Snapping nuclear fuel

When Channel Systems of Pinawa, Canada needed a novel ultraviolet transmitting zoom lens for use in their imaging system for the non-intrusive verification of spent nuclear fuel they turned to Chesham, Bucks-based Resolve Optics to help them out.

Inspectors for the nuclear industry are responsible, in part, for the regulation of nuclear materials to ensure that they are not being diverted for clandestine purposes. Spent nuclear fuel emits a faint ultraviolet Cerenkov light when gamma rays from fuel assemblies interact with electrons in the cooling pond water.

Cherenkov radiation itself is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to rigorously characterize it.

The Channel Systems' product, a so-called Digital Cerenkov Viewing Device (DCVD), is an instrument that captures a Cerenkov image of spent fuel for analysis and comparison. The instrument provides inspectors with an effective tool to differentiate irradiated fuel items from non-fuel items.

Resolve Optics was selected to design and manufacture the UV-zoom lens for the system - it was required to have a focal length of 80 - 200 mm, and designed and wavelength-corrected to view a nuclear fuel assembly situated 13 metres away from the lens and through 10 metres of water.

To meet the challenges of the application the UV-Zoom lens was designed to optimally operate from 10 to 55ºC. Miniature motors allow accurate remote setting of both zoom and focus functions. A filter slide was incorporated in the design enabling the lens to be switched between UV (270 to 350 nm) and visible (400 to 700 nm) without the need to refocus the lens. By using a telescopic focus the lens is able to image objects from 3m to infinity.

The lens achieves high image resolution with low distortion throughout the zoom range without refocusing (image tracking). A top-siding adjustment was provided on the lens so that the motor cables could be conveniently positioned and a unique lockable C mount ensures the lens cannot come loose during operation.