ML18059A454

Responds to 931008 Request Re Addl Info for Engineering Analysis & Root Cause Evaluation Rept of Recent Pressurizer Safe End Crack. Status & Suggested Course of Action for Nondenting-Related Primary-Side IGSCC of Westinghouse....

ML18059A454
Person / Time
Site:	Palisades
Issue date:	10/20/1993
From:	Rogers D CAROLINA POWER & LIGHT CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML18059A455	List: ML18059A454 ML18059A456 ML18059A470 ML18059A472 ML18059A473
References
TAC-87760, NUDOCS 9310260198
Download: ML18059A454 (9)

Text

consumers Power David W. Rogers Plant Safety and Licensing Director POWERIN&

ll/llCHl&AN'S PRO&RESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043 October 20, 1993

_:Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT - REPLY TO NRC REQUEST FOR ADDITIONAL INFORMATION REGARDING THE PRESSURIZER SAFE END CRACK (TAC No. 87760) .

An NRC letter dated O~tober 8, *1993, requested additional information concerning the engineering'analysis and root cause evaluation report of the recent pressurizer safe end"c.rack. The letter requested that *riesponses to the 15 information requests be docketed prior to the October 12, 1993 meeting scheduled with the NRC on this subject. The information *was not docketed prior to the meeting.

At the October 12, 1993 meeting at NRC headquarters, CPCo committed to a schedule for responding to the 15 information requests. The schedule indicated that the results of our metallurgical evaluation report, item 2 of the request, along with most of the remaining information request items (1, 5, 6, 7, 8, 9, 12, 13, 14, and 15) would be replied to by October 20, 1993. The metallurgical evaluation report is not complete at this time but will be

• submitted by October 27, 1993.

Responses to information request items 3 and 4 were submitted to the NRC in a letter dated October 15, 1993. Responses to information request items 10 and

'11 ar~ scheduled to be submitted by November 15, 1993.

David W. Rogers Pl ant Safety and Licensing Di rec_tor CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades Attachment 250155

~\I I

( 9310260198 931020 PDR ADOCK 05000255

1. P .. . PDR A CM5 ENERGY COMPANY

ATTACHMENT Consumers Power Company Palisades Plant Docket 50-255 PRESSURIZER SAFE END CRACK ADDITIONAL INFORMATION October 20, 1993 7 Pages

1 Request for additional information regarding Consumers Power's engineering analysis and root cause evaluation report dated October 7, 1993 for the pressurizer safe-end crack at Palisades plant.

NRC request:

1. Provide reference (1).

CPCo response:

A copy of EPRI Report, "Status and Suggested Course of Action for Nondenting-Rel ated Primary-Side IGSCC of Westinghouse-Type Steam Generators," EPRI NP-4594-LD, May 1986, is provided in Enclosure 1.

NRC request:

2. Provide metallurgical evaluation report including Huey test results.

CPCo response:

Metallurgical evaluation reports are being prepared by both Consumers Power Company and ABB-Combustion Engineering metallurgical personnel. Although originally expected to be complete by October 20, 1993, the ABB-Combustion Engineering report is still in process of being finalized and will be submitted by October 27, 1993. The Consumers Power Company metallurgical evaluation report will be available approximately one week later, November 3, 1993.

NRC request:

5. Provide a discussion of the safety significance of postulated failures of the PORV, spray relief valve, and surge lines.

CPCo response:

Safety Significance of a failure of a pressurizer PORV line.

The inner diameter of the pressurizer relief valve nozzle is 3 inches. This diameter equates to an area of 0.05 ft 2

• Reference 1 indicates that the maximum fuel cladding temperature predicted for a small break LOCA of this size is 660°F. The maximum thickness of cladding oxidation is predicted to be 0.002% of the cladding wall thickness. These values are much less than the limits of 2200°F and 17% wall thickness, respectively from 10 CFR 50.46.

2 Safety Significance of a failure of a pressurizer spray line.

There are two different sizes of spray line. 3-inch, Schedule 160 pipe comes off of two PCS cold legs. The two lines join to form a single 4-inch, Schedule 120 pipe that connects *to the pressurizer. The 3-inch lines have an inner area Qf 0.03755 ft 2 The 4-inch line has an inner area of 0.0716 ft 2 Failure of a 3-inch line would have the effect of opening an area in the PCS equal to two ~ipe areas (0.0751 ft 2 }. A break of this size would be bounded by the 0.1 ft break analyzed in Reference 1. This is the limiting small break LOCA. For the 0.1 ft 2 break, the predicted peak clad temperature is 1855°F with a maximum cladding oxidation thickness of 4.1%. These values are below the limits discussed above.

Failure of the 4-inch line would have the effect of opening an area in the PCS equal to two pipe areas (0.1432 ft 2 }. A break of this magnitude is also bounded by the 0.1 ft 2 case.

Safety Significance of a failure of a pressurizer surge line.

The sur~e line has an inner area of 0.6013 ft 2 This case is bounded by the 0.65 ft case analyzed in Reference 1. For the 0.65 ft 2 break, the maximum cladding surface temperature is predicted to be 1577°F. The maximum cladding oxidation thickness is predicted to be 0.60 % of cladding thickness. These values are below the limits discussed above.

[Additional Note: Reference 1 analyzes small breaks located on the bottom of the cold legs. Per the CE analysis referenced below, breaks on the top of the PCS (i.e., pressurize~ steam space} are much less limiting due to the lower mass ejection rate.]

Reference 1 CENPD-137 Supplement 1-P, "Small Break Model Calculative Methods for the CE Small Break LOCA Evaluation Model," January 1977.

NRC request:

6. Provide 3rd party evaluation of the RT examinations for the PORV line.

What were the conclusions from review of the original Radiography (RT}? Was the crack present from construction?

3 CPCo response: provides the third party evaluation of the RT examinations for the PORV line. As stated in the attached letter from PCI Energy Services, " ... the original construction radiographs dated October 21, 1969 and the film does not meet the code requirements for density, etc. Interpretation with this radiograph in the area of concern was not feasible . . . . "

NRC request:

7. Provide dimensional sketches of the PORV repair weld, spray safe end (SE} and surge line SE.

CPCo response:

A dimensional sketch of the pressurizer relief valve (PORV) line weld is provided in Enclosure 3. Dimensional drawings of the pressurizer spray nozzle safe-end and surge line safe-end are provided in the following Combustion Engineering drawings which are also included in Enclosure 3:

E-231-988-4, Vessel Assembly and Final Machining for Consumers Power Pressurizer E-231-985, Nozzle Details for Consumers Power Pressurizer E-231-986-5, Nozzle Details for Consumers Power Pressurizer NRC request:

B. What is the source of the reported yield strengths?

CPCo response:

The yield strengths reported were obtained from the Certificate of Material Test Reports (CMTR). for the specific heat/lots of Inconel 600 used in the fabrication of the nozzle safe-ends.

NRC request:

9. We have data to show the heat affected zone (HAZ) is harder than base metal. How was the value for strength of HAZ in crack growth rate analysis determined?

4 CPCo response:

Our data show the heat affected zone (HAZ) to be softer, rather than harder, than the base metal (see Enclosure 4 for microhardness test results for the base metal and fracture area). In the crack propagation analysis we assumed the yield strength of the HAZ to be 77 ksi, from the CMTR, and assumed that no softening of the HAZ had occurred. The hardness data would suggest some softening (reduced yield strength) had occurred. The assumed 77 ksi yield strength in the crack propagation analysis is conservative since the higher stress equates to a higher crack propagation rate.

NRC request:

12. Were the three pressurizer safety valve line safe-ends subject to weld repairs during or after fabrication? If they were subjected to repairs or if it cannot be determined that they were not subject to repairs, perform analyses as described in question 11, above, for these lines.

CPCo response:

Based on our review of available construction and non-destructive examination records for the pressurizer safety valve flanges, we are unable to determine if weld repairs were performed. However, after the shop weld joining the pressurizer safety relief valve nozzle to the safety relief valve flange were completed, approximately 7/32 inch of material was machined from the root side of the nozzle flange removing the notch effect. In addition, the weld was stress relieved at 1150°F along with the pressurizer. These differences in the joint design (machining to provide a smooth surface and stress relief heat treatment) make these welds much less susceptible to primary water stress corrosion cracking. Combined with the negative results from the extensive non-destructive examination (described below) performed on these welds during the 1993 refueling outage it is concluded that development of a primary water stress corrosion crack in these nozzles is extremely unlikely. Based on the above, analysis of the pressurizer safety valve nozzles from a leak before break perspective as described in our response to question 11 above is judged to be unnecessary.

During this outage the welds were subjected to the following examinations:

radiography; dye penetrant examinations from the inside diameter and outside diameter surfaces, and; optimized IGSCC ultrasonic examinations performed by three independent individuals. The ultrasonic examination personnel from Virginia Corporation and ABB-Combustion Engineering were certified by EPRI for detection and sizing of IGSCC.

5 NRC request:

13. Are there any plans to instrument the pressurizer spray line in order to better quantify the thermal stresses in that line?

CPCo response:

Palisades does not intend to instrument the pressurizer spray line. The temperature of the line during power operations is well understood, as are the thermal stresses in the line. Palisades operates the pressurizer heaters and spray system differently than other CE plants. Palisades operates with all backup heaters on continuously. This results in 1350 kw of electric heater capacity being continuously energized. The 150 kw proportional heater capacity is controlled by the PCS pressure control system.

During steady state operations at normal PCS pressure and temperature the pressurizer spray flow is approximately 60 to 70 gallons per minute. This is calculated by performing a mass and energy balance on the pressurizer with the pressurizer backup heaters on. During transients or start-up conditions, pressurizer spray flow may be reduced to 2 or 3 gallons per minute. While this may cause a temperature gradient to develop in the spray line, the duration of these transients will be short.

Low system pressure conditions will automatically cause the pressurizer spray flow to be reduced by closing the pressurizer spray valves. The pressurizer spray valves may also be manually controlled by the operator. There are small manual bypass valves installed in the spray lines to prevent thermal shocking these lines. The Palisades pressurizer spray line will normally be at 535°F (PCS cold leg temperature) due to the 60 to 70 gpm pressurizer spray flow.

NRC request:

14. Discuss the potential for water hammer in the PORV line and the pressurizer spray line.

CPCo response:

The operation of the PORVs was reviewed to assess the potential for water hammer given the possibility of water collecting in the bottom of the PORV inlet pipe. The PORVs serve two functions: low temperature-over pressure protection (LTOP) and post-accident once through cooling (OTC). The PORVs are isolated by their manual block valves and not used to provide over pressure protection during power operation.

The LTOP function primarily involves operation with the primary coolant system solid with water at relatively low temperatures (<430°F per plant Technical Specification 3.1.8.1.) The fluid dynamic forces associated with water discharge through the PORV line to the quench tank is explicitly addressed in the stress analysis and piping stresses are well within code allowable values.

6 The PORV may be relied upon for LTOP protection for a brief period of time after drawing a bubble in the pressurizer during plant heat up. During this period of time the potential exists for water to collect in the bottom of the PORV line. However, the likelihood of a PORV actuation during this period is low given the short time duration and the existence of the steam bubble which serves to moderate system pressure transients. Recent inspection of PORV line supports shows no evidence of support damage from any such loadings. In addition, while not explicitly addressed in the stress analysis of this operating mode, tha fluid dynamic forces associated with discharging steam and water through the PORV are addressed in the once through cooling (OTC) mode of operation discussed below.

The OTC function involves manual operation of the PORVs and their block valves during an accident sequence in which the secondary heat sink is lost.

Operation of the PORVs permits primary system pressure to be reduced to allow high pressure safety injection to cool the reactor. This operating mode involves the discharge of a water and steam mixture through the PORV lines at relatively high pressure (2,250 psia is assumed in the analysis). This operating mode has been explicitly addressed in the stress analysis and piping stresses are within code allowable values.

In summary, potential fluid dynamic loadings associated with discharge of water through the PORVs have been addressed in the piping stress analysis for the relevant operating modes of the PORVs.

Pressurizer Spray Line The pressurizer spray line is not considered susceptible to water hammer for reasons of operating strategy and operating experience.

The operating strategy for the pressurizer spray line is discussed in the response to question 13 above. In summary, the spray line is normally operated with relatively high spray flow on the order of 60 gpm.

Consequently, the spray line normally operates full of water and thus water hammer is not possible. There are brief and infrequent periods, however, during which pressurizer spray flow may be relatively small, approximately two gallons per minute. During these periods, there is the potential for water hammer in the spray line due to the presence of a steam-water interface in a horizontal run of the pressurizer spray piping. However, the potential for water hammer is greatly reduced during this period due to the relatively small temperature difference (50°F - l00°F) that exists between the spray water and the steam in the pressurizer. A water hammer may be caused by a hydraulic instability between the water and steam in the pressurizer spray line leading to a the rapid collapse of a steam bubble in the spray line. Without a significant temperature difference between the spray water and the steam in the spray line the potential for this hydraulic instability is greatly reduce.

Operating experience also indicates that the pressurizer spray line is not susceptible to water hammer. The spray is very loosely supported indicating that water hammer transients would be expected to result in significant

7 deflection of the p1p1ng, and likely resulting in damage t~ pipe supports.

However, recent inspections of the piping and pipe supports as a result of the safety related piping reverification project have shown no indication of pipe support damage attributable to water hammer. Therefore it is concluded that water hammer is not a concern with the pressurizer spray line.

NRC request:

15. Describe the examinations performed on the pressurizer surge line nozzle during the 1987 inspection. If these examinations were not optimized for detection of IGSCC, will IGSCC optimized examinations be performed prior to plant restart?

CPCo response:

The examinations performed in the 1987 inspection of the pressurizer surge line nozzle to safe end weld and safe end to elbow welds were a dye penetrant test and standard ultrasonic examination. The ultrasonic examination was not optimized for detection of IGSCC.

An optimized IGSCC ultrasonic examination was performed on October 10, 1993 and no recordable indications were identified. This examination was performed by a CPCo Level III inspector and a Level II inspector from ABB-Combustion Engineering (CE). The ABB-CE Level II inspector has been certified by EPRI for detection and sizing of IGSCC. The data from this examination is provided in Enclosure 5 to this letter.