Velocity ratio (V_L/V_s) changes as per the concentrations of H/D in different locations along the length of pressure tube. A hydride blister which forms at the pressure tube and calandria tube (CT) contact point is a distinct zone containing hydrogen more by 2-3 order of magnitude and hence well distinguishable from its surrounding matrix. Formation of a blister is detrimental to the integrity of an operating pressure tube. Failure of such pressure tube has occurred in Canadian reactor. Therefore, detection of a blister at an early stage is very important. Data generated from post irradiation examination will be helpful in designing an in-situ blister detection system.

A 10 MHz focused probe under immersion is used to get at least four backwall echoes. The probe parameters are such that the mode converted shear wave peak is also obtained with maximum amplitude. By measuring the time of flights, the ratio of the two velocities (V_L/V_s) is computed using cross correlation technique. Velocity ratio increases with H/D concentration in the pressure tube. Testing has been carried out on from irradiated pressure tubes including PT/CT contact region. The contact region did not show any significant increase in velocity ratio, indicating no blister has formed. Later, this observation was confirmed by neutron radiography and metallography. A brief account of the achievement made in the measurement of ultrasonic velocity ratio at Radiometallurgy Division, BARC will be discussed in this paper.