Where angels fear to tread

Aug. 1, 2004
Imaging in harsh environments provides critical data to scientists and engineers, and can guide dangerous operations.

Imaging in harsh environments provides critical data to scientists and engineers, and can guide dangerous operations.

On July 21, 1982, I stood with a group of nuclear engineers around a small video screen at the Three Mile Island (TMI) Nuclear Power Station, just south of Middletown, PA. We were watching the live feed from a 1 1/4-in.-wide camera as it was snaked down a control-rod drive mechanism and into the core of the Unit 2 reactor vessel. The near-catastrophic nuclear accident had taken place in the early morning of March 28, 1979, and it had required more than three years to sufficiently stabilize and decontaminate the plant so that workers—heavily clothed in protective suits and breathing gear—could enter the vaulted containment building, climb above the reactor vessel, and lower an imaging system into the stainless-steel structure. Their mission was to see what had actually happened during the accident to the 177 nuclear fuel assemblies in the core.

I was at TMI that July because I was a technical writer and had been sent to the site by Bechtel (San Francisco, CA), which was managing cleanup operations. Until that time, no one was sure of the reactor-core conditions. The consensus was that there might be significant damage to the upper part of the core. However, most engineers and managers at the site discounted any worst-case damage scenarios.

As the radiation-hardened camera was lowered into the core, I remember the growing sense of disbelief and even awe from the group gathered around the video monitor and the black-and-white images taken through murky water. When the camera reached 5 ft. into the 14-ft.-deep core region, the first images of the rubble finally appeared—shattered pieces of fuel rods and assemblies, and evidence of melting. Later video inspections revealed that not only had the 3100 K heat of the accident shattered and melted much of the core, but tens of thousands of kilograms of debris had flowed out of the core area and into the bottom of the vessel, where fortunately it had cooled and had not burned through the vessel bottom in a "China Syndrome" event.

All 133,000 kg of highly radioactive debris was painstakingly removed from the vessel and the cleanup essentially completed by 1990. In a later "lessons-learned" report, one of the biggest lessons was that even when accurate predictions could be made, people "were reluctant to accept the bad news until it was seen on a video screen."

Cameras—and robots—were important tools at Three Mile Island. Relatively simple by today's standards, they gathered visual and radiation data from otherwise-inaccessible contaminated spaces. Early plans for removing the reactor-core debris with a giant robotic arm gave way to the pragmatism of workers manipulating long-handled tools and cutting equipment while monitoring progress with underwater cameras.

The ability to image conditions in harsh environments was important in gathering information of analytical value and in supporting ongoing operations. The need for cameras to perform in such environments continues to provide opportunities for specialized imaging technologies and products that are far more sophisticated than the technology I saw 14 years ago.

For example, since CCD sensors can be seriously damaged by radiation, Symphotic TII (Camarillo, CA) recently developed a color, charge-injection-device (CID)-based camera with a ring of white LED lights to capture radiation-intense underwater scenes. This AquaRAD camera uses the CID sensor from CIDTEC (Liverpool, NY), a division of Spectra-Physics (Mountain View, CA), and was developed with a consortium of companies, including Roper Resources (Victoria, B.C., Canada), Japan Nuclear Fuel Industries (Tokyo), and Inuktun Services (Nanaimo, B.C., Canada).

CEDIP Infrared Systems (Croissy-Beaubourg, France) is working with the French manufacturer of nuclear plants, Framatone ANP, to develop an IR-camera and CW-laser system to replace the use of dye-penetration techniques in detecting surface cracks in reactor vessels. CEDIP is also developing an IR-imaging system to measure plasma temperature in the fusion reactor under development by the Joint European Torus facility (Culham, England).

Harsh environments are also found in chemical refineries, incinerators, sterilization procedures, and non-nuclear power plants. These industries are targeted by vision-system manufacturers such as Greene, Tweed Technologies (Hatfield, PA), which manufactures imaging systems that are contained in a shroud and have sapphire windows. The lessons about imaging learned at Three Mile Island continue to pay off.

CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: [email protected].

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