all. For examples, the bricks, stones, cements and drywalls that we use for the building of our homes, schools, offices frequently contain uranium ores and are thus sources of radon.

The human production of tobacco products introduces another way for us to get exposure to radiation. Smokers recieve a dose of radiation from polonium-210 which is naturally present in tobacco. Smokers also recieve an additional dose of radiation from the decay product of radon gas, polonium-218. Polonium-218 clings to aerosols such as tobacco smoke, and eventually winds up in the lungs. Once in the lungs, polonium decays by alpha particle emission and in the process may damage cells.

Strontium-90 and cesium-137 are the radioisotopes which should be most closely gaurded against release into the environment. They both have intermediate halflives of around 30 years, which is the worst range for half-lives of radioactive contaminants. It ensures that they are not only highly radioactive but also have a long enough halflife to be around for hundreds of years. Strontium-90 mimics the properties of calcium and is taken up by living organisms and made a part of their electrolytes as well as deposited in bones. As a part of the bones, it is not subsequently excreted like cesium-137 would be. It has the potential for causing cancer or damaging the rapidly reproducing bone marrow cells.

Strontium-90 is not quite as likely as cesium-137 to be released as a part of a nuclear reactor accident because it is much less volatile, but is probably the most dangerous components of the radioactive fallout from a nuclear weapon.

Iodine-131 is a major concern in any kind of radiation release from a nuclear accident because it is volatile and because it is highly radioactive, having an 8 day half-life. It is of further concern in the human body because iodine is quickly swept up by the thyroid, so that the total intake of iodine becomes concentrated there. The thyroid has a maximum uptake of iodine, however, so some protection against iodine releases can be afforded by taking potassium iodide tablets to load up the thyroid to capacity so that radioactive iodine would be more likely to be excreted.

Most of the radionuclides produced in nuclear tests, accidents and in the normal fuel cycle are short lived.

As the amount of spent nuclear fuel increases, the contribution to the total plutonium in the environment could become more significant over a longer time, especially if nuclear waste disposal sites release actinide elements slowly to the environment. Whatever the sources of plutonium and other actinides, their presence a contamination of the environment by highly toxic material.

The majority of the plutonium from weapons testing was injected initially into the stratosphere. The plutonium originally in the weapon which survived the explosion would have been formed into high-fired oxide which would be expected to remain insoluble as it returned to earth. Such insoluble particles would have sunk in a rather short time into the bottom sediments of lakes, rivers, and oceans or would become incorporated in soils below the surface layer.

These observations indicate that the speciation of speciation of radionuclides in the atmosphere is dependent on their source, their mechanisms of production and the nature of the particular environment. While some species are gaseous, others are associated to particles with properties and suspencion times that are strongly dependent on the particle size and density.

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