Nuclear mattersNew device helps radiation mapping in nuclear power plants

A U.K. company has developed a radiation-mapping device which allows repairs to be carried out in small contaminated areas of nuclear power plants. It was designed for use in difficult-to-access areas such as research glove boxes or active cells (areas with high levels of contamination). The RadBall, designed by Dr. Steven Stanley, senior research technologist at Stonehouse, Gloucestershire, U.K.-based Nexia Solutions is based on technology used for radiation therapy delivery. It is a spherical polymer plastic module the size of a tennis ball which sits inside a thin lead casing pierced with about 120 collimation holes. “Customers would typically map a cell if they needed to make a repair in the cell or they were looking to decommission the area and wanted to take the kit down and decontaminate it — so they would need to know where the radiation problems are,” Nexia team leader Richard Taylor told the Engineer.

When the RadBall is placed in a radioactive environment, the radiation penetrates the holes in the device and more opaque, dark green lines form in the solid polymer, which is a transparent green colour before exposure. “Radiation passes through the holes and only changes the opacity in the polymer of particular parts of the polymer, so it is almost like a holographic image captured inside the ball,” said Stanley. “The information that is captured in there can be used to determine where radiation is, how much there is, and it can also tell you what the radiation is.” He explained how polymer is made up of chains of molecules, and as the radiation passes through the polymer, it makes the chains cross-link. This changes their optical characteristics and thus their opacity.

After the device’s deployment, Nexia scientists then create a 3D computer image of the opacity within the ball to create a map of the radiation to interpret the results into information required by the customer. “We take the ball and put it in an optical tomography system, and it sends light through the ball and you measure the light intensity on the other side of the ball to give you a 2D image. If you rotate the ball around slowly and look at it from many different angles, you can produce a 3D image. That is how brain scans work, but they use X-rays, whereas we are using light,” said Stanley. He developed the RadBall after meeting a group of medical imaging doctors and consultants who used the materials used in the ball in medicine. “In radiation delivery therapy, they rotate the radiation source around you and it delivers targeted radiation to a particular piece of your body to kill a tumour. They use these materials to obtain information on directional dosage uptake in a patient. So what they might do is make a mould of a human head and put it through a brain scanner, which tells you the doses delivered on to the patient over the whole of the head. So I thought, if they use these materials in the medical arena, they must be useful in the nuclear sector,” he said.

In addition to patenting the combination of the polymer ball and the lead casing, Nexia developed the optical CT analysis software that digitises and turns the ball into an image. “We are also using our existing expertise to interpret that information and convert it into a meaningful image for the customer, which would be an image of the ceiling, an image of the four walls with radiation, type and intensity on it. We would overlay our data perhaps on to a CAD model of a cell or glove box,” said Stanley. Nexia has designed lead casings with holes ranging from 2mm2 to 8mm2, so the bigger the size of contaminated area, the smaller the holes. It has also tested the balls up to 40 sieverts, although it aims to expose the device to three sieverts to create an optimum image. “The device needs to be exposed to a specific amount of radiation, and if the radiation intensity is low, we need to leave it in there for a longer time, and if the radiation intensity is high, we put it in there for a shorter time,” said Stanley. While the outer lead sheath is reusable, the polymer ball can only be used once. The sheath is the main expense incurred making the device, but Nexia claims that the RadBall will be relatively cheap compared with devices such as the Rad Scanner — a collimated camera — which costs about £125,000.