High-radiation environments, such as nuclear facilities, provide unique challenges in vision systems. Courtesy of iStock.com/Bim. In an active nuclear power plant or a nuclear fuel recycling facility, the area within and surrounding the reactor or other potentially unstable structures must be carefully inspected for the safety of employees and the proper operation of the equipment. For this purpose, closed-circuit television cameras with radiation-tolerant lenses are required for imaging these sensitive areas — some of which may or may not be safe for human inspectors — to determine if leaks in containment have occurred. These locations within the facility can range from control panels to valves and pipes in and around the reactor itself, wherein anomalous readings can provide a warning of an existing or potential problem. But therein lies the rub from a machine vision standpoint: High levels of radiation degrade typical lenses, making them unsuitable for integration into a nuclear inspection environment. Traditional lenses will turn brown after sustained exposure. Hence the integration of radiation-tolerant, or nonbrowning, lenses, which are doped with materials, such as cerium oxide, to substantially expand their capacity to withstand radiation. A camera incorporating a radiation-resistant lens. Courtesy of Resolve Optics. “The companies and institutions that integrate these nonbrowning lenses into their systems want to be able to use them for a long time,” said Rob Watkinson, a sales manager at Resolve Optics, which produces and supplies these specialist optical components to camera and sensor companies servicing the nuclear industry. Resolve Optics offers a range of fixed-focus and zoom nonbrowning lenses optimized for small sensor cameras that provide a clear picture with a wide field of view. Most importantly, they enable imaging with minimal distortion at 100 million rad — a unit that indicates the amount of energy that has been absorbed. These glass lenses are used within vision cameras at active nuclear sites, both those that are currently generating power and those performing environmental cleanup and monitoring. “The nuclear industry is definitely our biggest market for nonbrowning lenses,” Watkinson said. “Use of these radiation-tolerant lenses in space satellites and medical imaging systems used in x-ray machines are also growth markets. These are highly specialized applications for which we can adapt and provide custom solutions.” A sampling of radiation-resistant lenses. Courtesy of Resolve Optics. Watkinson said that the issue of radiation degradation to camera and sensor-based optical systems relates to more than just the lenses, since the CMOS sensors used in nuclear inspection applications also must be more radiation tolerant. Camera manufacturers, such as Custom Cameras Ltd., incorporate nonbrowning lenses into their camera designs for highly sensitive, active nuclear settings. Lessons learned Nuclear disasters, though relatively rare, have released uncontained radiation into the surrounding environment that tragically caused deaths and illness and necessitated extensive cleanup. Well-documented emergencies included the tsunami that hit the coast of Japan and damaged the nuclear plant at Fukushima-Daiichi in 2011, as well as the nuclear meltdown at Three Mile Island in Pennsylvania in 1979, and the explosion that destroyed a nuclear reactor at Chernobyl in Ukraine in 1986. As a result of these emergencies — and the threat of varying temperature and radiation levels to the health and safety of those working at these sites — the demand increased for autonomous robots and robust instrumentation that can inspect nuclear power facilities. But there is a long-standing concern regarding gamma-induced degradation of sensors, lenses, and other components of imaging systems used for this purpose. One research collaboration from the University of Essex and the University of Birmingham examined the effects of this radiation a few years ago on both dark and illuminated images produced by CMOS cameras. Their study documented damage that produced significant changes in the functioning of electrical circuitry, as well as the visual odometry of the robotic imaging system. Once the radiation reached 3 grays/min (a measurement of a dose of ionizing radiation, recognized by the International System of Units), the data became unreliable1. The whole picture Andy Brownlow, managing director at Custom Cameras, said that nonbrowning lenses, such as the Model 390 from Resolve Optics, are key components in their radiation-tolerant cameras deployed at nuclear facilities around the world. Its cameras include the VHR Puffin UTC 300 Series, contained within a 2/3-in. cathode ray tube and split-head design that protects many electrical components that might otherwise be exposed to radiation. Designed for a long lifespan, the camera’s architecture includes a beam-current stabilization circuit that adjusts as the ray emission declines. Inspectors monitor nuclear facilities for possible radiation leakage. Courtesy of Resolve Optics. “There are a very small number of companies in the world that produce radiation-tolerant cameras and lenses of sufficient capability for an extremely demanding nuclear industry,” Brownlow said. “Together, Custom Cameras and Resolve Optics deliver high quality, reliability, and longevity of operational use imaging systems to our customer base around the world.” Since only a small subset of the vision industry can adapt to a niche specialty such as the nuclear industry, word-of-mouth is generally sufficient to attract customers for these specialized cameras, he said. “Our predominant sales are cameras to provide detailed inspection tasks within nuclear facilities during planned outage maintenance and inspection projects,” he said. “The critical data is recorded, and thorough analysis will be conducted by industry expertise.” An evolving industry Brownlow said that a reckoning for imaging in nuclear applications in the future could occur, because the manufacturing of cathode ray tubes has effectively been stopped by major producers. The most recent large-scale producer of cathode ray tubes, Videocon, stopped manufacturing them in 2015. Radiation-resistant endoscopes provided for vision applications in the nuclear industry. Courtesy of Resolve Optics. This has particular significance for monitoring nuclear sites, because studies have shown that most solid-state cameras are not “rad-hard” — meaning they have been designed to be radiation resistant. And even those that are built to be resistant have lower tolerances for radiation than cathode ray tube-based cameras2. Custom Cameras has stockpiled enough tubes to last a few decades, according to estimates from the company, but ultimately, various chip and LED-based imaging technologies will need to be built to a robust enough standard to accommodate these challenging environments with elevated radiation levels. “There has definitely been progress made, but more is needed,” Brownlow said. “The companies that work in this space are watching what the next technology will be and how effective and robust these emerging imaging solutions cope within the high-radiation-dose environments.” ? References 1. Z. Khanam et al. (2022). Gamma-Induced Image Degradation Analysis of Robot Vision Sensor for Autonomous Inspection of Nuclear Sites. IEEE Sensors Journal, Vol. 22, No. 18, pp. 17378-17390. 2. F.K. Reed et al. (2020). Radiation-Hardened Electronics for Reactor Environments. Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. Department of Energy. ORNL/TM-2020/1776.
