Simulation studies for optimisation of phased array parameters for examination of roll-joints in pressurised heavy water reactor coolant channels

The roll-joint area in the pressure tubes of pressurised heavy water reactors (PHWRs) is prone to delayed hydride cracking (DHC). This region is monitored during in-service inspection to avoid any catastrophic failure. The in-service inspection involves ultrasonic angle beam examination using a single-crystal high-frequency immersion probe. The inspection is limited to a region of 30-40 mm just ahead of the roll-joint where DHC is expected to occur. An ultrasonic phased array technique has been developed to replace the conventional approach. Using phased array, it is possible to examine the roll-joint region for DHC in a single rotation of the inspection head without axial movement. As a result, there is a huge saving in the inspection time. Moreover, the focusing of the sound beam provided by the phased array helps to achieve enhanced sensitivity and resolution. This paper describes the simulation studies carried out for optimisation of the aperture size and focal laws for the phased array examination of the roll-joints of coolant channels of PHWRs. The results of the simulation are validated by carrying out experiments on a sample piece with reference EDM notches 6 mm long, 0.2 mm wide and 0.15 mm deep.