The Wonderful World of Radiography: How Radioactive Sealed Sources Check Welds
January 29, 2014
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Sr. Health Physicist
During construction of pipelines and fabrication of large metal structures, welding is used to join the parts. It is very important to know the welds are structurally sound and the whole piece will be strong enough for its job. For example, the pipes used to transport natural gas must be properly welded together so that the gas does not leak from the pipes.
And metal I-beams used in constructing a parking garage must be properly welded so that the structure can hold the weight of the vehicles in the garage. How can this be done? Often, workers perform “radiography” using sealed sources to inspect the weld to see if it is correct.
What is radiography? It is term used to describe using gamma rays or x-rays to inspect the structure of some large dense material. Although x-ray machines may be used for this, they are limited by their need for an electrical source, and because x-rays can only penetrate certain materials. A radiography device (sometimes referred to as a radiography camera) uses sealed sources that emit gamma radiation that can penetrate very dense materials such as metal. Radiography devices can be used without electricity and are portable, making them handy to use at work sites.
Because a radiography source, while small in size, emits gamma radiation that can penetrate several inches of metal, it must be stored in its shielded container. When a weld needs to be inspected, a long guide tube is connected to the device that allows the source to travel to the location that needs to be inspected. A long drive cable also is attached to the other end of the device. This allows the radiographer performing the inspection to stand far away from the radiation source during the inspection. Typically, the source is in the guide tube only a few seconds to a few minutes, depending on what is being inspected.
Radiography is used to inspect welds on pipes for oil rigs; large tanks that hold gasoline; airplane engines; and other large metal structures. So the next time you use natural gas, or park your car in a multi-story garage, you might remember the important role a radioactive source plays in keeping you safe.