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Spectrometer Goes the Distance Inside Nuclear Reactor

Photonics Spectra
Dec 2000
Daniel C. McCarthy

Like most industries, the nuclear power sector relies on materials analysis for process control and plant monitoring. There are, however, many cases in which the material of interest or its location is highly radioactive, making removal of samples both infeasible and impractical. The alternative is in situ analysis, but conventional instrumentation is either too bulky or ill-suited to evaluating nuclear materials within the plant.

To remedy this problem, Applied Photonics Ltd. developed an instrument based on Q-switched Nd:YAG lasers from Big Sky Laser Technologies Inc. to perform in situ, quasi-nondestructive analysis within hostile industrial environments. Using a technique called laser-induced breakdown spectroscopy, 10-ns laser pulses create a small microplasma on the sample surface. As the plasma cools, it emits light of characteristic wavelengths based on the ablated ionic and atomic species. An optical detector collects these wavelengths to enable a computer to deliver results within seconds. Although the laser's peak power is in the megawatt range, less than 1 W interacts with the sample, making the technique quasi-nondestructive.

Some samples are within sight, allowing an optical telescope configuration to aim and deliver the laser beam. Other samples, however, are within a nuclear reactor and are accessible only via a convoluted route. Applied Photonics encountered this problem at two advanced gas-cooled nuclear power stations run by British Energy. Lifetime reviews of the reactors had identified microscopic cracks within the superheater, a primary factor in determining the risk of component failure.

Materials analysis within advanced gas-cooled nuclear reactors once required removal of samples to a laboratory for analysis. A new laser-based spectroscopy technique can evaluate reactor materials in situ by using a fiber optic cable to deliver laser light to locations deep within the superheater structure. Courtesy of Applied Photonics Ltd.

The engineers were aware that the materials in question exhibited abnormally high copper content during bifurcation, compared with cracks forming within stainless steel. However, distinguishing the two groups of materials proved difficult by conventional means. A 75-m fiber optic cable threaded into the reactor delivered the laser pulses to the site and quickly identified the affected components. This allowed the engineers to target their inspection and repair efforts within the pressure vessel.

The beam quality and pulse length of flashlamp-pumped, Q-switched Nd:YAG lasers make them well-suited for fiber optic delivery, said Andrew Whitehouse, managing director of Applied Photonics. Shot-to-shot pulse stability, reliability, accessible 110-or 240-V power requirements and ease of service were other factors in their selection.

Whitehouse attributed his selection of Big Sky's Ultra CFR laser to its portability and robustness. "The laser is a quality product and may be regarded almost as 'black box' technology with very little maintenance required by the user. ... We have transported our two lasers to a number of industrial sites throughout the UK, and on every occasion the lasers worked perfectly."

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