Radioactive Source Materials and its Applications
Radioactive sources may contain one of the dozens of radionuclides, and their activities can range from ~105 to 1017 Bq. The half-lives of typical isotopes can range from a few days to thousands of years. Radioisotopes are widely used in various commercial applications where these are not subject to the stringent level of security measures, as compared to fissile materials. A few important applications of these isotopes are given below along with an introduction to the nuclear reactors where these radioisotopes are produced:
1. Nuclear Power Reactor and Research Reactor
The most important application of nuclear energy is the generation of electric power in special types of reactors made for this purpose. Power is generated using the principle of nuclear fission. Normally, natural or enriched uranium is used as fuel in these reactors. Plutonium or a mixture of Pu-U (mixed oxide) is also used as fuel in some reactors. In future, Thorium will also be used as fuel in power reactors. There is another type of reactor known as research reactor whose main purpose is to produce radioisotopes for industrial, medical and other uses, in addition to research work.
2. Industrial and Other Applications
One of the important applications of radioisotopes is in the field of industrial radiography, a nondestructive technique, used for testing the integrity, porosity, cracks, etc., of welding joints. Iridium-192 is the γ-source commonly used for this purpose. Cobalt-60 and Cs-137 sources of γ-radiation are used in mining as well as in oil and gas well logging. The most common industrial applications of γ-sources are in level and thickness gauging as well as process control. If these gauges are not removed, when the facility is closed, they can end up in metal recycling facilities. Americium-241/Be and Californium-252 neutron sources are also used for many applications. The activity of some Am-241/Be neutron sources used in well logging can be as high as several hundreds of GBq per source (usually within the range of 1–800 GBq). Neutron sources are also used in machines at the airports to scan the presence of explosives and other prohibited/restricted materials. Radioisotope Thermoelectric Generators (RTGs) use heat generated by the decay of radioactive isotopes to produce electric power. They have no moving parts and can operate for decades without refuelling. They are used as a power supply where frequent maintenance or refueling is expensive or impractical. The largest known RTG used 25PBq of Sr-90 (typically about 2PBq of Sr -90). Industrial irradiators containing Co-60 or Cs-137 as γ-Sources are used to sterilise medical products, meat, fresh vegetables, fruits, spices and other foodstuffs. Although physically small (approximately 1 cm x 50 cm), the radioactive sources or ‘pencils’ in irradiators are highly radioactive. Individual cobalt pencils can have an activity of 500 TBq and an irradiator facility may have an array of cobalt pencils totalling up to a few hundred PBq. The highest activity Cesium irradiators may contain as much as 8 PBq of Cs -137.
3. Medical Applications
Hospitals and medical facilities are among the largest users of radioactive sources, typically for teletherapy and brachytherapy applications Gamma radiation is extensively used in the treatment of cancer patients. Until the 1950s, the only significant radioactive sources produced were Ra-226 sources that were used for brachytherapy. Most of the old radium sources used in brachytherapy have been replaced by Co-60, Cs-137, and Ir-192. Cobalt-60 is the most common radionuclide used in teletherapy, although some Cs-137 sources are also in use.
4. Applications in Agriculture
Sealed sources of Co-60 from a few TBq to 100 TBq range are used for food preservation and for removing of microorganisms from food products. Unsealed sources of P-32 with a few KBq are used for soil- and plant-uptake studies. By applying the process of mutation, breeding yield per acre of many types of seeds have been increased considerably.
5. Applications in Research and Education
Radioactive sources used in education and research contain a wide variety of radionuclides. During the 1960s, gamma irradiators containing large quantities of Co-60 were used for research purposes. Soil moisture gauges used for agriculture research contain Cs-137 sources and neutron producing Am-241/Be sources. Sealed sources of a few TBq are also used for agricultural research. Unsealed sources of C-14, S-35, H-3, etc., having strength of a few kBq are used in biomedical research.
6. Concern over Possible Malevolent Uses of Radioisotopes
Keeping in mind, the magnitude of the radioactivity of sources of Teletherapy Units (which in itself is a strong deterrent) and the nature of the special tools/skills needed to get access to these sources, the probability of diversion of sources from Teletherapy Units for malevolent purposes is considered very low. The more vulnerable equipment such as radiographic cameras (about 1,100 units of which are presently being used in India) containing 400 GBq to 4,000 GBq of Ir- 192 or Co-60 sources have a high risk of loss or theft, given the need to transport these radioisotopes to and from the radiography fields. Owners sometimes lack adequate control over these sources that puts them at the risk of getting lost or stolen. Many a time, commercial users abandon the radioactive sources, once their commercial interests are over. The radioisotopes can be used as Radiological Dispersal Device (RDD) or ‘dirty bomb’, which would disperse the radioactive material in the public domain, when exploded together with a conventional explosive device.
Radioactive decay may produce photons, which are light, but the photons are not in the visible portion of the spectrum. So radioactive elements do not glow in any color we can see. There are radioactive elements that impart energy to nearby phosphorescent or fluorescent materials and thus appear to glow.
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