ML18059A422

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Pressurizer Safe End Crack Engineering Analysis & Root Cause Evaluation.
ML18059A422
Person / Time
Site: Palisades Entergy icon.png
Issue date: 10/07/1993
From:
CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:
Shared Package
ML18059A421 List:
References
NUDOCS 9310120430
Download: ML18059A422 (25)
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Text

  • ENCLOSURE 1 Consumers Power Company Palisades Plant Docket 50-255 PRESSURIZER SAFE END CRACK ENGINEERING ANALYSIS AND ROOT CAUSE EVALUATION
  • October 7, 1993
  • 9310120430 931007___1:'.

PDR ADOCK 05000255 / : :

s .. PDR L

TABLE OF CONTENTS 1.0

SUMMARY

.. 1

2.0 BACKGROUND

.. 1 3.0 SAFETY ASSESSMENT 2 .

4.0 DESCRIPTION

OF THE PORV NOZZLE AND LINES 2 5.0 METALLURGICAL EXAMINATION RESULTS . . . 3 6.0 CAUSE OF THE PORV LINE SAFE-END CRACK 3 6.1 MATERIAL . . . . . . . 4 6.2 ENVIRONMENT . . . . . 5 6.3 STRESS . . . . . . . . 5 6.4 CRACK GROWTH ANALYSIS 7 7.0 EVALUATION OF OTHER PRESSURIZER NOZZLES 7 8.0 RELATIONSHIP TO OTHER INCONEL 600 ISSUES .. 11 9.0 OTHER PCS WELDS OF DISSIMILAR METALS . 12 10.0 PIPING STRESS ANALYSIS . . . 12 10.1 STRESS ANALYSIS SCOPE . 12 10.2 PORV LINE ANALYSIS . . . . . . . . . . . . 13 10.3 SAFETY RELIEF VALVE DISCHARGE LINE ANALYSIS . . . . 13 10.4 PRESSURIZER SPRAY LINE ANALYSIS ... . 13 10.5 PRESSURIZER SURGE LINE ANALYSIS 13 10.6 MECHANICAL MAINTENANCE LOADS . 14 11.0 REPAIR OF PORV LINE WELD . . . . . . . . 14 11.1 METHOD OF REPAIR . . . . . . . . 14 11.2 EVALUATION OF REPAIR LIFETIME . . 15 12.0 NON-DESTRUCTIVE EXAMINATIONS . . . ; 15 12.1 RELIEF VALVE NOZZLE ASSEMBLY WELDS (PORV} 15 12.2 SPRAY NOZZLE ASSEMBLY WELDS . . . . . . . . . 16 12.3 SAFETY RELIEF VALVE NOZZLE ASSEMBLY WELDS 16 12.4 EXAMINATION METHODS . 16 13.0 LONG TERM CORRECTIVE ACTIONS . . . . . . . . . . 17

1.0

SUMMARY

A leak occurred on a Palisades pressurizer relief valve nozzle safe-end made of Inconel. A metallurgical examination was conducted which showed the leak was caused by a primary water stress corrosion crack (PWSCC) in the heat affected zone of the safe-end to pipe weld and that the crack initiated from the inside wall of the safe-end. Analysis was performed to identify other potentially susceptible safe-end welds. Safe-end welds on the top of the pressurizer have been inspected with code acceptable radiography. To gain additional confidence, dye penetrant examination methods have also been used to examine potentially susceptible safe-end welds to the extent possible. In addition, ultrasonic examination techniques using EPRI qualified examiners and best available technology was also performed where possible. A repair was made to the pressurizer relief valve nozzle safe-end to pipe weld to restore structural integrity. Improvements were made to the weld design to address deficiencies noted in the original weld. An evaluation of the root cause of the PWSCC has been performed. This evaluation addressed each of the three factors necessary to cause PWSCC: material susceptibility, environment, and stress. Corrective actions associated with correctable factors have been completed.

2.0 BACKGROUND

The following is a summary of the events associated with the Palisades pressurizer relief valve nozzle safe-end crack.

The Palisades inservice inspection program identified a possible flaw in the pressurizer power operated relief valve (PORV) nozzle safe-end to stainless steel header pipe weld during the refueling outage in June 1993. This

  • possible flaw indication was evaluated and dispositioned at that time as an original construction flaw which would not compromise structural integrity.

This evaluation was performed by several qualified non-destructive examination (NOE) examiners.

On September 16, 1993, Palisades was in the process of heating up following a refueling outage. Tbe plant's primary coolant system (PCS) was in a hot shutdown condition (532°F and 2060 psia) when plant operations personnel identified an increasing trend in containment sump level indication. A few minutes later, an auxiliary operator conducting rounds in containment reported a steam leak near the pressurizer. Closer inspection found an unisolable leak in the power operated relief valve (PORV) line near the pressurizer nozzle.

The plant was returned to cold shutdown. While cooling down, a second visual examination of the leak was performed near 200 psig. This visual inspection characterized the leak to be a circumferential crack, in or very near to the Inconel 600 safe-end on the pressurizer nozzle.

On September 17, 1993 the plant achieved cold shutdown and direct visual and NOE examination of the crack area were performed. The leak area found the circumferential crack to be approximately 3-inches in length (about 30 percent of the circumference) in the Inconel safe-end at the safe-end to pipe weld (see Figure 3).

Review of containment sump level *information during the event indicates the steam leak was on the order of 0.2 gpm equivalent water .

  • 1

A'section of safe-end containing the crack and a portion of th~ PORV line was removed for repair purposes and metallurgical examination. This section was then divided; one part was evaluated by Consumers Power Company and ABB-Combustion Engineering metallurgical personnel and the other part was provided to the USNRC for their evaluation.

The pressurizer nozzle safe-end and PORV line were repaired by removing the cracked weld and heat affected zone of the safe-end, rewelding the stainless steel PORV line to the safe-end and performing Code required examinations of the repair.

3.0 SAFETY ASSESSMENT The cause of the pressurizer relief valve safe-end crack was primary water stress corrosion cracking in the weld heat affected zone (HAZ) of the Inconel 600 base metal.

The following actions have been taken to provide confidence that the Palisades Plant can be safely operated during the next operating cycle:

1. Identify the root cause(s) of the PWSCC crack in the pressurizer relief valve nozzle safe-end. (See sections 5 and 6)
2. Provide assurance that the repaired safe-end will be suitable for service until a more permanent fix can be implemented or until a supporting analysis demonstrating continued operation with the existing configuration can be completed. (See section 11)
3. Demonstrate that the failure scenario is unlikely to be repeated in other pressurizer nozzles with Inconel 600 safe-ends or in other primary coolant system nozzles containing Inconel 600 or other safe-end materials. (See sections 7, 8 and 9)
4. Assure that the Palisades primary coolant system leakage monitoring systems are capable of detecting leakage from this type of crack. Among the monitoring systems available to detect primary coolant system leakage are the containment humidity and sump level monitors. The plant response to the leak in the pressurizer relief valve safe-end clearly shows that leaks of this nature can be detected so that the plant can be safely shut down. The leakage was readily recognized on containment sump level instrumentation within approximately an hour after the leak began.

4.0 DESCRIPTION

OF THE PORV NOZZLE AND LINES Nozzle: The PORV nozzle is located on top of the pressurizer approximately 2-feet from the center of the head(See Figure 1).

The nozzle is constructed from stainless steel clad, 3-inch ID, 1.75-inch wall carbon steel pipe (See Figure 3).

Safe-end: The safe-end is a 4-inch long section of 1.5-inch wall Inconel-600 which is welded to the carbon steel nozzle (see Figure 3). The safe-end material is: Inconel-600, SB-166, .08%

Carbon, 77,500 psi yield strength, NX-5222 heat number .

  • 2

PQRV lines: The 4-inch schedule 120 stainless steel piping is then welded to the safe-end. Two parallel branch lines from the header each lead to a motor-operated block valve and a PORV in series (See Figure 2). The discharge piping is anchored to the structure a short distance downstream of each PORV.

5.0 METALLURGICAL EXAMINATION RESULTS The following are the results of the metallurgical examination of the PORV line to safe-end weld crack:

1. The PORV line to safe-end weld failure has been attributed to primary water stress corrosion cracking (PWSCC).
2. The metallurgical examination of the fracture surface showed the cracking to have initiated at the inner diameter (ID) of the pipe in the heat affected zone of the Inconel safe-end.
3. The cracking mode was intergranular and followed through the Inconel safe-end heat affected zone until failure occurred at the outside surface of the pipe.
4. The cracks ran perpendicular to the inner surface of the pipe wall, i.e., radially, without significant propagation in the axial or azimuthal direction.
5. The outer diameter of the crack location consisted of six discontinuous cracks about a half inch long. The total crack length on the outside
  • 6.

diameter was about 3-inches.

The cracking remained almost entirely in the Inconel 600 base metal and propagated only slightly into the Inconel 600 weld metal near the outside diameter (OD) of the weld.

7. Evidence of an original weld root repair, apparently made from the inside of the pipe, was found in the segment of the weld which is adjacent to the crack initiation point in the safe-end heat affected zone.
8. There was evidence of grinding on the root of the weld.
9. The presence of a black oxide on the fracture surfaces indicates that the crack had existed for some period of time and had not developed during the current refueling outage.
10. A significant mismatch (1/16 inch) existed between the inner-diameter of the safe-end and the inner-diameter of the PORV header line.
11. Shallow intergranular penetrations were present on the inner wall of the heat affected zone.

6.0 CAUSE OF THE PORV LINE SAFE-END CRACK Stress corrosion cracking (SCC) requires the simultaneous presence of three factors:

1. A metallurgical condition that is susceptible to sec, 3

2; An aggressive environment. High temperature water or steam are capable of causing SCC in nickel base alloys. When SCC occurs in this environment without the presence of a contaminant, it is referred to as PWSCC.

3. A stress level in the alloy exceeding some threshold value which is dependent on the material and environment. For Inconel 600, the threshold stress for PWSCC is estimated to be 40 ksi {Reference 1).

The PORV line safe-end involved each of these factors:

1. The PORV safe-end was fabricated from a heat of material with high yield stress and metallurgical condition which made it prone to PWSCC.
2. The Inconel 600 safe-end was exposed to an environment known to cause PWSCC {high temperature steam).
3. While stresses due to system operation were relatively low, the original field welding process and the associated repairs, the mismatch between the stainless pipe and Inconel 600 safe-end sizes and the nozzle/pipe material thermal mismatch, when accompanied by even low piping loads, can result in high local stresses which can cause the initiation of cracking at shallow intergranular penetrations in the heat affected zone.

6.1 MATERIAL The material used in the safe-end a~plication was NiCrFe Alloy 600 {lnconel 600) procured to the requirements of SB-166 and supplied as a solid bar forging. Details of the post-forging heat treatment are not available, but, based on a review of available data from hot-finished products produced prior to 1987, it was probably performed at 1600-1700°F {Reference 2). Processing of this particular heat {NX-5222) of material produced a coarse grain product with a continuous intergranular carbide network and numerous fine intragranular carbides, a high hardness {HRC-22), and a very high yield strength {77.5 ksi). This was consistent with the procurement requirements which specified a minimum yield strength only.

Stress corrosion cracking of Inconel 600 in nozzle applications is well documented for both domestic and foreign units. The Combustion Engineering Owners Group, as a result of pressurizer instrumentation nozzle failures, conducted an extensive Inconel 600 investigation. One result was the identification of forged or hot-rolled product as being susceptible to PWSCC.

Susceptible material was deemed to have one or more of the following characteristics:

1. Yield strength greater than 50 ksi.
2. Post-forge heat treatment of less than 1850°F.
3. Areas of cold-work {plastic deformation) as a result of fabrication.

As indicated above, the Palisades safe-end nozzle material has very high yield strength {77.5 ksi). The post-forge heat treatment was probably low and the machining and possibly repair welding operations may have produced layers of cold-worked material. The heat affected zone, where the cracks were located,

  • may have been softened {strength lowered) significantly as a result of welding but the low heat treatment temperature would still result in material susceptible to PWSCC.

4

6~2 ENVIRONMENT The environment experienced by the PORV nozzle safe-end during plant operations was stagnant steam phase at approximately 640°F and 2060 psia.

Oxygen and hydrogen, which will increase the potential for cracking, may have been present. The plant had operated for approximately 80,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> at the time the leak developed. Numerous investigations and field experience have confirmed that high temperature water and/or steam will cause PWSCC in Inconel 600 with hydrogen and possibly oxygen enhancing the tendency to crack.

Within industry pressurizers, most (but not all) events of cracking have been in vapor space applications. However, PWSCC has occurred in other locations in other plants indicating that Inconel 600 is susceptible to cracking in water as well as steam environments.

6.3 STRESS Stresses contributing to PWSCC can be generally categorized as operating stress and residual weld stress. The extent and geometry of the crack (approximately 30% of the circumference) indicates that the stresses were asymmetric.

Calculated Stresses - Operating Loads Operating, intensified stresses in the safe-end due to internal pressure, deadweight and normal system thermal expansion total approximately 19,000 psi.

This number reflects membrane and bending stress as calculated per the Code of Record (B31.l). When these loads are evaluated more critically, the calculated axial tensile loads that exist at the azimuthal location of the crack are on the order of 15,000 psi.

Material Mismatch Stress It is noted that there are major boundaries of dissimilar materials at the

- following locations:

1. Stainless steel piping-to-Inconel safe-end weld
2. Inconel safe-end-to-carbon s~eel nozzle base weld
3. Stainless steel-to-carbon steel nozzle base (weld-deposited clad)

A normal heatup (e.g., from 70°F to 650°F) will create axial and compressive stresses simply on the basis of the different thermal expansions associated with these materials. Stainless steel has a coefficient of thermal expansion of 9.87E-06 in/in/°F at 650°F; Inconel has a coefficient of thermal expansion of 7.95E-06 in/in/°F at 650°F; and carbon steel has a coefficient of thermal expansion of 7.33E-06 in/in/°F. (Values are from the 1971 ASME Code.)

The primary area of concern is the heat affected zone in the Inconel b~se metal, located just below the weld to the stainless steel piping .

  • 5

In order to ascertain the nature of the axial stress in the area of concern, an ANSYS axisymmetric model of the nozzle-to-piping configuration, with welds, was developed using 8-noded elements. The nozzle was then analyzed per the

-* following conditions: dimensions were set at 70°F (i.e., stress-free state);

the temperature of the entire nozzle was ramped from 70°F to 650°F; and the bottom edge of the nozzle was constrained in the axial direction.

Analysis results indicate that an axial tensile stress on the order of 10 ksi exists along the inside surface of the safe-end, from the safe-end-to-piping weld down to the next transition in geometry in the inside surface of the safe-end. This area includes the heat affected zone.

Weld Stress These stresses could have resulted from several sources. The original weld and the weld repairs made to the inner diameter of the weld were a likely source of stresses. There was a significant structural mismatch (i.e. poor fitup) between the line and safe-end of nozzle which could have also resulted in additional stresses. The magnitude of each of these stresses have not been quantified, but if additive, they could be significant.

Other Factors One other source of loading that is not currently part of the design basis for the PORV piping may be thermal stratification, caused by a volume of sub-cooled water becoming trapped in the PORV piping as the pressurizer heats up and a bubble is drawn in the pressurizer. *A resultant temperature difference from the top to bottom of the pipe could then exist during the heating up of the pressurizer as the water would not heat as quickly as the pipe. The differential temperature resulting from this stratification would tend to cause the pipe to bend and produce axial loadings on the PORV nozzle. These loadings are not normally considered in the operating stress evaluation and are very difficult to quantify without measured temperature data. Preliminary calculations with conservative thermal gradient assumptions indicated that:

stratification stresses are less than system thermal expansion stresses, they are not at a maximum value simultaneous with system thermal expansion stresses and their primary moment orientation does not yield significant stresses at the crack location.

Another stress factor is the shallow intergranular penet~ations which were present in.the heat affected zone but not.elsewhere. These have been observed in other Inconel 600 applications and are not necessarily the result of stress. Their presence, however, would serve to locally increase stresses (stress risers) and they probably were initiation sites for the through-wall crack.

6

~

6~4 CRACK GROWTH ANALYSIS Crack growth calculations were used to estimate the period during which the crack was propagating. This calculational methodology has been used previously to estimate lifetimes of Control Element Drive Mechanism nozzles in Combustion Engineering designed plants. In the absence of definite stress analysis data, the calculation assumed yield strength level stresses. Two cases were considered; (1) yield strength of 77 ksi and (2) yield strength of 40 ksi (for a softened HAZ). Crack aspect ratios (length/depth) of 2:1, 4:1 and 6:1 were analyzed. Time to through wall propagation from a 3-mil deep I.D. intergranular penetration ranged from 20 months (time at temperature and pressure) for a yield strength of 77 ksi and aspect ratio of 2:1 to approximately 65 months for the 40 ksi yield strength and 6:1 aspect ratio.

This indicates that the crack age is greater than 20 months. This analysis does not include the time to actually initiate crack propagation.

The same calculational procedure was used to determine the maximum size defect that could be present and would propagate essentially through wall over a 15 month operational period. For 77 ksi material with 77 ksi stress and an aspect ratio of 6:1, a 0.039-inch deep x 0.234-inch long initial crack will propagate through wall in 15 months. Using an aspect ratio of 4:1, which is more typical of sec, a 0.059-inch deep x 0.236-inch long crack will propagate through wall during the same period.

7.0 EVALUATION OF OTHER PRESSURIZER NOZZLES There are 136 nozzles that contain Inconel 600 in the Palisades pressurizer.

Of these, 120 are for heater wells, 8 for level instruments, 2 for temperature instruments, 3 for safety valve flanges, 1 for the spray line, 1 for the surge line, and 1 for the PORV line (which experienced the cracking).

The pressurizer heater sleeves (120) and lower shell and upper head temperature nozzles (2) are attached to the pressurizer by partial penetration welds. These welds did not receive a post-weld stress relief heat treatment.

To date, all of the Inconel 600 penetration cracking problems have involved partial penetration welds. CEOG studies of Inconel 600 cracki~g in partial penetration welds indicate that: the most susceptible locations can be identified by material and environmental conditions, the cracks will. be axially oriented, the penetrations will not fail catastrophically, and their occurrence is not a safety issue. These technical opinions have been presented by the Combustion Engineering Owners Group to the USNRC staff (Reference 2). In addition, inspection recommendations were provided for pressurizer instrumentation nozzles and heater sleeves.

The remaining top and bottom head level instrument nozzles (8) in the pressurizer use Inconel 600 spool pieces welded to Type 316 stainless steel safe-end pieces. The Inconel 600 used in these small diameter nozzle assemblies was procured to SB-166 (forged or hot rolled bar) requirements, with a yield strength of 46;2 ksi. The Inconel 600 to Type 316 stainless steel butt weld was not stress relieved. Concern about these nozzles prompted Consumers Power Company to sponsor a study of residual stresses associated with the butt weld in these nozzles. The study showed only compressive or relatively low tensile residual stresses. These nozzles are not restrained during welding and this accounts for the low residual stress.

7

The remaining pressurizer nozzles (6) are identified on Table 1. This table lists key material, environmental and stress factors important to stress corrosion cracking as well as inspections that have been performed on each of the nozzle welds .

  • The pressurizer spray line nozzle safe-end was fabricated from the same high strength Inconel heat (NX 5222) as the cracked PORV line safe-end. This safe-end has a greater wall thickness, but a similar weld geometry. Axial stresses may also be present near the safe~end to pipe weld. However, the coolant flowing through the spray line comes from the cold leg of the PCS and thus is at cold leg temperature (approximately 540°F). Laboratory studies of lnconel 600 PWSCC have demonstrated that approximately 20°F decreases in temperature will double the lifetime of an application assuming all other conditions remain unchanged (Reference 1). This explains why in steam generator tubing, the vast majority of ID initiated cracks are on the hot leg tubes with only isolated occurrences on the cold leg tubes. Applying this demonstrated temperature dependency to the spray line nozzle safe-end would result in a predicted lifetime of about 30 times that of .the cracked PORV safe-end based on temperature alone.

The three code safety nozzles are also fabricated from Inconel heat NX-5222 and experience high (640°F) temperatures in the pressurizer vapor space.

These nozzles have been extensively inspected this outage by ID dye penetrant techniques, radiographic techniques and IGSCC sensitive UT techniques with no indications observed. The nozzles were stress relieved in the shop during the fabrication process. The stress relief will reduce weld induced residual stresses. The absence of defects indicates these nozzles can remain in service for at least an additional cycle of operation .

  • The surge line nozzle safe-end was fabricated from a different heat (26190) of Inconel 600 to SB-166 requirements. The yield strength of this heat was 51.2 ksi, the carbon content was 0.065% and the final heat treatment is not available. The nozzle safe-end was welded to the stainless steel surge line using the same general procedures as was employed for the other safe-end welds. The surge line nozzle safe-end is exposed to primary coolant that is probably near the pressurizer operating temperature. The spray rate of the Palisades pressurizer is higher than in most ABB-CE pressurizers because the backup heaters are continuously energized. The resulting increased flow will reduce stresses imposed on the safe-end.weld which might otherwise result from flow stratification in the surge line.

Although the surge line nozzle safe-end weld is exposed to a high temperature environment, the weld region is less susceptible to PWSCC than the PORV nozzle because the safe-end weld is exposed to a water environment. The majority of observed PWSCC occurrences, and all leaks, in industry pressurizers involving SB-166 (hot finished or forged) material have been in the vapor space. This is probably the result of increased hydrogen and oxygen in the vapor space.

In addition, the safe-end material yield strength is significantly lower than the nozzle safe-end (51.2 ksi vs 77.5 ksi} .. The work of Woodman (Reference 3) shows a distinct relationship between yield strength and the time to PWSCC in SB-166 partial penetration welded pressurizer nozzles. This is probably the result of near yield strength level stresses being present. Field experience shows time-to-failures to be a function of yield stress to the -3.5 power.

Consequently, the projected lifetime for the surge nozzle safe-end weld will

  • be approximately four times that of the PORV nozzle safe-end weld.

8

T~e surge line is connected to the PCS piping by a nozzle with a Inconel-600 safe-end. The Inconel 600 is the same heat/lot as in the surge line nozzle safe-end on the pressurizer and comments about yield strength effects are also applicable here. In addition, the temperature experienced by this nozzle will be approximately hot leg temperature (-600"F). The reduced temperature will significantly increase the lifetime of this nozzle safe-end relative to the pressurizer nozzle safe-end.

After the shop welds joining the safe-ends to the PORV, safety valve, spray line, and surge line nozzle were completed, the nozzles received a 1150°F stress relief heat treatment. Thus, the in-board welds of the safe-ends and the nozzle to pressurizer welds were all stress relieved and have reduced residual stresses. This heat treatment will reduce the probability of PWSCC at these weld locations .

  • 9

TABLE 1 NOZZLES POTENTIALLY MORE SUSCEPTIBLE TO STRESS CORROSION CRACKING

SUMMARY

OF KEY MATERIAL, ENVIRONMENTAL AND STRESS FACTORS AND INSPECTION RESULTS PORV Shop 77 ksi 640°F, Vapor Yes UT, RT, Negative PT ID , PT OD PORV Field 77 ksi 640°F, Va or No Failed Weld Positive S ray Sho 77 ksi 540°F, Water Yes RT, PT OD Ne ative S ray Field 77 ksi 540°F, Water No Ne ative Safety Valves(3) Shop 77 ksi 640°F, Vapor Sur e Sho 51 ksi 640°F, Water Sur e Field 51 ksi 640°F, Water

  • 1987 Inspection 10

a;o RELATIONSHIP TO OTHER INCONEL 600 ISSUES Inconel 600 is used throughout the PCS in NSSS units supplied by ABB-CE and

-* other vendors. In addition to the pressurizer applications discussed in Section 7, the alloy is also used in:

1. Reactor pressure vessels - CEDM/CRDM nozzles, instrumentation nozzles, Vent pipes, leakage monitoring tubes and bottom head instrumentation nozzles.
2. Steam generators - primary head drain lines and instrument nozzles (not applicable to Palisades).
3. PCS piping - instrument/RTD nozzles, surge nozzles, shutdown cooling nozzles, drain nozzles, sample nozzles.

As is the case for the pressurizer applications, both cold drawn and annealed (SB-167) and forged or hot rolled (SB-166) products are used and are attached to the major components by both partial penetration and butt welds.

Occurrences of PWSCC first occurred in 1986 in pressurizer instrument nozzles at San Onofre and St. Lucie~2. Since 1989, there have been numerous occurrences of PWSCC in *pressurizer instrument nozzles, pressurizer heater sleeves, hot leg piping sample lines, steam generator drain lines and CROM nozzles. All of these previous events have been associated with partial penetration welds. Material procured both as pipe and as solid bar forgings have experienced PWSCC. Heats of Inconel 600 with yield strengths as low as 36 ksi and as high as 77.5 ksi have failed. Heats with microstructures

  • considered undesirable (fine grain, no intergranular carbides) and heats with desirable microstructures (coarse grain, continuous intergranular carbides) have failed. Basically, all types of Inconel 600 pipe, forgings and tubes have been demonstrated susceptible to PWSCC in high temperature water and steam given sufficient exposure time and sufficiently high stresses. In this respect, the material in the PORV line safe-end is representative of other Inconel 600 that has cracked and is another example of the continuing industry experie~ce with Inconel 600.

There are three characteristics of the Palisades PORV line safe-end failure that are unique: crack orientation (circumferential), weld type (butt weld),

and location (heat affected zone).

Regarding crack orientation, in all of the previous occurrences of PWSCC in Inconel 600 penetrations around the industry, the cracks were oriented axially, not circumferentially. Experimental and analytical residual stress measurements conducted by the CEOG and EPRI have indicated that the highest residual stresses in J-groove partial penetration welds will be in the circumferential direction and, as a result, cracks that develop will be axially oriented. However, the butt weld configuration in the Palisades PORV line, when the m~mbers are constrained during weldi~g, will produce greater residual axial stresses. In addition, for the weld repair noted earlier, the structural discontinuity between the Inconel 600 and stainless steel, any bending stresses present, etc. will also produce higher axial stresses. These axial stresses can account for the circumferential crack orientation in the Palisades PORV line.

11

With regards to location, the various partial penetration weld cracks to date have been predominantly located outside, but immediately adjacent to, the heat affected zone (HAZ). The Palisades crack, however, was completely within the HAZ. The various stresses from fabrication and installation will be greatest

~*

near the weld fusion zone and may be sufficiently high to eliminate any benefit received from the weld process. In this case, the crack would be expected in the HAZ.

9.0 OTHER*PCS WELDS OF DISSIMILAR METALS In addition to the pressurizer discussed in Section 7, there are numerous other dissimilar welds throughout the PCS at Palisades and at other PWRs.

Many of these are partial penetration welds of Inconel 600 nozzles to carbon or low alloy steel. The carbon and low alloy steels are clad with corrosion resistant materials which prevent exposure of the material to the primary coolant. Even if the cladding is breached, stress corrosion cracking (SCC) of the low alloy steel is unlikely because the oxygen level is low (<5 ppb),

dissolved hydrogen is present, and the corrosion potential is so low (-600 mv SHE) that SCC is unlikely in PWR applications.

There are also numerous applications in which Inconel 600 nozzles and pipes are butt welded to 300 series stainless steels. There have been numerous instances in boiling water reactors of intergranular stress corrosion cracking (IGSCC) in the heat affected zone of austenitic stainless steel welds. The root causes of these failures have been high weld residual stresses, sensitization of the stainless steel and high oxygen (- 200 ppb) content in the coolant. Although high residual stresses and some sensitization may be present in the stainless steels in the PWR circuit, the oxygen level is low

(<5 ppb) and dissolved hydrogen is present producing a low electro-chemical potential (ECP) that is below the corrosion potential. This being the case, IGSCC of the of the stainless steels is unlikely. There have been several events of IGSCC in PWRs, however, including CEDM housings and seal housing and canopy seal weldments. All of these involved geometries where aerated water was trapped.

With respect to BWR failures of stainless steels, a solution has been the introduction of a hydrogen water chemistry designed to lower the ECP to

-220 mv or lower. In operating PWRs, the normal ECP is on the order of

-600 mv, well below the critical potential for IGSCC.

10.0 PIPING STRESS ANALYSIS 10.l STRESS ANALYSIS SCOPE Existing pipe stress analyses were reviewed for the PORV line, safety relief valve (SRV} discharge lines; pressurizer spray system and pressurizer surge line. The analyses were reviewed in order to determine the intensified normal operating stresses at the six associated Inconel safe-ends. Summaries of our conclusions are contained in the following sections.

This review incorporated a revised calculation on the PORV line to reflect the as-found thickness of the elbow nearest the PORV nozzle.

The revised calculation on the PORV line included revised thermal pressurizer anchor movements and a parametric evaluation of pipe temperature distributions from the pressurizer to the block valves.

12

In addition, mechanical maintenance loadings were reviewed in order to determine any potential impact they may have upon piping safe-end structural integrity.

10.2 PORV LINE ANALYSIS The PORV line analysis was evaluated from the pressurizer nozzle to the anchor just past the PORVs to current FSAR criteria. The maximum normal operating stresses were at the second elbow from the pressurizer. The sustained plus thermal expansion stresses met FSAR allowables.

The normal operating stresses at the Inconel safe-end (nozzle) were less than half the FSAR allowables for the appropriate loading combination.

For both locations, thermal expansion stresses dominated the sustained (axial pressure and deadweight bending) loads. The support load increase was acceptable at the anchor.

10.3 SAFETY RELIEF VALVE DISCHARGE LINE ANALYSIS The three safety relief valve discharge lines realize very low sustained and thermal expansion loads at the safe-end. The stresses are low because of:

the geometry of the safe-end connection, low relief valve discharge loads relative to PORV loads (hence low system stiffness), low system temperature and the broad use of variable support hangers in the deadweight support system. The composite PORV/SRV line analysis model will be subject to a complete reanalysis with respect to all applicable loading combinations in the Safety Related Piping Reverification Program.

10.4 PRESSURIZER SPRAY LINE ANALYSIS The pressurizer spray line is a flexibly supported system with relatively high sustained loads with respect to.thermal expansion loads at the nozzle safe-end. The thermal anchor movements are easily accommodated in the relatively long piping arrangement and the temperature is lower than in the PORV system. The stresses all meet FSAR limits.

10.5 PRESSURIZER SURGE LINE ANALYSIS .

The surge line has similar sustained and thermal expansion stresses at the safe-end. The safe-end configuration requires intensification. Although the system has no solid supports between the pressurizer and cold leg (it has three spring hangers), thermal expansion stresses are relatively high because the line is of sufficient size (stiffness) and the thermal anchor movements at the pressurizer and cold leg are of sufficient magnitude to generate significant piping reaction forces. However, all safe-end stresses are well within FSAR allowables.

P~r Generic Letter 88-11, thermal stratificati-0n stresses have been evaluated-for the pressurizer surge line per Combustion Engineering Owner's Group Task 662 and Palisades is bounded by the results of that work scope. Those results will be reported to the Commission and incorporated in the pipe stress analysis as committed per the generic letter .

  • 13

10.6 MECHANICAL MAINTENANCE LOADS Mechanical maintenance loads are usually applied to piping systems during the

-* maintenance of valves and other in-line components. Such loads are not expected to have been applied to the simple pressurizer surge line configuration.

Mechanical maintenance loads have been applied at regular intervals to the pressurizer spray and safety relief valve lines. The loads applied to these lines have not been quantified. However, these piping systems are very flexible and removal and reinstallation of components is achieved easily with no significant impact on the safe-ends.

The PORV line near the pressurizer is relatively stiff as may be expected for a system designed to accommodate two phase flow. Therefore, PORV removal has been evaluated by code compliance analysis in order to ensure system structural integrity and functionality with respect to the application of maintenance loads. The results of these analyses have been incorporated into the maintenance procedures.

11.0 REPAIR OF PORV LINE WELD 11.1 METHOD OF REPAIR Figure 3 shows the approximate dimensions of the original pressurizer PORV nozzle, safe-end and pipe connection. Figure 4 shows the location of the repair welds made during the repair process.

At the start of the repair, two cuts were made in the original piping and pressurizer nozzle safe-end. The first cut was made at the first elbow weld coming from the pressurizer (location of weld 3). The second cut was made approximately 1/2-inch below the lower toe of the safe-end to pipe weld (location of weld IA on Figure 4 and shown as "cut line" on Figure 3). While the first cut location could have been as close as 1/2-inch above the safe-end to pipe weld, making the cut at the elbow allowed easy access to the inner surface of the safe-end and raised one of the repair welds to a more accessible height.

The first cut was made at the elbow in anticipation of possible cold spring in the pipe. It was felt that, if present, cold spring might cause the pipe to "pop" apart during cutting and possibly damage the crack if the first cut were made at the safe-end. There was only a minor amount of spring present in the piping and the pipe rose 1-1/2-inches when cut.

After the second cut was made, the entire cracked safe-end to pipe weld was removed (along with about 1-foot 6-inches of stainless pipe) for metallurgical examination.

The remaining safe-end was then machined for welding to a replacement spool piece of stainless pipe. The location of the safe-end weld preparation is shown on Figure 3. After machining the safe-end, the machined surfaces along with the original safe-end ID were liquid penetrant tested.

Because initial examinations of the ID of the original safe-end to pipe weld indicated that a flush root ID on the repair.weld would be desirable, it was decided to fabricate the replacement stainless piping in two pieces - one 3-inches long and the other approximately 1-foot 3-inches long.

14

F~rst, a 3-inch spool piece was welded to the safe-end using Inconel-600 filler metal. This 3-inch piece allows access to the root of the repair weld (weld IA). After welding, the root between the safe-end and 3-inch spool piece was ground flush followed by flapping of the ID in the area of the root and safe-end counterbore.

After a successful radiographic examination (RT) of the root of the safe-end to pipe weld, a second spool piece approximately I-foot 3-inches long was welded in place (welds 2 and 3 on Figure 4) using stainless filler metal. The spool piece was sized so as to match the distance between the pressurizer nozzle and the first pipe elbow in the original design.

Weld IA (see Figure 4) was made with inconel filler metal. Gas Tungsten Arc Welding (GTAW) was used for the root and first 5/16-inches to 7/16-inches of weld. Shielded Metal Arc Welding (SMAW) was used for the remainder of the weld.

Welds 2 and 3 were made using austenitic stainless steel filler metal by automatic GTAW.

  • 11.2 EVALUATION OF REPAIR LIFETIME The defective safe-end has been repaired using techniques and procedures that will enhance its survivability. The weld prep has been re-machined to provide material for the new weld that is away from the heat affected zone of the original weld to ensure that the heat affected zone of the new weld will not have intergranular penetrations or discontinuities that could act as stress risers when the plant returns to service. The fit-up between the PORV line stainless steel pipe and the Inconel 600 safe-end has also been improved, further reducing stress. Furthermore the ID surface of the new weld was ground and the new weld has no ID repair welds which ensures lower residual stress. The improvements and changes will extend potential crack initiation time significantly and this will result in increased lifetime for the repaired weldment as compared to the original weldment. Since crack propagation required more than 20 months for the original weld (see section 6.4), the lifetime of the repaired weld will be more than one cycle of operation. This lifetime analysis only includes the time for crack propagation, not the time to initiate crack propagation.

12.0 NON-DESTRUCTIVE EXAMINATIONS 12.l POWER OPERATED RELIEF VALVE NOZZLE ASSEMBLY WELDS Nozzle to Safe-end Weld - Examinations performed on this weld joint include:

Dye penetrant (PT) examination of the prepped inside diameter (ID) of the weld when 'the joint was exposed to create the new safe-end to pipe weld; Radiography (RT); and PT examination of the outside diameter (OD) surface.

Results of the PT and RT did not reveal any cracking. With the ability to do a complete PT from the ID surface coupled with the RT of the weld volume, we have a high degree of confidence that there is no cracking generating from the ID surface or any other cracking within the weld volume. Consequently, there is no need to perform additio~al examination of this weld .

  • I5 L_

Safe-end to Pipe Weld - This is the weld joint that failed and has been replaced. During the replacement of this weld the prep areas of this joint were PT examined before welding. The weld was examined by radiography and accepted. The ID surface of the weld joint was ground smooth to remove any surface roughness which may contribute to the growth of potential flaws. The acceptance of this new joint is based on standard RT and OD-PT methods.

No further NOE examinations are scheduled for this weld joint.

12.2 SPRAY NOZZLE ASSEMBLY WELDS Nozzle to Safe-end Weld - This weld joint has been examined by RT and OD-PT examination. The ID of this joint is not accessible for ID-PT examinations.

The geometry of this weld joint precludes the use of ultrasonic techniques for the examination of this weld joint. Results of the RT and OD-PT did not reveal any cracking and the joint is acceptable.

Safe-end to Elbow Weld - This weld joint has been examined by RT and OD-PT examination. The ID of this joint is also not accessible for ID-PT examination. The geometry of this weld joint also precludes the use of ultrasonic techniques for examination of this weld joint. Results of the RT and OD-PT did not reveal any cracking and the joint is acceptable.

12.3 SAFETY RELIEF VALVE NOZZLE ASSEMBLY WELDS There are a total of three nozzle to lnconel flange welds. The pressurizer code safety relief valves mount to the flanges. The ID surface of these weld joints have been machined smooth.

The following examinations have.been performed for these three weld joints:

standard RT, PT of the ID and OD surfaces after the surfaces have been cleaned by a mechanical process, and UT of the weld joint from the OD surface using IGSCC techniques for the Inconel side of the weld only. This UT examination was performed by three separate individuals; CPCo's UT Level III examiner, Combustion Engineering's UT Level II and Virginia Corporation's UT Level III.

The Combustion Engineering and Virginia Corporation employees are certified through EPRI for their ability to detect and size IGSCC types of indications.

Results of examinations did not reveal any cracking and the weld joints are acceptable.

12.4 EXAMINATION METHODS Dye penetrant (PT) examination, from both the inside (ID) and outside (OD) weld surfaces, radiography (RT) and ultrasonic (UT} techniques were utilized for these examinations. The ID-PT, OD-PT, and RT examinations are considered "code acceptable" examinations. It is recognized that there are certain limitations to the UT used for these examinations. While not "code acceptable" in all respects, the UT examination represents an effective effort by Consumers Power Company to detect IGSCC in susceptible Inconel 600 safe-ends. The UT, as well as the ID-PT examinations, are in addition to the code required RT and OD-PT examinations and they provided added assurance that the safe-end welds are satisfactory for service.

Section 6.4 identifies minimum calculated crack sizes that could grow through wall during the next plant operating cycle. These calculated crack sizes were based on conservative assumptions relative to material stress and crack aspect (length/depth) ratio. Consumers Power Company is confident that these cracks 16

can be detected using the non-destructive examination techniques that are presently available to. us.

13.0 LONG TERM CORRECTIVE ACTIONS Based on this event, and the evaluation of the root cause of the crack in the pressurizer relief valve nozzle safe-end, a number of long term corrective actions have been identified. These actions are necessary to assure the long term suitability for service of the pressurizer relief valve nozzle safe-end and PORV line as well as other Inconel 600 components:

1. The design of the pressurizer relief valve nozzle safe-end and PORV line will be reviewed and appropriate modifications will be made during the next refueling shutdown to ~ssure a suitable lifetime for the pressurizer relief valve nozzle safe-end. This review will address the material properties of the safe-end and stresses imposed on the safe-end by the PORV line. The review will also be coordinated with the safety related piping reverification project (SRPRP) review of the PORV line.
2. A comprehensive program to deal with Inconel 600 issues at Palisades will be developed. The program will guide future inspections and replacements of Inconel 600 components in the primary coolant system.
3. Further evaluation and qualification of non-destructive examination techniques for detection of PWSCC will be conducted. This effort will include:
i. Evaluation of the June, 1993 NDE results and implementation of
  • ii.

lessons learned.

Review of past (se~ond interval) inservice inspection results to assure effectiveness of radiography interpretation. A sample of past radiography performed on other Class 1 welds will be reviewed by an independent Level III examiner.

iii. Development of an appropriate mockup and qualification of ultrasonic examination techniques for PWSCC .

  • 17

REFERENCES

1. J. A. Gorman, "Status and Suggested Course of Action for Nondenting-Related Primary-Side IGSCC of Westinghouse-Type Steam
  • Generators," EPRI NP-4594-LD, May 1986.
2. CEN-406-P, "A Status Report on CEOG Activities Concerning Primary Water Steam Corrosion Cracking of Inconel 600 Penetrations," May 1991.
3. B. Woodman, "Correlation Between Yield Strength and Time to PWSCC for Alloy 600," 1992 Workshop on PWSCC of Alloy 600 in PWR's, Orl~ndo, December 1992 .
  • 18

3" Safety Valve Nozzle 3"

Relief Valve Nozzle Manway 3" 3" safety Safety Valve Nozzle Valve Nozzle

~---121 f' 00 (Ref 1----....-1 110!6' DIA (Inside Cladding Top View l"

Temp Nozzle Surqe Nozzle Su~rt Skirt Bottom View Elevation rltJUR£ I

2'1,q"H FROM N-S AXIS I FOR CC-II-~"

SEE DWG Mlle SH.1806

~

'6'-.

FIGURE 2

w a...

a...

I I

CRACK INCONEL WELD AREA~

I I

LINE NEW WELD PREP INCONEL 1~

SAFE-END 6 I 3Yz"

(.'.)

z 0

Y4" 0

<[

_J u

(/')

.. C.S.NOZZLE

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  • FIGURE 3

> TO PORV'S I t::::=::=j.._- wELD # 3

.... 1'-3 11 S.S. PIPE i-311 t::::=::=j.._- WELD tt 2 t ,________,'4----WELD ttlA INCONEL SAFE ENO~----;~

~-WELD ttt 1--------l C.S. NOZZLE T-72 PRESSURIZER FIGURE 4

PRESSURIZER SHED GENERAL EQUIPMENT LAYOUT V-61A P'lJl Sl'llAY PORVN01ZLE sec detail above PflESSURIZER

  • f/GURE 5
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