Ways to Minimized Safe Controlled Area for Gamma Ray Radiography Fion Zhang/ Charlie Chong
Fion Zhang/ Charlie Chong
Site instruction Minimization of Radiation As per the requirements from the NDE Steering Committee regarding the RT governance procedure it has been highlighted that there are many ways that RT contractors can minimize radiation to the surrounding areas. It shall be noted that it is the responsibility of all RT operators and associated contractors to work to an ALARP basis for both safety and interference with all parties. In consideration with this objective wherever possible RT operators and associated contractors shall employ the following practices
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1.Performing radiography on lowering into trenched
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2.Transport the shop weld spools to centralized bunker or excavated low ground. And performs RT in the bunker.
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3.Enclosed the joint with Pb Lead Shield on critical RT works involving road closure and interface problems.
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http://www.eichrom.com/PDF/gamma-shielding-attenuation-guide.pdf
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4.Used internal crawler with/or panoramic shot, reducing exposure time for each joint and ease of shielding the target (Circular PB Lead enclosure)
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5.Consideration of use of SCAR / SafeRad type processes (See Item 8/9)
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6.Used Gamma Radiography. Employed lower voltage (with weaker radiation) on sensitive areas
https://www.nde-ed.org/EducationResources/CommunityCollege/Radiography/Physics/HalfValueLayer.htm
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7.Used of high speed digital radiography over traditional film base radiography
http://www.acutech.gr/media/pdf/Succesful%20conversion%20from%20film%20to%20CR.pdf
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8. Used of well-designed collimator
Well-designed collimator
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Well-designed collimator
The collimator shown, could be more robust, partly enclosed the outward radiation, focusing the useful directional beam.
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9.Used of Selenium-75 Gamma Radiography Sources Selenium-75 provides advantages of longer half-life, improved operator safety, smaller exclusion zone and high image quality, particularly in the 5-30mm steel working range. The advantage of the longer half-life reduces decay loss due to transportation and minimizes the costs associated with managing frequent source exchanges of other isotopes. This is particularly beneficial when sources are used in remote geographical regions and most particularly in offshore applications. Safety compliance, operator dose and storage logistics are made easier using Selenium-75. The operational exclusion zone for Selenium-75 is reduced relative to that of Iridium-192 due to the softer emission spectrum and lower gamma ray constant. Sources can be used in more restricted areas, affording considerable operational and safety advantages. The half-life of Selenium-75 is 120 days; this is almost double that of Iridium-192 and four times Ytterbium-169.
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Physical & Chemical Properties of Elemental Selenium Elemental selenium is highly toxic, volatile, reactive and corrosive. It comes in three physical forms: amorphous, monoclinic and metallic with densities of 4.3g/cc, 4.5g/cc and 4.8g/cc respectively and it has a very large coefficient of expansion close to its melting point of 2170C. This is shown in Figure 1. Early source designs containing elemental Selenium had to contain some void space inside the capsule to allow for this expansion during reactor irradiation when the Selenium melted. The Selenium vapour pressure increases rapidly with temperature as shown in figure 2. This exceeds atmospheric pressure above the boiling point of 6900C and at 8000C (the temperature of the special-form capsule integrity test) the pressure is about 3 atmospheres. At higher temperatures, the internal pressure continues to rise as extrapolated in the figure.
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During the development of the advanced, second-generation design, the corrosion reaction between Selenium and Vanadium or Titanium was found to occur above about 400-4500C. Corrosion compounds of the following type were formed: xM + ySe (r) MxSey where M=Ti or V.
http://www.ndt.net/article/wcndt00/papers/idn655/idn655.htm
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