Some methods based on ultrasonic, have been used for this purpose. The main disadvantage in the ultrasonic method is that the air gap, due to the poor bonding between the steel structure and the lead, results in uncertain evaluation. The ultrasonic testing will underestimate the error if there is more than One cavity or void in the line of sight.

Under such specific needs, radiometric method is being commonly employed. In this method, a strong gamma source is located inside the vessel and the external surface is scanned by dose measuring instruments. These instruments have limited accuracy at low dose rate levels which necessitates use of very strong source. The surface does rates need to be of the order of a few mR/h to successfully use such devices. A lower dose rates, the uncertainty in the detector reading masks the small variation in lead thickness. For a 4% deviation in a lead thickness of about 20 cm, the variation in dose rate is 50 to 60%. This implies measurement which is difficult to achieve with such detectors. However, in actual conditions where these vessels are designed to carry radioactive materials of 10⁴ to 10⁶ Ci, a 4% tolerance when missed under such scanning would result in hot spots of 300 to 350 mR/h. A specification of 2% tolerance is impossible to detect by radiometric method unless source strength is very much increased. The source handling problems get aggravated further when the vessel to be tested has large thickness but short height (comparable to outer diameter of the vessel) which is the case with some of the FBTR shielding containers.

To overcome the above difficulties, a new technique has been adopted in the method developed at IGCAR. IN this method, instead of scanning the transmitted dose rate, the transmitted virgin flux is measured using a NaI(Tl) detector and a Single Channel Analyser (SCA) with a window set for the photopeak region of the source energy. Depending on the material and thickness, a suitable source of low, medium or high energy can be chosen. Even a stringent specification of 1% tolerance can be evaluated. the method in detail, the advantages, the experiences gained over the years are described in the paper. This paper also highlights application of the method for other measurements such as level estimation for liquids including Na and NaK in field conditions.

This method was successfully tested on a mock up for possible application in a reactor environment, like identifying cracks propagating in end shield of a PHWR reactor. The major handicap in such studies is the presence of high radiation levels interfering with the measurements. The method has confirmed that such a handicap does not distort the inference. The details of this study are also highlighted in the paper.