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Radiation safety

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About

In the United States, the person within an organization responsible for the safe use of radiation and radioactive materials as well as regulatory compliance. An organization licensed by the Nuclear Regulatory Commission to use radioactive materials must designate a Radiation Safety Officer in writing.

Responsibility

The Radiation Safety Officer is responsible for recommending or approving corrective actions, identifying radiation safety problems, initiating action, and ensuring compliance with regulations.

The Radiation Safety Officer (hereafter referred to as the RSO) is also responsible for assisting the Radiation Safety Committee in the performance of its duties and serving as its secretary.

Duties

  • Annual review of the radiation safety program for adherence to ALARA (as low as reasonably achievable) concepts.
  • Quarterly review of occupational exposures. The RSO will review at least quarterly external radiation exposures of authorized users and workers to determine that their exposures are ALARA.
  • Quarterly review of records of radiation level surveys. The RSO will review radiation levels in unrestricted and restricted areas to determine that they were at ALARA levels during the previous quarter.

Educational Responsibility

  • The RSO will schedule briefings and educational sessions to inform workers of ALARA programs.
  • The RSO will ensure that authorized users, workers, and ancillary personnel who may be exposed to radiation will be instructed in the ALARA philosophy and informed that the management, the Radiation Safety Committee, and the RSO are committed to implementing the ALARA concept.

Establishment of investigational levels in order to monitor individual occupational external radiation exposures

  • An institution must establish Investigational Levels for occupational external radiation exposure which, when exceeded, will initiate review or an investigation into the over exposure of the worker or authorized user.

Types of Radiation

"Ionizing" radiation passes through matter and can cause some of its atoms to become electrically charged, or ionized. In living tissues, the ions caused by such radiation can affect normal biological processes. Ionizing radiation comes in several different forms:

Alpha particles - are positively charged particles. They are easily stopped by paper or skin, and are only hazardous if alpha-emitting materials are swallowed or breathed into the body.

Beta particles - are electrons and have a greater penetrating power than alpha particles, but can be stopped by thin layers of water, glass or metal. However, beta emitting material can be hazardous if taken into the body.

Gamma and X rays - are electromagnetic radiations similar to light and radio waves but with shorter wavelengths. They are very penetrating and heavy shielding materials like lead and concrete are needed to stop them.

Neutrons - are particles with no charge; they are neutral, and because of this they can penetrate many materials very easily. They do not produce ionization directly, but their interaction with atoms can give rise to alpha, beta, gamma or X rays which produce ionization. Neutrons can only be stopped by thick masses of concrete, water or paraffin.

Ionizing radiation from radioactive materials diminishes over time at various rates as the atoms change into other atoms. Often, there is not just the disappearance of one kind of radiation, but the production of different radiations if the new atoms are radioactive. The time for half the radioactivity to dissipate is called the "half-life". Half-lives vary from a small fraction of a second to many millions of years.

Measuring Radiation

The amount of radiation the `dose' received by people is measured in millisieverts (mSv). This unit belongs to the same family as the litre and kilogram, the most commonly accepted, international system of units.

Sources of Natural Radiation

Everyone is exposed to radiation, and for most people nature is the largest source of exposure.

Cosmic radiation comes through the earth's atmosphere, some from the sun and energy sources in our galaxy or outside it. Those from the sun are more intense during solar flares but the others are fairly constant in number. However, the density is affected by the earth's magnetic field, which makes it greater nearer the poles than the equator. The radiation dose people receive increases therefore with latitude. In addition, the earth's atmosphere is a partial shield to the radiation. As one goes higher there is a lower shielding effect and the dose increases as the altitude increases. Buildings and the fuselages of aircraft provide little protection. The global yearly average dose is 0.39 millisieverts.

The Earth's Crust is made up of materials that are naturally radioactive. Uranium, for instance, is dispersed throughout rocks and soil, mostly at very low concentrations. So are thorium and potassium-40. They nearly all emit gamma rays which irradiate the whole body more or less uniformly. Since building materials are extracted from the earth, they can be slightly radioactive, and people are irradiated indoors as well as out of doors. The radiation doses vary according to the rocks and soils of the area and the building materials in use but the global yearly average is 0.46 millisieverts.

Radon is a naturally radioactive gas that comes from the uranium that is widespread in the earth's crust. It is emitted from rocks or soil at the earth's surface and disperses in the atmosphere unless it enters a building, in which the concentration can build up. Radon decays to form other radioactive atoms which, when inhaled, can lodge in the lung and irradiate tissue. The global yearly average dose is 1.3 millisieverts but in high radon areas the doses can be many times higher. The radiation dose can most easily be reduced by preventing the radon gas from entering it in the first place.

Food and Drink. Since radioactive materials occur everywhere in nature it is inevitable that they get into drinking water and food, giving a global yearly average dose of 0.23 millisieverts. Potassium-40 in particular is a major source of internal irradiation, but there are others. Potassium-40 in the body varies with the amount of muscle, for instance, being twice as high in younger men than in older women. Some foods, for example shellfish and Brazil nuts, concentrate radioactive materials so that, even when there is no artificial radioactivity, people who consume large quantities can receive a radiation dose significantly above average.

Sources of Artificial Radiation

Doses from artificial radiation are, for most of the population, much smaller than those from natural radiation but they still vary considerably. They are in principle fully controllable, unlike natural sources.

Medical. Radiation is used in medicine in two distinct ways: to diagnose disease or injury; and to kill cancerous cells. In the oldest and most common diagnostic use, X rays are passed through the patient to produce an image. The technique is so valuable that millions of X ray examinations are conducted every year. One chest X ray will give 0.1 mSv of radiation dose. For some diseases, diagnostic information can be obtained using gamma rays emitted by radioactive materials introduced into the patient by injection, or by swallowing or by inhalation. This technique is called nuclear medicine. The radioactive material is part of a pharmaceutical chosen so that it preferentially locates in the organ or part of the body being studied. To follow the distribution or flow of the radioactive material a gamma camera is used. It detects the gamma radiation and produces an image, and this indicates whether the tissue is healthy or provides information on the nature and extent of the disease.

Cancerous conditions may be treated through radiotherapy, in which beams of high energy X rays or gamma rays from cobalt-60 or similar sources are used. They are carefully aimed to kill the diseased tissue, often from several different directions to reduce the dose to surrounding healthy tissue. Radioactive substances, either as small amounts of solid material temporarily inserted into tissues or as radioactive solutions, can also be used in treating diseases, delivering high but localised radiation doses.

Medical uses of radiation are by far the largest source of man-made exposure of the public; the global yearly average dose is 0.3 millisieverts.

Environmental Radiation. Radioactive materials are also present in the atmosphere as a result of atomic bomb testing and other activities. They may lead to human exposure by several pathways external irradiation from radioactive materials deposited on the ground; inhalation of airborne radioactivity, and ingestion of radioactive materials in food and water.

Radioactive fall-out from nuclear weapons tests carried out in the atmosphere is the most widespread environmental contaminant but doses to the public have declined from the relatively high values of the early 1960s to very low levels now. The global yearly average dose is 0.006 millisieverts. However, where tests were carried out at ground level or even underground, localised contamination often remains near weapons sites.

Nuclear and other industries, and to a small degree hospitals and universities, discharge radioactive materials to the environment. Nearly all countries regulate industrial discharges and require the more significant to be authorized and monitored. Monitoring of such effluent may be carried out by the government department that authorizes the discharges as well as by the operator.

The nuclear power industry releases small quantities of a wide variety of radioactive materials at each stage in the nuclear fuel cycle. For the public the global yearly average dose is 0.008 millisieverts. The type of radioactive materials, and whether they are liquid, gaseous or particulate depends upon the operation of each process. For instance, nuclear power stations release carbon-14 and sulphur-35, which find their way through food chains to humans. Liquid discharges include radioactive materials that people may ingest through fish and shellfish.

The yearly dose to individuals living close to a power plant is small - usually a fraction of a millisievert; doses to people further away are even smaller. Reprocessing nuclear fuel produces higher doses which vary greatly from plant to plant. For the most exposed members of the public, they can be as high as 0.4 millisieverts, but for most of the population they are very much smaller.

World-wide, there are estimated to be four million workers exposed to artificial radiation as a result of their work, with an average yearly dose of about 1 millisievert. Another five million (mostly in civil aviation) have yearly average doses due to natural radiation of 1.7 millisieverts.

Non-nuclear industries also produce radioactive discharges. They include the processing of ores containing radioactive materials as well as the element for which the ore is processed. Phosphorus ores, for instance, contain radium which can find its way into the effluent. A very different industry, the generation of electricity by coal-fired power stations, results in the release of naturally-occurring radioactive materials from the coal. These are discharged to air and transfer through food chains to the population. However, the radiation doses are always low - 0.001 millisieverts or less.

Accidental releases of radioactive materials. Apart from contamination due to the normal operations of the nuclear industry, radioactivity has been widely dispersed accidentally. The most significant accident was at Chernobyl nuclear power station in the Ukraine, where an explosion caused the release of large amounts of radioactivity over a period of several days. Airborne radioactive material dispersed widely over Europe and even further afield. Contamination at ground level varied considerably, being much heavier where rain washed the radioactivity out of the air. Radiation doses therefore varied significantly from normal. More than 100,000 people were evacuated during the first three weeks following the accident. Whole body doses received from external radiation from the Ukrainian part of the 30-km exclusion zone showed an average value of 15 millisieverts. (source OECD, 1995)

Radiation in Consumer Products. Minute radiation doses are received from the artificial radioactivity in consumer goods such as smoke detectors and luminous watches, and from the natural radioactivity of gas mantles. The global yearly average dose is extremely small (0.0005 millisieverts).

Biological Effects of Ionizing Radiation

The health effects of radiation may be divided into those that occur early and those that occur late.

Short term: It has long been recognized that exposure to high levels of radiation can harm exposed tissues of the human body. Such radiation effects can be clinically diagnosed in the exposed individual; they are called deterministic effects because once a radiation dose above the relevant threshold has been received, they will occur and the severity depends on the dose.

Long-term: Studies of populations exposed to radiation, especially of the survivors of the atomic bombing of Hiroshima and Nagasaki, have shown that exposure to radiation can also lead to the delayed induction of cancer and, possibly, of hereditary damage. Effects such as these cannot usually be confirmed in any particular individual exposed but can be inferred from statistical studies of large populations: they appear to occur at random in the irradiated population.

Information on the biological effects of ionizing radiation is assembled and published periodically by a number of expert bodies. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) is an inter-governmental Committee made up of prominent scientists from many countries around the world and is charged with assembling, studying and disseminating information on the observed levels and the effects of ionizing radiation, both natural and man-made. The International Commission on Radiological Protection (ICRP) was established nearly 70 years ago, and is an independent, non-governmental group of experts whose recommendations are generally adopted as the basis for national regulations governing radiation exposure.

Measuring Exposure

For radiation protection purposes, exposure to ionizing radiation is most often measured in terms of "effective dose." This is based on the energy deposited in tissue by radiation, taking into account the type of radiation and the sensitivity of the tissues irradiated. It is thus a measure of the overall risk arising from the exposure. The unit is the sievert, but millisieverts (mSv) are commonly used.[1]

References

  1. International Atomic Energy Agency . Radiation Safety. http://www.iaea.org/Publications/Booklets/Radiation/radsafe.html#one

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