Radioactive materials shielding

Nuclear utiUties have sharply reduced the volume of low level radioactive waste over the years. In addition to treating wastes, utiUties avoid contamination of bulk material by limiting the contact with radioactive materials. Decontamination of used equipment and materials is also carried out. For example, lead used for shielding can be successfully decontaminated and recycled using an abrasive mixture of low pressure air, water, and alumina. 

The nuclear reactor also must be shielded against the emission of radioactive material to the external environment. Suitable radiation controls include both thermal and biological shielding systems. Radiation from alpha particles (a rays) and beta particles ((3 rays) has little penetrating power, but gamma rays have deep penetration properties. Neutron radiation is, however, the primary area of risk. Typically, extremely thick concrete walls are used as a neutron absorber, but lead-lined concrete and special concretes are also used. 

The beneficial use of radiation is one of the best examples of how careful characterization of the hazard is essential for its safe use. A radioactive substance can be safely stored or transported if appropriately contained. Depending on the characteristics of the radioactive material, it can be safely handled by using appropriate shielding and safety precautions. Laboratory workers usually wear special badges that quantify radiation exposure to ensure that predetermined levels of exposure, which are considered safe, are not exceeded. Unfortunately, after more than 50 years, society has not yet been able to design and implement a safe way to dispose of radioactive waste. The hazardous properties of radiation are explored further in a subsequent chapter. 

This technology is applicable only to radioactive material. It acts as radiation shielding only, it does not treat waste. 

When handling radioactive materials, the operator as well as the apparatus, especially the electron detector should be shielded from radioactive contamination. 

ALARA principle time, distance, and shielding. We should minimize our time working with radioactive materials, maximize the distance between us and the source of radiation, and use proper shielding to minimize our exposure. Finally, we must make sure that we understand and follow all the facility procedures and regulations regarding the use of radioactive materials so that our safety and that of our patients are assured. 

The most often used unit to quantify the activity of any radioactive material is the curie (Ci). For most level detection applications, source strengths of 100 millicuries (mCi) or less are satisfactory. A 1 Ci source will produce a dose of 1 roentgen (r) at a receiver placed 1 m (3 ft) away from the source for 1 h. Radiation is attenuated when it penetrates liquids or solids, and the rate of attenuation is a function of the density of the material. The higher the density, the more attenuation the shielding material will provide. Figure 3.122 shows how various thicknesses of different materials will attenuate (reduction factor—NB) the intensity of radiation and result in different degrees of attenuation. 

Critics of nuclear power also worry about the amount of radioactivity released by nuclear power plants on a day-to-day basis. This concern is probably of less importance than is the possibility of a major disaster studies have shown that nuclear power plants are so well shielded that the amount of radiation to which nearby residents are exposed is no more than that of a person living many miles away. Nevertheless, some epidemiological evidence hints that the small amounts of radioactive material released during routine operation may have detectable medical effects on nearby populations. These claims are, of course, intensely disputed. 

When lead bricks or concrete blocks are used as shielding materials, shielding must be done as near radiation sources as possible and the shielding effect must be confirmed with a survey meter. Care must be taken in regard to leaking and scattering rays through clearances between piled bricks and blocks. A lead container is usually used to carry radioactive materials. 

Components for X-ray and radiation shielding (W, HM, HM-polymer) [7.20-7.23]. Typical examples are containers or flasks for radioactive materials and liquids, shielded syringes (Fig. 7.16), shielding eonstraction parts such as collimators in computer tomographic scanners (Fig. 7.17), radiation therapy instmments, and containers and shielding for oil prospecting using radioactive sources (Fig. 7.18). 

Figure 9.5. Lead barrier shield (L-block), behind which all formulation and handling of radioactive materials are carried out (Photo courtesy of Biodex Medical Systems Inc., NY).

As with any construction project, a construction permit must be obtained from the local authority. All local construction codes must be adhered to regarding the electricity and water supply and fire safety. A health physicist or a medical physicist should be consulted to address the issues of shielding and personnel traffic in restricted areas. A radioactive material license from the appropriate authority must be in possession for the use of PET radiopharmaceuticals, before the PET center goes into operation. Authorized physicians must be included on the license. 

Other organisms in the environment are just as vulnerable as humans to this danger. Because of these risks, radioactive materials are shipped in shielded containers. The risks for medical and food-processing applications that utilize radioactivity are known to be very small compared to the potential benefits, but this is considered by many to be a controversial topic. 

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