ML13113A218
| ML13113A218 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 04/23/2013 |
| From: | Jay Collins Piping and NDE Branch |
| To: | |
| Collins J | |
| References | |
| TAC ME7646 | |
| Download: ML13113A218 (12) | |
Text
TECHNICAL LETTER REPORT EVALUATION OF LICENSEES ALTERNATIVE TO 10 CFR 50.55A(G)(6)(II)(F) FOR LIMITATIONS TO VOLUMETRIC EXAMINATIONS OF DISSIMILAR METAL WELDS ENTERGY OPERATIONS, INC.
ARKANSAS NUCLEAR ONE, UNIT 2 - DOCKET NUMBER: 50-368 BACKGROUND By letter dated November 11, 2011, and with subsequent information in letters dated April 13, May 21, and September 10, 2012, the licensee, Entergy Operations, Inc., submitted an alternative to the examination requirements of Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(g)(6)(ii)(F) which, in part, requires licensees implement American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME) Code Case N-770-1, Alternative Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated With UNS N06082 or UNS W86182 Weld Filler Material With or Without Application of Listed Mitigation Activities. The CFR requirements include a baseline ultrasonic examination to be performed on each full penetration piping butt weld in the reactor coolant pressure boundary welded with Alloy 82/182 materials. Ultrasonic examination requirements are listed in ASME Code Section XI and ASME Code Case N-770-1, as modified by CFR.
The licensee submitted an alternative to volumetric examination coverage requirements for multiple dissimilar metal welds (DMW) at Arkansas Nuclear One, Unit 2 (ANO-2). The alternative applies only to limited coverage on circumferential scans (for detection of axially-oriented cracking), as full coverage was obtained on axial scans. The NRC requested that Pacific Northwest National Laboratory (PNNL) evaluate the licensees alternative with respect to claimed coverage and ultrasonic technique capabilities as applied to two reactor coolant pump (RCP)-to-primary loop piping welds identified as Weld 09-008 and Weld 10-014. The evaluation included theoretical modeling of the sound beams based on actual phased-array design parameters and component geometrical information provided by the licensee. It should be noted that currently modeled sound field extents and densities represent only isotropic material, i.e., actual grain sizes and structures, velocity ranges, and other material variables that will affect sound beam attenuation, re-direction, and signal-to-noise values have not been applied, and in some cases, are presently unknown.
Another consideration is the potential for inconsistent transducer coupling. This variable is not addressed in the current modeling software, as both CIVA1 and UltraVision1 tend to set perfect contact between the probe and the examined component. However, the actual component outside diameter (OD) weld surfaces, from where the manual phased-array examinations were performed, would typically possess varied waviness around the circumference of the pipe, as the weld crowns would have been manually ground or blended smooth during the time of 1
CIVA and UltraVision are trademarked acoustic modeling and phased array operating software of CEA and ZETEC, respectively.
fabrication. Depending on the extent of surface irregularities that exist, intermittent and unpredictable losses of ultrasonic transducer coupling may affect transmitted sound beam coherence. This can be more pronounced with phased-array probes, as the array is generally required to be adequately coupled over the entire primary axis in order for wave-fronts emitted from individual elements to constructively interfere to produce proper beam steering and focusing. In addition, the licensee stated that probe wedges with flat contact surfaces were used for these examinations, which could also contribute to coupling inconsistencies in wavy regions, or if the probe were to rock during circumferential scans on the component.
Based on the physical limitations described above, the models should be viewed as a best-case representation only.
As requested, PNNL performed individual assessments for each of the subject welds. These are provided in the following sections of this report.
EVALUATION Weld 09-008 Weld 09-008 is a full penetration DMW on the RCP discharge nozzle joining the carbon steel, inside diameter (ID) clad primary piping to a cast austenitic stainless steel (CASS) safe end.
The safe end is welded directly to the RCP pump housing. An idealized cross-sectional depiction of Weld 09-008 is shown as Figure 1. Note that this drawing shows a very flat OD surface in cross-sectional profile. Because the OD surface features of a ground weld crown can vary circumferentially, and no information could be obtained from the licensee to depict actual field conditions, PNNL modeled Weld 09-008 with this same perfectly flat OD surface.
Figure 1 Idealized Cross Section of RCP Weld 09-008 The licensee also submitted a volumetric coverage sketch, showing calculated beam plots for claimed circumferential scan coverage. This is shown as Figure 2, and a legend has been included to depict volumetric weld coverage. The licensees sketch indicates that inner one-third coverage could only be obtained in the buttering and portions of the weld nearest the fusion zones. The licensee further shows that coverage in the outer two-thirds of the weld is similar. However, a large center portion of the weld is shown as not examined, due primarily to OD surface features. The licensee has estimated volumetric coverage as being approximately 73.8%, which includes the inner one-third of the ID-clad carbon steel piping, but no coverage on the CASS safe end.
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Figure 3 PNNL Modeled -6 dB Coverage The phased array was operated with focal laws defined to produce steered beams from 0 to 80 degrees, at one-degree increments, each focused at approximately 122 mm of metal path after exiting the probe. This focal length is beyond the ID surface for steered beams less than around 20 degrees, and only produces 6 dB field densities at the ID for beams at approximately 20-25 degrees. In a similar manner, steered beams above about 65-70 degrees will not produce useful beam profiles for detecting flaws near the ID because they are focused at too short a metal path length (6 dB sound field limits produced by the array are shown in Figure 4).
Therefore, the model predicts that all sound beams above approximately 30 degrees to have less than 6 dB beam intensities near the ID of the weld. The 6 dB value represents a point where field intensity is diminished by 50% of the initial maximum; every 6 dB is an additional 50% reduction; for example, 12 dB is 4 times lower than initial sound energy. The model is a best-case scenario; that is, no material attenuation or backscattered noise from coarse grains in the weld is included. These factors, as well as other potential coupling issues described above, will typically lower the amplitudes of reflected energies from flaws, resulting in decreased signal-to-noise ratio (S/N), and making flaw detection significantly more challenging.
Figure 4 - Sound Beam Intensity Profiles for 20-80 Degree Beams Weld 10-014 Weld 10-014 is a full penetration DMW on the RCP suction nozzle joining the carbon steel, ID clad primary piping to a CASS safe end. The safe end is welded directly to the RCP pump housing. An idealized cross-sectional depiction of Weld 10-014 is shown as Figure 5. This weld varies from the discharge DMW 09-008 by an OD taper between the ferritic elbow to the CASS safe end, as shown in the figure.
Figure 5 Idealized Cross Section for Weld 10-014 As with previous Weld 09-008, the licensee submitted sketches showing areas of inner one-third, outer two-thirds, and no volumetric coverage obtained for Weld 10-014. One such sketch is provided as Figure 6. The licensee estimated combined circumferential scan coverage to be approximately 84.1% of the required volume, with the area of no coverage shown to be an ID-to-OD region of the weld nearest the CASS fusion zone. Note that the licensee did not take credit for electronically skewing the UT beam down into the weld ID region, although focal laws for a 10-degree lateral skew were used. As shown in the figure, the primary reason for limited coverage is the OD taper.
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- 08.
or Weld 10-01 n in Figure 7 and slightly i weld that do ngles (20-25 th, electronic beam by 10 roximate 4-d he array. Ho eled. Figure eld is the cro n the steered 14
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Figure 7 PNNL Modeled 6 dB Coverage
Figure 8 End View of PNNL Modeled Aggregate Sound Field Figure 9 Sound Field Intensity Profiles for 20-80 Degrees on Weld 10-014 Flaw Detection and Optimized ID Impingement
Based on the sound fields described above, it is shown that only certain lower (20-30 degrees projected) angles produce focused energies, at or above 6 dB, at the ID surface of these welds. Flaw detection in austenitic weld materials is complicated, but is generally believed to require a corner-trapped, or crack-face, specular response to be back-scattered to the detecting probe with sufficient energy to yield a minimum 2:1 S/N. This amplitude is affected by several factors, including material acoustic properties, and impedance mismatching and orientation of the flaw, with respect to the impinging sound beam. When attempting to detect axially-oriented, ID surfacing-breaking planar flaws, there is a theoretical optimum range of ID impingement angles that should be designed into the transducer/wedge combination. The desired impingement angle should be below a critical value (above which sound would not impact the ID surface) and can be calculated by the following:
( )
( )
OD sin sin ID
=
(1) where:
is the ID impingement angle, is the initial refracted angle from the probe on the OD surface, and OD/ID is the ratio of the outside-to-inside pipe diameters.
A graphical depiction of this relationship is shown as Figure 10. According to the industrys Performance Demonstration Initiative (PDI) generic DMW ultrasonic procedure 10 (PDI-UT-10),
the optimum ID impingement angle () for detecting PWSCC on the subject welds is in the range of 55-60 degrees, vis--vis, the transmitted refracted angle () should be in the range of 42-45 degrees.
Figure 10 Representation of Solution for ID Impingement Angle CONCLUSIONS With respect to volumetric coverage extent, and a few minor differences, ultrasonic ray trace drawings provided by the licensee are in general agreement with PNNL modeling results.
However, there are several key observations provided by the theoretical model that could impact the quality of the subject examinations. When one considers the optimum range of impingement angles for flaw detection in ANO-2 RCP discharge and suction Welds09-008 and 10-014, and the best-case theoretical sound field intensities modeled by PNNL, it would seem that insufficient acoustic energy at the correct angles (42-45 degrees) is being generated by the phased-array probe and focal laws used for detecting axially-oriented PWSCC. This would be especially true for shallow cracks, on the order of 20-30% through-wall and smaller. Further, the model predicts that only angles below about 25-degrees appear to provide adequate
( 6 dB) sound fields to facilitate detection. It is presently unclear how well these lower transmitted angles, and resultant impingement angles, will perform on ID surface-breaking flaws.