African Fusion March 2015

AFRICAN FUSION

March 2015

Phased-array ultrasonic testing (UT)

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-

Small bore pipe socket weld inspection

using phased-array UT

Byungsik Yoon*, Yongsik Kim and Jeongseok Lee

At the 67

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.

Figure 1: The access offset caused by a phased array probe wedge for small bore

piping, (a) with wedge, (b) without wedge.