Regulatory Guide 1.150

Ultrasonic Testing of Reactor Vessel Welds During Preservice and Inservice Examinations

ML12305A248
Person / Time
Issue date:	06/30/1981
From:	Office of Nuclear Regulatory Research
To:	NRC Region 1
References
Task SC 705-4 RG 1.150
Download: ML12305A248 (16)

U.S. NUCLEAR REGULATORY COMMISSION June 1981 REGULATORY GUID)E

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 1.150

(Task SC 705-4)

ULTRASONIC TESTING OF REACTOR VESSEL WELDS DURING

PRESERVICE AND INSERVICE EXAMINATIONS

A. INTRODUCTION

Criterion XVII, "Quality Assurance Records," of Appen- dix B requires, in part, that sufficient records be maintained Criterion 1, "Quality Standards and Records," of Appen- to furnish evidence of activities affecting quality. Consistent dix A, "General Design Criteria for Nuclear Power Plants," with applicable regulatory requirements, the applicant is to 10 CFR Part 50, "Domestic Licensing of Production and required to establish such requirements concerning record Utilization Facilities," requires, in part, that components retention as duration, location, and assigned responsibility.

important to safety be tested to quality standards commen- surate with the importance of the safety functions to be performed. Where generally recognized codes and standards This guide describes procedures acceptable to the NRC

are used, these codes and standards must be evaluated to staff for implementing the above requirements with regard determine their adequacy and sufficiency and must be sup- to the preservice and inservice examinations of reactor plemented or modified as necessary to ensure a quality pro- vessel welds in light-water-cooled nuclear power plants by duct in keeping with the required safety function. Criterion 1 ultrasonic testing (UT). The scope of this guide is limited to further requires that a quality assurance program be imple- reactor vessel welds and does not apply to other structures mented in order to provide adequate assurance that these and components such as piping.

components will satisfactorily perform their safety functions and that appropriate records of the testing of components important to safety be maintained by or under the control

B. DISCUSSION

of the nuclear power unit licensee throughout the life of the unit. Reactor vessels must periodically be volumetrically examined according to Section XI of the ASME Code.

Section 50.55a, "Codes and Standards," of 10 CFR which is incorporated by reference, with NRC staff modifica- Part 50 requires, in part, that structures, systems, and tions, in § 50.55a of 10 CFR Part 50. The rules of Section Xl components be designed, fabricated, erected, constructed, require a program of examinations, testing, and inspections tested, and inspected to quality standards commensurate to evidence adequate safety. To ensure the continued with the importance of the safety function to be performed. structural integrity of reactor vessels, it is essential that Section 50.55a further requires that American Society of flaws be reliably detected and evaluated. It is desirable that Mechanical Engineers Boiler and Pressure Vessel Code results from prior UT examinations be compared to results (ASME B&PV Code) Class 1 components meet the require- from subsequent examinations so that flaw growth rates ments set forth in Section XI, "Rules for Inservice Inspection may be estimated. Lack of reliability of UT examination of Nuclear Power Plant Components," of the ASME Code. results is partly due to the reporting of ambiguous results, such as reporting the length of flaws to be shorter during Criterion XII, "Control of Measuring and Test Equipment," subsequent examinations. This lack of reproducibility arises of Appendix B, "Quality Assurance Criteria for Nuclear because the Code requirements are not specific about Power Plants and Fuel Reprocessing Plants," to 10 CFR many essential variables in the UT procedures. Recommenda- Part 50 requires, in part, that measures be established to tions of this guide provide guidance that would help to ensure that instruments used in activities affecting quality obtain reproducibility of results. Reporting of UT indications are properly controlled, calibrated, and adjusted at specified as recommended in this guide will help to provide a means periods to maintain accuracy within necessary limits. for assessing the ambiguity of the reported data.

USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Regulatory Guides are issued to describe and make available to the Attention: Docketing and Service Branch.

public methods acceptable to the NRC staff of implementing specific parts of the Commission's regulations, to delineate tech- The guides are Issued in the following ten broad divisions:

niques used by the staff in evaluating specific problems or postu- lated accidents or to provide guidance to applicants. Regulatory 1. Power Reactors 6. Products Guides are not substitutes for regulations, and compliance with 2. Research and Test Reactors 7. Transportation them is not required. Methods and solutions different from those set 3. Fuels and Materials Facilities 8. Occupational Health out in the guides will be acceptable if they provide a basis for the 4. Environmental and Siting 9. Antitrust and Financial Review findings requisite to the issuance or continuance of a permit or 5. Materials and Plant Protection 10. General license by the Commission.

Copies of issued guides may be purchased at the current Government This guide was Issued after consideration of comments received from Printing Office price. A subscription service for future guides in spe- the public. Comments and suggestions for improvements in these cific divisions is available through the Government Printing Office.

guides are encouraged at all times, and guides will be revised, as Information on the subscription service and current GPO prices may appropriate, to accommodate comments and to reflect new informa- be obtained by writing the U.S. Nuclear Regulatory Commission, tion or experience. Washington, D.C. 20555, Attention: Publications Sales Manager.

Operating and licensing experience 2 ' 3 and industry performance characteristics (amplitude linearity and tests4 have indicated that UT procedures that have been amplitude control linearity) is to be verified at the beginning used for examination of reactor vessel welds may not be

4 of each day of examination. Requirements in Article 4, adequate to consistently detect and reliably characterize Section V, 1977 edition, which is referenced by Section XI,

flaws during inservice examination of reactors. This lack of for the periodic check of instrument characteristics (screen reproducibility of location and characterization of flaws has height linearity, amplitude control linearity, and beam resulted in the need for additional examinations and spread measurements) for UT examination of reactor evaluations with associated delays in the licensing process. pressure vessels have been relaxed. The interval between periodic checks has been extended from a period of I day

1. INSTRUMENT SYSTEM PERFORMANCE CHECKS to a period of extended use or every 3 months, whichever is less. This change has not been justified on the basis of Instrument system performance checks to determine the statistically significant field data. Performance stability of characteristics of the UT system should be performed at automated electronic equipment is dependent on system intervals short enough to permit each UT examination to be performance parameters (essential variables), and the ASME

correlated with particular system performance parameters to Code has no quality standards to control these performance help compare results. These determinations will help make it parameters. Until the performance stability of UT systems possible to judge whether differences in observations made can be ensured by the introduction of quality standards, at different times are due to changes in the instrument system it is not reasonable to increase the period between calibration characteristics or are due to real changes in the flaw size and checks. Therefore, recommendations have been made to characteristics. Determinations for "Frequency-Amplitude check instrument performance parameters more frequently Curve" and "Pulse Shape" recommended in regulatory posi- than is specified in the ASME Code.

tions 1.4 and 1.5 may be made by the licensee's examination agent by using any of the common industry methods for Requirements of Appendix I, Article 1, 1-4230,Section XI

measuring these parameters as long as these methods are of the ASME Code, 1974 edition, state:

adequately documented in the examination record. These measurements may be performed in the laboratory before "System calibration shall be checked by verifying the and after each examination, provided the identical equip- distance-amplitude correction curve (1-4420 or 1-4520)

ment combination (i.e., instrumentation, cable, and search and the sweep range calibration (1-44 10 or 1-45 10) at the unit) is used during the examination. start and finish of each examination, with any change in examination personnel, and at least every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> during

4 These determinations are to aid third-party evaluations an examination."

when different equipment is used to record indications on subsequent examinations and are not intended to qualify In the 1977 edition, these requirements were changed.

systems for use. According to Article 4 (T-432.1.2),Section V of the ASME

Code, 1977 edition, the following applies:

The intent of regulatory position 1.5 is to establish the instrument pulse shape in a way that actual values of pulse length and voltages can be observed on an oscilloscope. The "A calibratio'n check on at least onerof the basic reflectors calibrated time base does not necessarily have to follow the in the basic calibration block or a check using a simulator time base of the distance-amplitude correction (DAC) curve but shall be made at the finish of each examination, every may be chosen to suitably characterize the initial pulse. The 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> during the examination and when examination pulse shape record will assist in analyzing potential differences personnel are changed."

in flaw response between successive examinations (i.e., is the This requirement has several minor deficiencies, including difference due to flaw growth or system change).

the following:

Pulse shape is best determined by using a high-impedance oscilloscope with the transducer disconnected from the a. One-Point Check instrument.

A calibration check is now required on only one of the

2. CALIBRATION basic reflectors. As a result, the accuracy of only one point on the DAC curve, and not the accuracy of three points as According to Appendix I, Article 1, 1-4230,Section XI of previously required, is checked. This alteration would the ASME Code, 1974 edition, instrument calibration for permit the instrument drift for other metal path distances to go unnoticed, which is not desirable.

l"Ultrasonic Reinspection of Pilgrim 1 Reactor Vessel Nozzle N2B," John H. Gieske, NUREG-6502.

2

b. Secondary Reference

"Summary Hatch Nuclear Plant Unit 1 Reactor Pressure Vessel Repair," 1972, Georgia Power Company.

The change allows a one-point check by a mechanical

3

"Summary of the Detection and Evaluation of Ultrasonic or electronic simulator instead of a check against the basic Indications - Edwin Hatch Unit 1 Reactor Pressure Vessel," Jan uary

1972, Georgia Power Company.

4 Round robin tests conducted by the Pressure Vessel Research Committee (PVRC) of the Welding Research Council for UT of calibration block. A mechanical simulator could be a plastic, steel, or aluminum block with a single reference reflector, which may be a hole or a notc

h. Without specified

4 thick section steels. details, the electronic simulator could be any device that

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provides an electrical signal. With the resulting uncertainty, dropped) during transport than those parameters that there may be errors in checking against the secondary served as a basis for defining the error band.

reference (simulator), the magnitude of which is undefined and unknown. Use of electronic simulators would be permissible if they can check the calibration of the UT system as a whole c. Electronic Simulator and the error band introduced by their use can be relied on and taken into consideration.

Subarticle T-432.1.3 of Article 4,Section V of the ASME Code, 1977 edition, allows the use of an electronic d. Static Versus Dynamic Reflector Responses simulator and also permits the transducer sensitivity to be checked separately. Both these provisions may introduce With some automated systems, the DAC curve is errors that will be very difficult to detect. manually established. In these cases, the signal is maximized by optimizing the transducer orientation toward the To avoid the introduction of errors and to ensure calibration holes. Subsequently, detection and sizing of repeatability of examinations at a later date, it would be flaws are based on signals received from a moving transducer advisable to check the calibration of the entire system where no attempt is made (or it is not possible) to maximize rather than that of individual components. Checking system the signal even for significant flaws. This procedure neglects calibration without the transducer and the cable is not several sources of error introduced by the possible variation advisable because these tests do not detect possible leakage in signal strength caused by:

or resistance changes at the connectors. This is especially important when the UT examination is performed under (1) Differences between the maximized signal conditions of high humidity or under water and the connec- and the unmaximized signal.

tors may not be waterproof or moistureproof. Checking the transducer sensitivity separately (sometimes weeks in (2) Loss in signal strength due to the separation of advance) also neglects the effects of possible damage due to the transducer from the metal surface because transport or use. The transducer characteristics may change of the viscosity of the coupling medium (plan- because of damage to or degradation of internal bonding ing effects).

agents or inadvertent damage to the transducer element.

Further, the use of an electronic block simulator (EBS) as a (3) Variation in contact force and transducer secondary standard introduces an error band in the calibra- coupling efficiency.

tion process. The error band may depend on, among others, the following factors: (4) Loss in signal strength due to structural vibra- tion effects in the moving transducer mount

(1) Drift due to ambient temperature change. and other driving mechanisms.

(2) Drift due to high temperature storage.

(3) Drift due to high humidity storage. (5) Loss in signal strength due to the tilting caused

(4) Drift due to vibration and shock loading during by the mounting arrangement in some trans- shipment. ducer mounts.

(5) Degradation of the memory device used to store the reference signal information due to vibra- Because of the above, it would be advisable to establish tion, shock, aging, or heat effects. the DAC curve under the same conditions as those under which scanning is performed to obtain data for detection To ensure stability, computer systems are generally and sizing. It would be acceptable to establish a DAC curve kept in an air conditioned environment; however, EBS by maximizing signal strength during manual scans when systems are not usually kept in a controlled environment. signals are also maximized for flaw sizing. However, it

5 would not be advisable to use manually maximized signals Error band for one particular type of instrument to establish the DAC curve when data are obtained later by was determined to be in the range of +/-6 percent. The error mechanized transducers (where signals cannot be maximized)

band for other instruments may be in a different range and for the detection and sizing of flaws without adjustment for may vary for the same instrument if memory devices or the potential error introduced. In these situations, an components of different quality are used at a later date. acceptable method would be to establish DAC curves using The error band is dependent on the temperature extremes, moving transducers or to establish correction factors that shock loadings, and vibrations suffered by the instrument. may be used to adjust signal strength. It would be prudent Since the error band value depends on these parameters, it to use care and planning in establishing correction factors.

would be advisable to ensure, through recording instruments, For example, establishing a ratio between a dynamic and that the EBS was not subjected to higher temperatures static mode under laboratory conditions using a precision (container lying in the sun) and greater shock (container transducer drive and stiff mounting may have very little in common with the transducer mounting and traverse condi- tions of the actual examination setup. If correction factors

5

"Calibration Verification of Ultrasonic Examination Systems with are to be used, it would be worthwhile to build either the Electronic Block Simulator," D. J. Boomgard et al., August 1979, full-scale mockups or consider the variation of all the Report No. WCAP-9545, Westinghouse Electric Corporation, Nuclear Service Division, P.O. Box 2728, Pittsburgh, PA 15230. important parameters in a suitable model taking into

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consideration scaling laws on variables such as mass, vibration, holes; however, if the block or these holes are polished, this and stiffness constants. It would be advisable to confirm the fact should be recorded for consideration if a review of the scaling law assumptions and predictions for vibration and viscosity effects before correction factors are used for setting scanning sensitivity levels.

UT data becomes necessary at a later date.

3. NEAR-SURFACE EXAMINATION AND SURFACE

RESOLUTION

4 Differences in the curvature and surface finish between calibration blocks and vessel areas could change the dynamic Sound beam attenuation in any material follows a response, so it may be advisable to establish correction factors decaying curve (exponential function); however, in some between dynamic and static responses from the indications cases the reflection from the nearest hole is smaller than the that are found during examination. This would avoid the reflection from a farther hole. This makes it difficult to difficulties associated with establishing a dynamic response draw a proper DAC curve. In such cases, it may be desirable DAC curve and still take all the factors into consideration. to use a lower frequency or a smaller transducer for flaw detection near the beam-entry surface to overcome the e. Secondary DAC difficulty of marginal detectability.

During some manual scans, the end point of the DAC Near-field effects, decay time of pulse reflections, curve may fall below 20 percent of the full screen height. shadow effects, restricted access, and other factors do not When this happens, it is difficult to evaluate flaws on the permit effective examination of certain volume areas in the

20 percent and 50 percent DAC basis in this region since component. To present a clear documentation and record the 20 percent and 50 percent DAC points may be too close of the volume of material that has not been effectively to the baseline. To overcome this difficulty, it is advisable examined, these volume areas need to be identified. Recom- that a secondary DAC curve using a higher-gain setting be mendations are provided to best estimate the volume in the developed so that 20 percent and 50 percent DAC points may region of interest that has not been effectively examined, be easily evaluated. For this purpose, it is advisable that the such as volumes of material near each surface (because of gain be increased sufficiently to keep the lowest point of near-field effects of the transducer and ring-down effects of the secondary DAC curve above 20 percent of screen height. the pulse due to the contact surface), volumes near interfaces between cladding and parent metal, and volumes shadowed The secondary DAC curves need not be generated by laminar flaws.

unless they are required. If electronic DAC is used and amplitudes are maintained above 20 percent of full screen height, a secondary DAC would not be necessary.

4. BEAM PROFILE

Beam profile is one of the main characteristics of a tians-

4 f. Component Substitution ducer. It helps to show the three-dimensional distribution of beam strength for comparing results between examinations A calibration check should be made each time a and also for characterizing flaws. The beam profile needs to component is put back into the system to ensure that such be determined and recorded so that comparisons may be components as transducers, pulsers, and receivers were not made with results of successive examinations.

damaged while they were in storage. This will ensure elimination of the error band and mistakes in resetting the 5. SCANNING WELD-METAL INTERFACE

various control knobs.

The amount of energy reflected back from a flaw is g. Calibration Holes dependent on its surface characteristics, orientation, and size. The present ASME Code procedures rely on the Comparison of results between examinations performed amplitude of the reflected signal as a basis for judging flaws.

at different times may be facilitated if the same equipment This means that the size estimation of a defect depends on is used and if the reflections from growing flaws can be the proportion of the ultrasonic beam reflected back to the compared to the same reference signal. Reference signals probe. The reflection behavior of a planar defect, which obtained from a calibration block depend on, among other largely depends on the incident beam angle when a single things, the surface roughness of the block and the reflector search unit is used to characterize the flaw, is thus a decisive holes. Therefore, these surfaces should be protected from factor in flaw estimation. The larger the size of a planar corrosion and mechanical damage and also should not be defect, the narrower is the reflected sound beam. The altered by mechanical or chemical means between successive narrow reflected sound beam makes the flaw very difficult 6 7 examinations. If the reference reflector holes or the block to detect in most cases (unless the beam angle is right). '

surface are given a high polish by any chemical or mechanical means, the amplitude of the reflections obtained from these

6 reflector holes may be altered. Polishing the holes or the "Probability of Detecting Planar Defects in Heavy Wall Welds by block surface is not forbidden by the ASME Code. However, Ultrasonic Techniques According to Existing Codes," Dr. lng. Hans- Jurgen Meyer, Quality Department of M.A.N., Nurnberg, D 8500

this possibly altered amplitude could affect the sizing of indications found during any examination. At this time, no recommendations are being made to control the surface Nurnberg 115.

7

"Reflection of Ultrasonic Pulses from Surfaces," Haines and Langston Central Electricity Generating Board, U.K. (CESB) Report I

roughness of the block or the above-mentioned reflector Number RD 18/N4115.

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8 Therefore, the beam angles used to scan welds should be has to be considered in judging the significance of flaws.

optimized and should be based on the geometry of the It is therefore recommended that only signals with a total weld/parent-metal interface. At least one of these angles transducer travel movement greater than the beam spread should be such that the beam is almost perpendicular (+/-l 5 should be considered significant.

degrees to the perpendicular) to the weld/parent-metal interface, unless it can be demonstrated that large (Code-

7. REPORTING OF RESULTS

unacceptable) planar flaws unfavorably oriented, parallel to the weld-metal interface, can be detected by the UT tech- This guide gives recommendations for recording the charac- nique being used. In vessel construction, some weld preps are teristics of the UT examination system. This information essentially at right angles to the metal surface. In these cases, can be of significance in later analysis for determining the use of shear wave angles close to 75 degrees is not recom- location, dimensions, orientation, and growth rate of flaws.

mended. Two factors would make the use of shear wave angles close to 75 degrees inadvisable, - first, the test distances Records pertaining to UT examinations should be con- necessary become too large resulting in loss of signal, and sidered quality assurance records. Recommendations on the second, the generation of surface waves tends to confuse collection, storage, and maintenance of these records are the interpretation of results. In these cases, use of alternative given in Regulatory Guide 1.88, "Collection, Storage, and volumetric nondestructive examination (NDE) techniques, Maintenance of Nuclear Power Plant Quality Assurance Re- as permitted by Subarticle IWA-2240,Section XI of the cords." Availability of these records at a later date will permit ASME Code, should be considered. Alternative NDE a review of the UT results from the data gathered during techniques to be considered may include high-intensity previous ultrasonic examinations.

radiograph or tandem-probe ultrasonic examination of the weld-metal interface. To avoid the possibility of missing When ultrasonic examination is performed, certain vol- large flaws, particularly those that have an unfavorable umes of material such as the following are not effectively orientation, it is desirable that the back reflection amplitude, examined:

while scanning with a straight beam, be monitored over the entire volume of the weld and adjacent base metal. Any a. Material volume near the front surface because of near- area where a reduction of the normal back-surface reflection field effects, cladding disturbance, or electronic gating.

amplitude exceeds 50 percent should be examined by angle beams in increments of +/-15 degrees until the reduction of b. Material volume near the surface because of surface signal is explained. Where this additional angle beam roughness or unfavorable flaw orientations.

examination is not practical, it may be advisable to consider examining the weld by a supplementary volumetric NDE c. Volumes shadowed by insulation or part geometry.

technique.

In some cases, as much as 1 inch (25.4 mm) or more

6. SIZING below the surface is not examined because of the electronic gate setting. This means that the unexamined volume may The depth or through-wall dimension of flaws is more contain flaws that would be unacceptable according to significant than the length dimension, according to fracture Section XI, ASME Code, as follows:

mechanics analysis criteria. Using the single-probe pulse-echo technique, it is possible, depending on flaw orientation, a. Without evaluation (deeper than approximately 0.2 that some large flaws may not reflect much energy to the inch).

search unit. 6 Because of this possibility, the depth dimen- sion of the flaw should be conservatively sized unless there is b. Even after evaluation (deeper than approximately evidence to prove that the flaw orientation is at right angles 0.85 inch).

to the beam. It is recommended that indications that are asso- ciated with through-thickness flaws and do not meet Code- Assuming an aspect ratio of 0.1, according to IWB-35 10.1, allowable criteria or criteria recommended in this guide be Section XI, ASME Code, flaws 0.2 inch deep would be sized at 20 percent DAC as well as at 50 percent DAC. unacceptable for a 9-inch wall thickness.

In certain cases, it is possible for various reasons that a Typically a BWR reactor pressure vessel (RPV) wall in flaw would not reflect enough energy to the search unit to the beltline region is 6 inches thick and a PWR-RPV wall is make the indication height 50 percent of the DAC curve 8.5 inches thick. During flaw evaluation, where the wall height. However, if such a flaw were large, a persistent temperature is high and the available toughness is high, and signal could be obtained over a large area. It is therefore the calculated critical surface flaw depth (ac) exceeds the wall recommended that all continuous signals that are 20 percent thickness (t), ac is taken9 as the wall thickness. According to of DAC with transducer travel movement of more than IWB-3600,Section XI, the allowable end-of-life size is af =

1 inch plus the beam spread (as defined in Article 4, non- 0. 1ac. Flaws exceeding this allowable value, which would mandatory Appendix B,Section V of the ASME Code,

8

1977 edition) should be considered significant and should "Ultrasonic Examination Comparison of Indication and Actual Flaw be recorded and investigated further. The beam spread in RPV," Ishi Kawajima-Harima Industries Co., Ltd., January 1976.

effect in some cases can make very small flaws appear to be 9

"Flaw Evaluation Procedures: ASME Section XI-EPRI," NP-719-SR,

large when judged at 20 percent DAC; hence, beam spread special report, August 1978.

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be 0.85 inch for a PWR and 0.65 inch for a BWR, will have 1.3 Amplitude Control Linearity to be repaired. The above example illustrates the importance of blanking out the electronic indication signals and not examining the surface volume to a depth of 1 inch. Since the flaws that can be missed because of electronic gating may Amplitude control linearity should be determined according to the mandatory Appendix II of Article 4,Section V of the ASME Code, 1977 edition, within the time limits specified in

4 be larger than the flaws permitted with or without evaluation, regulatory position 1.1.

this unexamined volume is important and needs to be identified.

In certain specific cases, areas were not examined 1.4 Frequency-Amplitude Curve because insulation was in the way and the transducer could not scan the volume of interest. NRC was not informed of A photographic record of the frequency-amplitude curve this situation until much later. In view of the above and to should be obtained. This record should be available for avoid licensing delays, it is advisable that the volume of areas comparison at the inspection site for the next two successive not examined for any or all of the above reasons be reported. inspections of the same volume. The reflector used in generating the frequency-amplitude curves as well as the The volumes of material that are not effectively examined electronic system (i.e., the basic ultrasonic instrument, depend on the particular part geometry and unique situa- gating, form of gated signal, and spectrum analysis equip- tions associated with each RPV. During identification of ment) and how it is used to capture the frequency-amplitude the material volumes that have not been examined, considera- information should be documented.

tion should be given to the types of flaws that are currently being reported in some of the operating plants. These include stress corrosion cracks in the heat-affected zone, 1.5 Pulse Shape fatigue cracks, and 'cracks that are close to the surface and sometimes penetrate the surface. These volumes of A photographic record of the unloaded initial pulse material should be identified and reported to NRC along against a calibrated time base should be obtained. The time with the report of welding and material defects in accordance base and voltage values should be identified and recorded with the recommendation of regulatory position 2.a(3) of on the horizontal and vertical axis of the above photographic Regulatory Guide 1.16, "Reporting of Operating Informa- record of the initial pulse. The method used in obtaining tion-Appendix A Technical Specifications." the pulse shape photograph, including the test point at which it is obtained, should be documented.

C. REGULATORY POSITION

2. CALIBRATION

4 Ultrasonic examination of reactor vessel welds should be performed according to the requirements of Section XI of System calibration should be checked to verify the DAC

the ASME B&PV Code, as referenced in the Safety Analysis curve and the sweep range calibration per nonmandatory Report (SAR) and its amendments, supplemented by the Appendix B, Article 4,Section V of the ASME Code, as a following: minimum, before and after each RPV examination (or each week in which it is in use, whichever is less) or each time any

1. INSTRUMENT PERFORMANCE CHECKS component (e.g., transducer, cable, connector, pulser, or receiver) in the examination system is changed. Where possible, The checks described in paragraphs 1.2 through 1.5 should the same calibration block should be used for successive in- be made for any UT system used for the recording and sizing service examinations of the same RPV. The calibration side of reflectors in accordance with regulatory position 6 and holes in the basic calibration block and the block surface should for reflectors that exceed the Code-allowable criteria. be protected so that their characteristics do not change during storage. These side holes or the block surface should not be modified in any way (e.g., by polishing) between successive

1.1 Frequency of Checks examinations. If the block surface or the calibration reflector holes have been polished by any chemical or mechanical means, As a minimum, these checks should be verified within 1 day this fact should be recorded.

before and within I day after examining all the welds that need to be examined in a reactor pressure vessel during one outage.

Pulse shape and noise suppression controls should remain at 2.1 Calibration for Manual Scanning the same setting during examination and calibration.

For manual scanning for the sizing of flaws, static calibra-

1.2 Screen Height Linearity tion may be used if sizing is performed using a static trans- ducer. When signals are maximized during calibration, they Screen height linearity of the ultrasonic instrument should also be maximized during sizing. For manual scanning should be determined according to the mandatory Appen- dix I to Article 4,Section V of the ASME Code, within the time limits specified in regulatory position 1.1.

for the detection of flaws, reference hole detection should be shown at scanning speed and detection level set accordingly (from the dynamic DAC).

I

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2.2 Calibration for Mechanized Scanning d. When a universal calibration block is used and some or all of the reference holes are larger than the reflector When flaw detection and sizing are to be done by holes at comparable depths recommended by Article 4, Sec- mechanized equipment, the calibration should be performed tion V, of the ASME Code, 1980 edition, a correction factor using the following guidelines: should be used to adjust the DAC level to compensate for the larger reflector holes. Also, if the reactor pressure vessel a. Calibration speed should be at or higher than the has been previously examined by using a conventional block, scanning speed. a ratio between the DAC curves obtained from the two blocks should be noted (for reference) with the significant b. The direction of transducer movement during calibra- indications data.

tion should be the same as the direction during scanning unless (1) it can be shown that the change in scanning direction 3. NEAR-SURFACE EXAMINATION AND SURFACE

does not make a difference in the sensitivity and vibration RESOLUTION

background noise received from the search unit or (2) these differences are taken into account by a correction factor. The capability to effectively detect defects near the front and back surfaces of the actual component should be c. For mechanized scanning, signals should not be estimated. The results should be reported with the report of maximized during the establishment of the DAC curve. abnormal degradation of reactor pressure boundary in accordance with the recommendation of regulatory posi- d. One of the following alternative guidelines should be tion 2.a(3) of Regulatory Guide 1.16. In determining this followed for establishing the DAC curve: capability, the effect of the following factors should also be considered:

(1) The DAC curve should be established using a moving transducer mounted on the mechanism that will be a. If an electronic gate is used, the time of start and stop used for examination of the component. of the control points of the electronic gate should be related to the volume of material near each surface that is

(2) Correction factors between dynamic and static not being examined.

response should be established using full-scale mockups.

b. The decay time, in terms of metal path distance, of

(3) Correction factors should be established using the initial pulse and of the pulse reflections at the front and models and taking scaling factors into consideration (assumed back surface should be considered.

scaling relationship should be verified).

c. The disturbance created by the clad-weld-metal

(4) Correction factors between dynamic and static interface with the parent metal at the front or the back response should be established from the indications that are surface should be related to the volume of material near the found during examination for sizing. For detection of flaws interface that is not being examined.

during the initial scan, correction factors may be assumed based on engineering judgment. If assumed correction d. The disturbance created by front and back metal factors are used for detection, these factors should later be surface roughness should be related to the volume of confirmed on indications from flaws in the vessel during the material near each surface that is not being examined.

examination. Deviation from the assumed value may suggest reexamining the dat

4. BEAM PROFILE

2.3 Calibration Checks The beam profile should be determined if-any recordable flaws are detected. This should be done for each search unit If an EBS is used for calibration check, the following used during the examination by a procedure similar to that should apply: outlined in the nonmandatory Appendix B (B-60), Article 4,Section V of the ASME Code, 1980 edition, for determining a. The significant DAC percentage level used for the beam spread. Beam profile curves should be determined for detection and sizing of indications should be reduced to each of the holes in the basic calibration block. Interpola- take into account the maximum error that could be introduced tion may be used to obtain beam profile correction for assess- in the system by the variation of resistance or leakage in ing flaws at intermediate depths for which the beam profile the connectors or other causes. has not been determined.

b. Calibration checks should be performed on the 5. SCANNING WELD-METAL INTERFACE

complete connected system (e.g., transducer and cables should not be checked separately). The beam angles used to scan welds should be based on the geometry of the weld/parent-metal interface. At least c. Measures should be taken to ensure that the different one of these angles should be such that the beam is almost variables such as temperature, vibration, and shock limits perpendicular (+/-15 degrees to the perpendicular) to the for which the EBS error band is determined are not exceeded weld/parent-metal interface unless it can be demonstrated during transport, use, storage, etc. that unfavorably oriented planar flaws can be detected by

1.150-7

the UT technique being used. Otherwise, use of alternative the site for examination by the NRC staff. If the size of volumetric NDE techniques, as permitted by the ASME an indication, as determined in regulatory positions 6.1 or Code, should be considered. Alternative NDE techniques may be considered to include high-intensity radiography or tandem-probe ultrasonic examination of the weld-metal

6.2, equals or exceeds the allowable limits of Section XI of the ASME Code, the indications should be reported as abnormal degradation of reactor pressure boundary in A

interface. accordance with the recommendation of regulatory posi- tion 2.a(3) of Regulatory Guide 1.16.

6. SIZING

Along with the report of ultrasonic examination test Indications from geometric sources need not be recorded. results, the following information should also be included:

6.1 Traveling Indications a. The best estimate of the error band in sizing the flaws and the basis for this estimate should be given.

Indications that travel on the horizontal baseline of the scope for a distance greater than indications from the b. The best estimate of the portion of the volume calibration holes (at 20 percent DAC amplitude) should be required to be examined by the ASME Code that has not recorded. Indications that travel should be recorded and been effectively examined such as volumes of material near sized at 20 percent DAC. Where the indication is sized at each surface because of near-field or other effects, volumes

20 percent DAC, this size may be corrected by subtracting near interfaces between cladding and parent metal, volumes for the beam width in the through-thickness direction shadowed by laminar material defects, volumes shadowed obtained from the calibration hole (between 20 percent by part geometry, volumes inaccessible to the transducer, DAC points) that is at a depth similar to the flaw depth. If volumes affected by electronic gating,1 and volumes near the the indication exceeds 50 percent DAC, the size should be surface opposite the transducer. 0

recorded by measuring the distance between 50 percent DAC levels without using the beam-width correction. The c. The material volume that has not been effectively determined size should be the larger of the two. examined by the use of the above procedures may be examined by alternative effective volumetric NDE techniques.

6.2 Nontraveling Indications If one of these alternative NDE techniques is a variation of UT, recommendations of regulatory positions I and 3 Nontraveling indications above 20 percent DAC level should apply. A description of the techniques used should that persist for a scanning distance of more than 1 inch plus the beam spread between 20 percent DAC points (as defined by nonmandatory Appendix B, Article 4,Section V

of the ASME Code, 1977 edition) should be considered be included in the report. If other volumetric techniques or variations of UT are used as indicated in regulatory posi- tion 5, the effectiveness of these techniques should be demonstrated and the procedures reported for review by

4 significant. The size of these flaws should be determined by the NRC staff.

measuring the distance between points at 50 percent DAC and between points at 20 percent DAC where the beam-

D. IMPLEMENTATION

width correction is made only for the 20 percent DAC size.

The recorded size of the flaw would be the larger of the Except in those cases in which an applicant proposes an two determinations. If it can be adequately demonstrated acceptable alternative method for complying with specified that a nontraveling indication is from a geometric source portions of the Commission's regulations, the method (and not a flaw), there is no need to record that indication. described herein will be used in the evaluation of (1)

the results of inservice examination programs of all operating The following information should also be recorded for reactors after July 15, 1981, and (2) the results of preservice indications that are reportable according to this regulatory examination programs of all reactors under construction position: performed after January 15, 1982.

a. Indications should be recorded at scan intervals no The recommendations of this guide are not intended to greater than one-fourth inch. apply to preservice examinations that have already been completed.

b. The recorded information should include the indica- tion travel (metal path length) and the transducer position The NRC staff intends to recommend that all licensees for 10 percent, 20 percent, 50 percent, and 100 percent modify their technical specifications to make them consistent DAC and the maximum amplitude of the signal. with the recommendations contained herein.

7. REPORTING OF RESULTS

lOlt should be noted that the licensee is required to apply for relief Records obtained while following the recommendations from impractical ASMECode requirements according to § 50.55a of of regulatory positions 1.2, 3, 5, and 6, along with discus- I

10 CFR. If the licensee is committed to examine a weld as per the inspection plan in the plant SAR, the licensee is required to file an sions and explanations, if any, should be kept available at amendment when the commitments made in the SAR cannot be met.

1.150-8

VALUE/IMPACT STATEMENT

1. PROPOSED ACTION demonstrated without doubt that the flaw will not grow and has not been growing, a rather large flaw can be tolerated.

1.1 Description Crack initiation and growth is also a potential problem in cases where it is probable that no crack exists, but where The present inservice examination procedures for there is a cluster of small rounded inclusions. These clusters ultrasonic examination require improvement in order to of inclusions should be monitored by UT to ensure absence consistently and reliably characterize flaws in reactor of cracks and crack growth.

pressure vessel (RPV) welds and RPV nozzle welds. The apparent low level of the reproducibility of detection, Where the rate of flaw growth is expected to be large or location, and characterization of flaws leads to lengthy is uncertain, even a small flaw may be of concern. To discussions and delays in the licensing process. Much permit determination of growth rate, the UT procedures attention is paid to the integrity of RPV welds during the should be such that results of successive UT examinations licensing process because the failure probability of a reactor can be compared. With present procedures, these results pressure vessel is considered to be sufficiently low to cannot be compared because of variation in instrument exclude it from consideration as a design basis accident. system characteristics. UT instrument system characteristics The assumption of a low probability relies heavily on depend on the characteristics of the system's different regularly repeated inservice examination by ultrasonic components. Variation in the characteristics of calibration testing (UT) of welds. blocks can also affect results.

1.2 Need for Proposed Action Guidelines are needed so that uncertainties in flaw charac- terization may be reduced or eliminated. The safety of the As more reactors start producing power, as those in components is evaluated with the help of fracture mechanics.

operation grow older, and as more inservice examinations Flaw sizes need to be known for fracture mechanics evalua- are performed, the number of detected flaws with uncertain tions. Uncertain determination of flaw sizes leads to uncer- characteristics (size, orientation, and location) is likely to tainties in the determination of the safety of the components.

increase. Flaw characterization is essential for flaw evalua- Uncertainties in component safety lead to delays in licensing.

tions required by the ASME Code and by NRC to determine There is a need to specify and standardize the performance the structural integrity of nuclear reactor components when required of most UT system components to achieve better such flaws exist. It is essential to have valid background consistency in UT results so that delays in the licensing data for the flaw evaluations required by Section XI of the process may be reduced.

ASME Code. Based on the information gathered according to ASME Code requirements, it is often difficult to assess This guide will provide supplementary procedures with whether or not the flaw has grown between examinations. the objective of improving conventional UT procedures, as The procedures now in use do not require the recording of defined in the ASME Code. This guide is based partly on the certain information that can be important in later analysis information available in literature concerning both U.S. and for determining the location, dimensions, orientation, and European procedures and partly on the judgment of the growth rate of flaws. NRC staff and their consultants. On the basis of support work being performed at the Oak Ridge National Laboratory, The lack of standardization in the use of UT equipment the staff plans to issue a revision to this guide that should and procedures leads to uncertainty concerning the results further improve flaw characterization.

obtained. For example, transducer characteristics such as beam spread, damping characteristics, and frequency The use of new techniques such as holography or synthetic for peak response are not defined, and there is no provision aperture imaging of flaws by UT that have not been imple- to keep track of these from one examination to the other. mented into practice and could considerably increase the Similarly, characteristics of other UT system components cost of inservice examination is not being proposed here.

such as the pulser, receiver, amplifier, and video display screen may vary from one examination to another, and all 1.3 Value/Impact of Proposed Action these characteristics can influence the magnitude of the flaw indications. Therefore, well-defined criteria for supple- 1.3.1 NRC

mentary UT procedures are needed so that it will be possible to correctly characterize flaws, estimate flaw growth, and Reporting UT examination results as indicated in this guide have reproducible results from inspections performed at would help the NRC staff and their consultants to better different times using different equipment. assess the results of the data. At present, the NRC staff must spend a great deal of time on controversy over deter- In many instances, the rate of flaw growth can be even mining the safety of components from inconsistent UT

more important than the flaw size. For example, if a flaw is results. Lack of faith in flaw size determination from found in an RPV nozzle or belt-line region and it can be uncertain UT results points toward the adoption of some

1.150-9

conservative safety measures that are undesirable, for the i. Providing more consistent UT procedures for flaw most part, to the industry managers. Licensing delays occur characterization, thereby leading to procedures that because decisions have to be made on the basis of uncertain information. Flaw size determination from consistent UT

results would help remove or reduce the uncertainties and ensure lower probability of missing large flaws and ensuring greater safety for the public, industrial workers, and government employees.

4 debates over the safety issues. Because of the above, NRC

staff time for review of reported data and interpretation of indications is likely to be reduced. 1.3.3.2 Impact. There will be major impact in the following three areas:

1.3.2 Other Government Agencies a. Quality control of the UT equipment Not applicable, unless the government agency is an At present, requirements in the ASME Code for quality applicant, such as TVA. control of UT equipment are marginal; for example, there are no direct requirements to control the quality of UT transducers. Criterion XII,"Control of Measuring

1.3.3 Industry and Test Equipment," of Appendix B, "Quality Assur- ance Criteria for Nuclear Power Plants and Fuel Repro- The value/impact on industry of the regulatory guide cessing Plants," to 10 CFR Part 50 requires, in part, that positions is stated by each position in the appendix to this measures be established to ensure that instruments used value/impact statement. Some highlights of the value and in activities affecting quality are properly controlled, impact of the regulatory guide positions are stated below. calibrated, and adjusted at specified periods to maintain accuracy within necessary limits. The recommendations of this guide will help to bring about uniformity in the

1.3.3.1 Value. This regulatory guide specifies supplemen- quality control procedures among different companies tary procedures that will lead to the following advantages: and will ensure that quality control measures are taken to ensure reliability and reproducibility of UT results.

a. Attaining greater accuracy and consistency in flaw No new UT equipment will be needed to follow the characterization. recommendations of this guide. However, the quality control measures recommended for UT equipment b. Providing information for consistent flaw characteriza- tion at NRC review time and thus reducing NRC staff effort in review of flaw indications.

will impose extra cost burdens that are difficult to estimate without feedback from industry. 4 b. Increase ifi examination time c. Helping assess flaw growth.

This guide would recommend, for the first time, that d. Providing a more reliable basis for flaw detection and indications with significant length of indication travel evaluation, which should help in the uniform enforce- (larger than the standard calibration holes) or with ment of rules and the avoidance of delay in licensing significant depth dimensions be recorded. It is not decisions. expected that the slag type of flaws, which are common among welds, or geometric reflectors will give signif- e. Reducing licensing time for reviewing examination icant traveling indications within the guidelines pro- results, which will aid in the reduction of reactor down- posed. Hence, no substantial increase in recorded time during examinations and will be of great benefit indications as a result of this recommendation is to industry. With present construction costs of about expected; however, the exact increase is difficult to

1.3 billion dollars for a 1000-megawatt reactor and the predict or estimate.

average size of a reactor running around 1100-megawatt capacity, the savings per day by eliminating reactor Reporting of indications associated with flaws larger downtime are likely to be $500,000 or more. than 1 inch (indications larger than 1 inch plus beam spread at 20 percent DAC level) is also new. RPV welds f. Avoiding unnecessary repairs due to flaw size uncer- are examined by radiography, and no flaws larger than tainties. three-quarters of an inch are acceptable in these welds.

Because of this acceptance length, only new service- g. Reducing radiation exposure to personnel by helping induced flaws larger than 1 inch, of which there should to eliminate unnecessary repairs. The radiation not be many, are expected to be identified and reported exposure during repairs is usually many times the as a result of this recommendation.

exposure during examination, so a net reduction in radiation exposure is expected. Because of the above two new reporting recommenda- h. Reducing margins of error in estimates of flaw growth and thus helping reduce overconservative estimates tions, there may be an increase in examination time and dollar cost that is difficult to estimate. This will depend on how many significant flaws are detected I

and decisions on flaw acceptance. and how large and complex they are.

1.150-10

c. Radiation exposure 2.3 Comparison of Technical Alternatives Recommendations of this guide apply to the examina- Imposing inservice examination of RPV welds by the use tion of RPV welds and RPV nozzle welds. RPV welds of holography, synthetic aperture imaging technique, or are usually examined by automated equipment, and acoustic emission, all of which are still in the stage of proto- data are collected on tape. Therefore, no increase in type development and have not been proved effective for radiation exposure is anticipated as a result of the field use, would not be justifiable on the basis of either regulatory guide positions addressing RPV weld cost or effectiveness.

examinations.

RPV nozzle welds are sometimes examined by 2.4 Comparison of Procedural Alternatives automated equipment but in most cases by manual UT. An increase in radiation exposure to examination Leaving the situation as it is would mean that continued personnel may be expected while RPV nozzles are attention and manpower would have to be devoted by the being manually examined. The probable percent NRC staff to investigate the uncertainties associated with increase in examination time or radiation exposure is flaw growth on a case-by-case basis. The low level of impossible to estimate without field data and research confidence in the present techniques means that excessive effort. Requirements for reporting traveling indica- margins would continue to be used in the flaw-acceptance tions and indications associated with flaws larger than criteria. Also, unnecessary cutting and repair attempts to

1 inch may lead to an increase in occupational remove suspected flaws may result.

exposure in those cases in which the above indications are found and additional examination is required. The The procedures recommended in this guide have been magnitude of this additional exposure can only be shown to be effective in practice, although they are not in assessed on a case-by-case basis. It should be noted general use in the United States. Including these procedures that radiation levels at vessel nozzle regions are as regulatory guide recommendations should result in their reported to range from 0.5 to 2.0 rem/hour. Total wider use and consequently their improvement. After these person-rem doses can be drastically reduced by procedures have been accepted by the industry, we will shielding and local decontamination. seek their inclusion in the ASME Code. Some of these procedures have already been sent to the ASME for considera- The guide is not expected to have any adverse impact on tion and inclusion in the present ASME Code procedures other government agencies or the public. for ultrasonic examinations.

1.3.4 Public

2.5 Decision on Technical and Procedural Alternatives No impact on the public can be foreseen. The only identifiable value is a slight acceleration in the review On the basis of the above, it appears desirable to issue a process. regulatory guide to provide recommendations for improving ASME Code procedures. These recommendations, which

1.4 Decision on Proposed Action are based on the advanced state-of-the-art UT procedures in current use by some organizations, would improve the The Office of Nuclear Reactor Regulation (NRR) has ability to detect and characterize flaws without imposing stated the need for this guide to help them and their new, unproved techniques for flaw detection on industry.

consultants in evaluating the size and significance of the flaws detected during inservice examination to ensure the 3. STATUTORY CONSIDERATIONS

integrity of reactor pressure vessels between periods of examination. It would therefore be advisable to issue this 3.1 NRC Authority guide.

The authority for this guide is derived from the safety

2. APPROACH requirements of the Atomic Energy Act of 1954, as amended, and the Energy Reorganization' Act of 1974, as implemented

2.1 Technical Alternatives by the Commission's regulations. In particular, § 50.55a,

"Codes and Standards," of 10 CFR Part 50 requires, in Alternatives would include requiring the use of holography, part, that structures, systems, and components be designed, synthetic aperture imaging, acoustic emission, neutron fabricated, erected, constructed, tested, and inspected to radiography, or a combination of the above during RPV quality standards commensurate with the importance of inservice examination. the safety function to be performed.

2.2 Procedural Alternatives 3.2 Need for NEPA Assessment One alternative is to leave the situation as it is. A second The proposed action is not a major action, as defined by alternative is to request change of the ASME Code require- paragraph 51.5(a)(10) of 10 CFR and does not require an ments. environmental impact statement.

1.150-11

4. RELATIONSHIP TO OTHER EXISTING OR PRO- difficult for the NRC staff or their consultants to review, POSED REGULATIONS OR POLICIES analyze, and assess the UT data to determine the flaw size Recommendations of this guide would be supplemental to the requirements of Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components," of the and evaluate the system safety when the data are made available to NRC at a later date. The present data obtained from UT equipment of uncertain and unspecified performance lead to discussions and delays in the review process resulting

4 ASME Code, which is adopted by § 50.55a, "Codes and in loss of NRC staff time and loss of plant availability Standards," of 10 CFR Part 50. and power generation capacity for the utilities. These situations definitely need to be avoided as much as possible.

5. SUMMARY This guide is aimed at achieving this purpose by issuing recommendations that will be supplementary to the existing This guide was initiated as a result of a request from ASME Code UT procedures. The issue remains whether to NRR. Preliminary results of the round robin UT examination wait for the development of advanced NDE techniques and procedures following ASME Code procedures indicate a continue with the present ASME Code procedures resulting need for additional guidelines to the existing ASME Code in uncertainties, delays, and discussions or to encourage procedures to control equipment performance, calibration improvement in the present state of the art of conventional block specifications, and scanning procedures to improve the UT. The decision appears to be obvious that we should use reproducibility of results and detectability of through-thick- conventional UT based on engineering judgment until some ness flaws. new techniques for flaw detection and sizing can be proved effective in the field. This guide is aimed at providing the Minimum ASME Code requirements do not specify the recommendations needed to improve on the ASME Code details of recording requirements that are essential to UT requirements until proven advanced NDE techniques evaluate flaws. This deficiency in the Code rules makes it are available.

4

1.150-12

APPENDIX TO VALUE/IMPACT STATEMENT

Values that will result from this regulatory guide are now apply to the examination of the RPV, require calibra- much easier to perceive than the impact. It is very difficult tion against the calibration block only "prior to use of the to assess the real impact because the kind of statistical data system." It is considered that the present 1977 ASME Code needed is simply not available at this time. One way in which rules are not adequate to control potential problems in the we hope to estimate the impact is through industry feed- variation of instrument performance characteristics. There- back after the guide has been issued. fore, the recommended calibration before and after each examination is a more reliable approach to instrument We have made an attempt, in a qualitative manner, to performance checks. The above position is not more con- estimate the value/impact of regulatory guide positions, servative than the previously accepted 1974 Code rules, but is position by position, as follows: more conservative if 1977 rules are considered.

1. INSTRUMENT PERFORMANCE CHECKS Considering the requirements of Article 4,Section V

(1977), the above position will mean a calibration check Recording the characteristics of the ultrasonic testing each week the system is in use or before and after each (UT) examination system will be useful in later analysis for RPV examination, whichever is less, instead of before each determining the location, dimensions, orientation, and examination. A calibration check against the calibration growth rate of flaws. System performance checks to deter- block takes 15 to 30 minutes for manual UT and for mine the characteristics of the UT system will be made automated UT equipment where provision is made to shortly before the UT examinations. Each UT examination calibrate the equipment without having to remove the trans- will therefore be correlated with a particular system per- ducers from the rotating scanning arm of the mechanized formance check. This practice will help to compare results. scanner. In some cases, transducers have to be removed These determinations will help make it possible to judge from the scanning arm for calibration of the UT instrument;

whether differences in observations made at different times in these cases, a calibration check may take from 30 to 60

are due to changes in instrument characteristics or are due minutes. The added cost of the above would be in terms of to real changes in the flaw size and characteristics. additional time spent by the examiner and would occur each week or once for each RPV examination, depending It is recommended that, as a minimum, instrument on whether or not the examination is completed in less checks should be verified before and after examining all the than a week. No additional radiation exposure is expected welds that need to be examined in a reactor pressure vessel because of this position.

during one outage.

3. NEAR-SURFACE EXAMINATION AND SURFACE

Performance of these instrument checks is likely to add RESOLUTION

a few thousand dollars to test equipment cost and to take 1 to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> of examination time before and after each reactor This position recommends that an estimation of the pressure vessel (RPV) examination. The examination equip- capability to effectively detect defects at the metal front ment is usually idle between examinations. Performance and back surfaces of the actual component should be made checks on the examination equipment could be performed and reported. This will not require any additional calibration during these idle periods. These performance checks are not or examination time but will simply require an estimate of likely to reduce the number of examinations that a particular this capability by the examiner, which will be reported to UT system could perform in a year. No additional radiation NRC. No additional radiation exposure is expected because exposure is expected because of this position. of this position.

2. CALIBRATION

4. BEAM PROFILE

According to this position, system calibration should be This position recommends that the beam profile (for checked to verify the distance-amplitude correction (DAC) each search unit used) should be determined if any signif- curve, as a minimum, before and after each RPV examina- icant flaws are detected during the RPV examination.

tion (or each week the system is in use, whichever is less) or each time any component (e.g., transducer, cable, connector, Assuming that no more than three search units are likely pulser, or receiver) in the examination system is changed. to be used during an RPV examination, this step is likely to require no more than 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> of examination time. No Subarticle 1-4230, Appendix I,Section XI, ASME B&PV additional radiation exposure is expected because of Code (1974 edition), which applied to the inspection of the this position.

RPV, required calibration using the basic calibration block at "the start and finish of each examination, with any change 5. SCANNING WELD-METAL INTERFACE

in examination personnel and at least every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> during an examination." However, the 1977 rules of Article 4 This position recommends that the beam angles used to (T-433),Section V, which are referenced by Section XI and scan welds should be based on weld/parent-metal interface

1.150-13

geometry and at least one of these angles should be such that on the screen larger than the indication on the screen from the beam is almost perpendicular(+/-1 5 degrees to the perpen- the calibration holes (1/2-inch hole for a 12-inch weld

4 dicular) to the weld/parent-metal interface, unless it can be thickness, 3/8-inch hole for an 8-inch thickness), this demonstrated that large (Code-unacceptable) planar flaws recommendation will not result in any more recording of unfavorably oriented can be detected by the UT technique. indications. If the RPV welds being examined have several indications with travel in excess of the calibration hole On the basis of information available, it appears that it is diameter, the examination and recording time will be difficult 1 ,2,3 to detect large planar flaws (e.g., service-induced increased for the investigation of these flaws, depending fatigue or stress corrosion cracks) oriented at right angles to on the number of these indications. Slag inclusions in welds the surface, using the ASME Code UT procedure. However, are generally long cylindrical defects and do not have much the option is being provided to demonstrate that such flaws depth unless they are associated with shrinkage or service- can be located by conventional methods or by using new induced cracks. These slag inclusions are not expected to advances in UT techniques. In these cases, the technique will increase the number of indications that will be recorded.

be acceptable as a volumetric examination method. Otherwise, Increase in examination time will depend on the number, the use of high-intensity radiography or tandem-probe UT size, and complexity of geometry of through-thickness technique, among other techniques, should be considered. indications.

The above type of flaw is the most significant but the For RPV girth or nozzle welds where examination is most difficult to detect. Because of this, the present recom- performed by automated equipment and data are recorded mendations are being made despite their potential impact on tape, this position will mean no increase in examination on cost and radiation exposure. time or radiation exposure; but interpretation, analysis, and reporting time for these depth indications will increase. The The potential impact may be as follows: extra burden in terms of dollar cost will depend on the number, size, and complexity of flaws, and there are no a. Additional NRC staff time may be needed to evaluate rational bases or data available at this time to estimate the the effectiveness of UT techniques on a generic basis to increase in the cost of examination.

detect perpendicular planar flaws. After techniques are recognized to accomplish the above, NRC staff time that is For RPV welds, mostly nozzle welds, where examination being spent currently on evaluating problems on a plant-by- is performed manually and data are not recorded on tape, plant basis is expected to be considerably reduced. this position will mean extra examination time and increased b. Reactor downtime may increase, depending on the examination time differentials between the conventional and refined techniques. This may, however, be offset by a radiation exposure to the examiners. Increase in dollar cost and radiation exposure will again depend on the number, size, and complexity of indications, and there are no bases or data available to estimate this increase.

4 reduction in the downtime currently needed for NRC

experts to evaluate data that sometimes requires further 6.2 Nontraveling Indications

2 4 clarification and reexamination. '

This position also recommends the recording of nontravel- c. Additional cost might be incurred in changes needed ing indications above 20 percent DAC level that persist for to add transducers or data-gathering capability to existing a distance of more than 1 inch plus the beam spread.

automated equipment or to automate current manual According to NB-5320, Radiographic Acceptance Standards, examinations. Automation of current manual techniques is Section III, Division 1, ASME Code, 1977 edition, flaws likely to reduce radiation exposure to personnel. larger than 3/4 inch for weld thicknesses above 2-1/4 inches are not acceptable. Because of this requirement, it is

6. SIZING AND RECORDING OF INDICATIONS expected that no flaws larger than 3/4 inch in length are present in the RPV welds, and if indications are detected

6.1 Traveling Indications that suggest flaws larger than 3/4 inch, there is a strong possibility that these may be service-induced flaws. Service- This position recommends the recording of traveling induced flaws are rare in RPV welds, and it is therefore indications. If RPV welds do not have any travel indications not expected that additional indications would have to be recorded because of this position. However, if such indica-

1

,Probability of Detecting Planar Defects in Heavy Wall Welds by tions (over 1 inch) are detected, examination time for Ultrasonic Techniques According to Existing Codes," Dr. Ing. Hans- automated recording and examination time plus radiation Jurgen Meyer, Quality Department of M.A.N., Nurnberg, D 8500 exposure for manual UT examinations will be increased.

Nurnberg 115.

2 There are no rational bases or data available to estimate the

"Interim Technical Report on BWR Feedwater and Control Rod impact of regulatory position 6.2.

Drive Return Line Nozzle Cracking," NUREG-0312, July 1977, p. 3.

3

"Analysis of the Ultrasonic Examinations of PVRC Weld Speci-

7. REPORTING OF RESULTS

mens 155, 202, and 203," R.A. Buchanan, Pressure Vessel Research Committee (PVRC) Report, August 1976.

This position recommends that the areas required to be

4,"Summary of the Detection and Evaluation of Ultrasonic Indica- tions - Edwin Hatch Unit I Reactor Pressure Vessel," January 1972, examined by the ASME Code that have not been effectively Georgia Power Company. examined and an estimate of error band in sizing the flaws

1.150-14

should be brought to the attention of the NRC when the of this guide, those inservice and preservice examinations results are reported. This effort may take about 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> in performed in the past. Such a policy would tend to be reportwriting time. overly conservative and would put a heavy burden on all plant owners. Although UT examinations have missed some

8. IMPLEMENTATION flaws in the past, there appears to be no immediate danger from the estimated flaw distribution probability to warrant It should be noted that the recommendations of this guide such a strong action. Therefore, this alternative was not are not intended to apply to those preservice examination adopted.

tests already completed. However, the licensees may consider repeating their preservice examination tests or 8.2.2 Second Alternative using the recommendations of this guide any time at their option to avoid possible flaw interpretation problems at a In the past, several instances have been noted where the later date. Flaw interpretation problems may occur if minimal Code UT examination procedures have not been traveling indications identified as significant according to adequate for detecting and sizing flaws. Discussions and the recommendations of this guide do not correlate with undesirable licensing delays were frequently the result. As preservice volumetric NDE results and hence would be more plants begin producing power and existing plants grow assumed to have been service induced. It would be difficult older, more flaws may be expected in the weld areas. These to show that these indications arose from fabrication flaws. flaws may be generated as a result of fatigue, stress corrosion, Therefore, licensees would be well advised to consider the or other unanticipated factors. It is imperative that the above possibilities. guide recommendations for supplementary UT examination procedures be used in the future to maintain an acceptable

8.1 Alternatives level of safety at these welds. The second alternative was therefore selected for applying this guide to the preservice The following alternatives were considered in applying and inservice examination of RPV welds.

the recommendations of this guide.

It is expected that inservice UT examinations will detect I. To apply the recommendations of the guide to all the flaws generated during plant operation, whereas preservice preservice and inservice examinations that have examinations will provide UT examination data for sub- already been performed. sequent comparisons. First, a radiographic examination is performed of all the vessel welds under Section III of the

2. To apply the recommendations of the guide to all ASME Code. After this examination, a UT preservice exam- future preservice and inservice examinations per- ination of welds is performed to serve as a supplementary formed after the issuance of the guide. volumetric examination. Because of the above, these pre- service examinations are not as important as inservice exam-

8.2 Discussion of Alternatives inations. It was therefore decided that the guide recommenda- tions should apply to judging the inservice examination results

8.2.1 First Alternative for those examinations performed immediately after the issuance of the guide; however, the guide recommendations Alternative I would infer that all RPV welds examined should apply to preservice examinations beginning 6 months as per the current code requirements are at a quality level after the issuance date. The NRC staff considered this that would not ensure an acceptable safety performance. approach best because of the difficulties being experienced This approach would also mean that all the plants would in reviewing inservice UT examination data from the have to repeat, in accordance with the recommendations different plants.

1.150-15

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