According to German nuclear power safety guidelines, the internal austenitic steel clad
of reactor pressure vessels is assumed to be a protective layer to prevent corrosion of
the carbon steel pressure vessel. Therefore, material properties and thickness of the
clad have never been considered as contributors to strength and/or toughness in design
and fracture mechanical regards. Inservice NDT inspection requirements only call for
the inspection of the bond between clad and base material using straight beam
longitudinal wave techniques. The volume of the clad is not inspected; however,
ultrasonic testing is used for detecting sub-clad flaws. Typically, this is done using dual
element transmit/receive search units to inspect the sub-clad material with refracted
longitudinal waves (usually 70°) at a focal point distance of 30 mm (1.2 inch).
The development and validation of new NDT techniques occurred with two goals in
mind: 1) To supplement standard techniques with diverse information to assist in
interpretation of potential nonconformance; and 2) To provide information to evaluate
aspects of component lifetime extension.
Currently in Germany, lifetime extension issues are not of actual concern. Recently,
however, the continued operation and subsequent licensing of Germany's oldest
pressurized water reactor (400 MW, two loops) was under discussion. The primary
concern was an assumed increase of neutron degradation of circumferential welds (in
the belt-line region) and field welds with increased copper content filler material. Due
to this condition and in accordance with the specific license directives, the clad areas of
and adjacent to the welds have to be inspected during each outage using a multi-frequency
eddy current technique. This technique employs frequencies above 20 kHz
and is limited to detection of surface-connected flaws. An optimized mixing algorithm
[1] was applied to suppress disturbing signals caused by sensor lift-off at the rough and
uneven cladding surface and by local 
-ferrite variations.
Considering the clad contribution to overall strength and ductility when applying
fracture mechanics, it was demonstrated that imbrittlement can be excluded. To secure
this approach, the NDT techniques selected to document the integrity of the cladding,
including detection and characterization of flaws in the volume of the clad and at the
interface with the base material, had to be validated and qualified.
Figure 1: Multi-Frequency
Eddy Current System
Figure 3:
Hard and
Software Interrelation |
Figure 2: Hardware Components
|
The new system is based on low-frequency eddy current testing [2] that is
complemented with a high-frequency component, providing a large frequency
spectrum that is more practical in eddy current testing. This allows for an all-inclusive
and detailed NDT assessment of the clad [3], [4].
The system employs the following features:
-
Wide signal dynamic range (> 85 dB) and strong long-range stability, for
detection of even the smallest measurement effects within a high-interference
environment.
- Effective computing ability using integrated digital signal processors (DSP).
- Powerful signal processing algorithm.
Figure 1 shows the inspection system. The analog eddy current electronic is contained
in the interface box to the right of the PC, which is used for data acquisition,
presentation, and documentation.
The analog boards and the DSP-board are the main components contained in the
interface box, as depicted in Figure 2 below. The DSP-board can combine up to four
analog boards to accommodate different operation modes, such as a time-parallel mode
with four frequencies, time-multiplexing mode, or multi-frequency mode. The transfer
of inspection data to the DSP-board and to the PC is performed via a serial interface.
The interrelation of the hardware and software is presented in Figure 3. The basic
concept is to control most of the system functions numerically by the digital signal
processing software in the DSP to minimize hardware components and expense.
The signal processing firmware is based on a multi-regression analysis. The available
algorithms can be interpreted as a numerical filter for different inspection tasks.
Figure 4: Calibration Variables
| Firmware
|
- Least-Square Fit
| Objectives
| - Suppression of Disturbing (non-relevant) Signals
- Flaw Detection, Classification, Characterization & Sizing
Procedure
- Collecting all Elements Required for "Calibration Data-Matrix"
- Calculation of Filter Coefficients of the Different Functions
- On-Line Signal Mix, using Distinct Filters to Suppress Noise
and Non-Relevant Input Data
| |
The objective of the signal processing is to suppress disturbing signals, which allows for
the quantitative evaluation of target functions (i.e., the characterization of flaw type and
size). The calibration of the inspection system requires well-defined calibration and/or
test blocks, which represent the complete set of relevant component parameters,
including all known component condition variations found during the actual
inspection.
Different frequency ranges combined with either absolute or differential sensors allow
inspection for a variety of flaw types or clad conditions, as presented below in Figure 5.
Figure 5: Inspection Techniques
| High-Frequency - Absolute Sensor (50/280/500 kHz)
|
- Surface Connected Flaws
- Surface Profiling
- -Ferrite Content and Distribution
| Low-Frequency - Absolute Sensor (500/2800/5000 Hz)
| - Clad Thickness Measurements
| Low-Frequency - Differential Sensor (500/2800/5000 Hz)
| - Subsurface Volumetric Flaws
- Subsurface Planar Flaws
- Sub-Clad Flaws
| | |
Figure 6: Surface Profiling, HF-Absolute Sensor
Figure 7: -Ferrite Content, HF-Absolute Sensor
Figure 8: Surface Connected Planar Flaws, HF-Absolute Sensor
Figure 9: Clad Thickness, LF-Absolute Sensor
Figure 10: Subsurface Volumetric Flaws, LF-Differential Sensor
Figure 11: Planar Flaws in the Clad Volume, LF-Differential Sensor
Figure 12: Sub-clad Planar Flaws, LF-Differential Sensor
|
When inspecting RPV clad, a number of deviant parameters have to be considered that
make processing of specific data quite difficult. On the other hand, comprehensive
information of the clad condition can be individually filtered for each of the inspection
tasks listed in Figure 5, using an appropriate firmware/software package. The
following Figures below present actual inspection results obtained using three
frequencies simultaneously.
The display of post-evaluation results using a numerical filter optimized for the
measurement of lift-off between the sensor and the clad surface is depicted in Figure 6.
The color-coded C-scan image on the left side closely represents the surface profile of
the scanned surface. The grooves of the strip-welded clad are clearly displayed along
with a localized repair (ground-out). The numerical amount (in millimeters) of the
sensor lift-off is indicated in the lower A-scan presentation at the current index-line
(horizontal line).
The spatial distribution of the -ferrite content of the scan area is displayed in Figure 7
on the following page. The numerical filter for the -ferrite prediction is applied to the
acquired raw data. Both A-scans indicate the percentage of the -ferrite content at the
cross-line cursor position.
To detect and analyze the scan area for surface-connected planar flaws, a filter is
applied that suppresses non-relevant signals caused by the lift-off effect and/or signals
due to -ferrite influences. The C-scan, presented in Figure 8, shows the indications of
surface-connected EDM notches contained in a calibration block. These notches---
oriented parallel, transverse, and oblique to the weld---are easily detected and
displayed in the C-scan presentation. The depth of each individual notch is indicated in
the amplitude display (A-scan display) as a function of amplitude height. Additional
filters can be applied to select the display of planar reflectors of specified depth. To
obtain the required quality of the above presented results (lift-off filter, -ferrite filter,
and surface-connected planar flaw filter), high-frequency techniques, operating with 50
kHz, 280 kHz, and 600 kHz are used for inspection.
For investigation of the clad volume, low-frequency techniques using 510 Hz, 2.8 kHz,
and 5 kHz are employed. Figure 9 presents clad thickness results furnished with the
numerical filter for clad thickness testing. Local thickness variations are clearly
indicated in the C-scan presentation, where clad thickness ranges from 6 mm up to 12
mm, whereas the nominal clad thickness should be 8 mm.
For the detection and characterization of volumetric flaws buried in the clad volume,
specific differential coil sensors operating in the low frequency range are applied.
Figure 10 displays evaluation results using a numerical filter for the detection of
volumetric flaws in the clad volume. The C-scan presentation reveals three side-drilled
holes. On top, the cursor location on the A-scan presentation shows the indication
amplitudes representing remaining ligaments of 2, 3, 4, and 6 millimeters as a function
of amplitude height. Again, additional filters can be applied to select for display of only
reflectors of a specified depth. Planar flaw indications in the clad volume and flaws in
the sub-clad volume are not detected with this filter type; these require application of a
filter described in the following paragraph.
Figure 11 presents results of the inspection for planar flaws in the clad volume,
applying yet another specific numerical filter. All unwanted signals, such as those
described previously, are suppressed so as to display planar flaws embedded in the
clad only. The cursor location on the A-scan presentation at the top shows the
amplitudes of the indications representing remaining ligaments of 4 and 6 mm as a
function of amplitude height. The filter applied for this flaw type can be expanded to
select the display of planar reflectors of specified depth also.
Finally, a numerical filter to detect and characterize planar flaws in the sub-clad volume
(under-clad cracking) is applied, as presented in Figure 12. The indication is detected
at a remaining ligament of 8 millimeters, representing the clad to base material interface
at nominal clad thickness. Due to the local increase of electrical conductivity at the clad
to base material interface, this inspection technique benefits from the enhancement of
the eddy current density, producing a high signal-to-noise ratio for the detection of
planar flaws extending to the clad to base material interface.
Figure 3: 
Figure 6: 
Figure 7: 
Figure 8:
Figure 9: 
Figure 10: 
Figure 11: 
Figure 12: