Detection of deposits adhered on the back surface of plate-like structures using a scanning laser source technique

An imaging technique using a scanning laser source (SLS) was applied to detect deposits inside pipes, which are necessary for the safe decommissioning of the Fukushima Nuclear Power Plant. Experimental results showed that the more firmly adhered the deposit on the back surface of a flat plate, the clearer it is to obtain an image of the deposit. This result is as expected because the imaging by an SLS technique is dependent on the bending stiffness of the thin plate structure. Furthermore, even epoxy putty as large as 50 mm in diameter adhered to the inner surface of the pipe could be imaged, and even when the receiver device was changed from a piezoelectric device to a non-contact laser doppler vibrometer, the image of the deposit could be obtained properly with some degradation of the image due to the effect of a lower signal-to-noise ratio.