African Fusion March 2015

Phased-array ultrasonic testing (UT)

Small bore pipe socket weld inspection using phased-array UT Byungsik Yoon*, Yongsik Kim and Jeongseok Lee At the 67 th IIW International Assembly and Conference, a team from the Korea Hydro and Nuclear Power (KHNP) Central Research Institute presented this paper about the use of phased-array ultrasonic testing (UT) for small bore pipe socket welds on nuclear power plants.

U nexpected failure of small bore pipingwelds is a recur- ring problem at nuclear power plants. In fact, socket welds account for more than 80% of weld failure in nuclear plants. Due to the lack of reliable inspectionmethods, in addition to the large number of welds, socket welds are not subjected tovolumetric inspectionona regular basis. However, socket weld cracking has caused unplanned plant shutdowns in several nuclear plants. The losses incurred by unplanned shutdowns are significant; consequently, early crack initia- tion and crack detection, including the detection of fillet weld manufacturing defects, is of the utmost importance. Current inspection techniques are not capable of reli- ably inspecting socket welds, therefore new approaches are needed. The new technique must be sensitive to socket weld cracking, which usually initiates from the triple point, in order to detect the cracking during the early failure phase. In this study, shear wave phased array ultrasonic inspec- tion techniques are applied to increase inspection speed and reliability. Additionally, a manually encoded scanner has been developed to enhance contact conditions and maintain constant signal quality. Introduction Failure of small bore piping socket welds installed in a nuclear power plant is a recurring problem. Although not a severe safety issue, it may cause an unexpected shutdown of power plant operation to repair the damage, which leads not only to economic loss but also to serious impacts on the operation of the power plant. To date, small bore piping socket weld dam- age has been extensively reported globally. For US nuclear power plants in the 1990s, the small bore piping accounted for 80% of 450 piping-related failures, and accordingly has been considered and managed as an important factor in operating nuclear power plants. In the case of Korea’s nuclear power plants, since the leak- age in a drain line piping socket weld of a steam generator

in Kori Nuclear Power Plant, Unit 3, in 2008, a management programme for the small bore piping socket welds presently installed in nuclear power plants has been established to implement ultrasonic inspection for volumetric examination as an addition to the surface examination requirements speci- fied by ASME Section XI. With the ultrasonic inspection currently being applied, however, inspectors experience difficulties in contacting the ultrasonic probe evenly owing to small installation space of the piping, limited access by examiners, and the small diam- eter of small-bore piping. Inspection has not, therefore, been implemented properly due to restricted access by the probe and interference with neighbouring structures. In this study, a shear wave phased array ultrasonic probe was designed andmanufactured and amanual encoder scan- nerwas developed inorder to solve theproblemof accessibility to the weld leg of the small bore piping socket welds by the ultrasonic beam. Additionally, an experiment to detect flaws on a specimen with fatigue cracks was carried out using the manufactured probe to develop a reliable inspection technol- ogy for small bore piping socket welds. Development of inspection technology In general, shear waves have shorter wavelengths compared to longitudinal ultrasonic waves and are sensitive to various material properties such as grain size and structural charac- teristics, while having a relatively low level of penetration into thematerial. However, thiswave type offers good sensitivity in contrast to the penetration offsets for carbon steel or stainless steel, and as a result a shear wave mode is generated and ap- plied for general ultrasonic testing. Nevertheless, in order to generate a shear wave for an object to be subjected to phased array ultrasonic inspection, which has recently attracted attention, as well as normal ultrasonic inspection, a wedge manufacturedwithdue considerationof Snell’s law is attached to the front of the probe. But the attachment of the wedge at the time of small bore piping socket weld inspection may widen the gap between the inspection area and the index point of the probe, and thereby cause poor accessibility, which in turn is likely to decrease flaw detection capability. The phased array ultrasonic probe can create a beam that is incident to a certain angle without using a wedge. For this reason, considering design parameters of a phased array ultrasonic probe, a shear wave ultrasonic probe was designed to gen-

Figure 1: The access offset caused by a phased array probe wedge for small bore piping, (a) with wedge, (b) without wedge.

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March 2015

AFRICAN FUSION