ML18102B500
| ML18102B500 | |
| Person / Time | |
|---|---|
| Site: | Salem, Hope Creek  |
| Issue date: | 08/07/1997 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML18102B499 | List: |
| References | |
| NUDOCS 9708140229 | |
| Download: ML18102B500 (7) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555--0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
.RELATED TO WELDING SA-351. GRADE CD4MCu MATERIAL PUBLIC SERVICE ELECTRIC AND GAS COMPANY ATLANTIC CITY ELECTRIC COMPANY PHILADELPHIA ELECTRIC COMPANY DELMARVA POWER AND LIGHT COMPANY SALEM NUCLEAR GENERATING STATION. UNITS 1 AND 2 AND
1.0 INTRODUCTION
HOPE CREEK GENERATING STATION DOCKET NOS. 50-272. 50-311. AND 50-354 By letter dated January 18. 1995. Public Service Electric and Gas Company (PSE&G) requested approval of an alternative to the 1989 Edition of the American Society of Mechanical Engineers (ASME).Section III Boiler and Pressure Vessel Code (the Code) for Salem Nuclear Generating Station. Units 1 and 2 (Salem). and Hope Creek Generating Station (Hope Creek).
PSE&G requested approval to weld repair SA-351. Grade CD4MCu material. which is not permitted by Section III. Table I~7.2. Note 5 of the 1989 Edition. but is allowed per Section II of the 1989 Edition of the Code.
By letter dated May 5, 1995. the NRC approved the use of the proposed alternative pursuant to 10 CFR 50.55a(a)(3)(i). However. the safety evaluation (SE) enclosed with the May 5. 1995 approval letter limited the usefulness of the initially requested alternative. Specifically, item number (2) under section 3.0 of.the SE states. "No repairs will be performed at the site. All repairs will be performed at the manufacturer's facilities where the parts can be solution heat treated after the repair."
In its letter of December 12. 1996. as supplemented by letters dated March 20.
and June 20. 1997. PSE&G requested approval to modify its alternative and relief from its commitment to post weld heat treat CPWHT) weld repairs on SA-351 Grade CD4MCu material for the service water pumps at Salem and Hope Creek.
The purpose of this evaluation is to determine the acceptability of an alternative to ASME Code Section III repair requirements proposed by PSE&G for its service water pumps at Sal em and Hope Creek.
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... 2.0 EVALUATION OF RELIEF REQUEST 2.1 Code Requirement for which Relief is Requested ASME Code Section I'II Requirements Table 1-7.2 (Appendices) of the 1989 Edition of the ASME Code restricts the use of welding processes on SA-351. Grade CD4MCu material with the following Note No. 5: "No welding is permitted on these materials."
ASME Code Section II Requirements ASME Code Section I I. Speci fi cation SA-351/SA-351M. Section 10. Repair by Welding, allows weld repairs on this class of materials if these repairs are I
made using procedures and welders qualified under Section IX of the ASME Code.
2.2 Licensee's Basis for Request The licensee stated that the basis for the request to perform on-site or off-site weld repairs to CD4MCu material without PWHT are 1) to support potential emergent plant conditions. and 2) to repair cavities or indications found after machining during the manufacturing process.
To support potential emergent plant conditions. PSE&G proposed to perform on-site or off-site weld repairs to CD4MCu material without PWHT to expedite the return to service of the Station Service Water System within the Allowed Out-of-service Times CAOT) imposed by the station Technical Specifications and avoid the risk of a plant shutdown transient. Although weld repairs of CD4MCu material without PWHT potentially reduces corrosion resistance. repairs without PWHT facilitate the timely return to service of the Service Water System within its AOT.
In addition. PWHT at a temperature of 1900-2050°F performed for weld repairs of machined castings could affect the machined tolerances and result in an unusable pump casting.
Due to the long lead time needed to manufacture new castings. weld repair without PWHT would support return of the Station Service Water System to service in a timely manner.
2.3 Justification for Request The licensee provided the technical justification below for its request.
"Post Weld Heat Treatment of weld repairs is performed to restore the optimum toughness and corrosion resistance properties of the CD4MCu duplex phase material. particularly in the Heat. Affected Zone CHAZ).
PSE&G recognizes that PWHT of weld repairs to CD4MCu is desirable to achieve a mix of 50% austenite and 50% ferrite.
However. the structural properties (e.g., Ultimate Tensile Strength CUTS). and Yie.ld Strength CVS)) of the CD4MCu material are not detrimentally affected by weld repairs as shown by the tables below."
Base Metal Characteristics Table 1 shows the tensile test results of the foundry's welding procedure qualification records for the Hope Creek castings.
PWHT No Yes SA/A 351 Grade CD4MCu specs Table 1 - Weld Joint Tensile Test Data UTS (ksi)
YS (ksi)
Break Location 121/121 94/~6 Base Metal 111-115 79-84 Base Metal 100 minimum 70 minimum Filler metals with compositions similar to that of E2553 electrodes used in the weld repair of CD4MCu castings were tested. The data shows that the strength of the base metal is higher without PWHT. and still meets the base metal mechanical properties. The weld joint tensile specimens broke in the base metal indicating the weld metal and the heat affected zone (HAZ) are stronger than the base metal.
Filler Metal Characteristics The filler metal chemical composition is designed to provide the desired 50%
ferrite and 50% austenite microstructure and associated mechanical properties in the as-welded condition and the PWHT condition. The tensile tests results of the E2553 filler metal from the Hope Creek material test reports are shown in Table 2 below.
Table 2 - E2553 Fi'l ler Metal Tensile Test Data PWHT UTS (ksi)
YS (ksi)
Elongation Ferrite(%)
(%)
Yes
_ 115-123 73-83 28-31 No 125-130 87-106 17-27 47-b~
SFA 5.4 110 minimum 16 minimum specs The above data was the result of testing of 4 different lots of E2553 filler metal. The tensile strength of the filler metal is higher and elongation is lower without PWHT.
The ferrite range in the as welded condition is from 47%
to 62% which approaches the desired 50% range.
The filler metal's higher nickel content and the addition of nitrogen which promote the formation of austenite. result in the desired dual phase microstructure in the weld metal.
The mechanical properties of the filler metal meet the base metal requirements for CD4MCu material and the SFA 5.4 Classification E2553 filler metal specifications.
PSE&G therefore concluded that the structural integrity of the CD4MCu castings would not be comprom_ised by either onsite or offsite weld repairs without PWHT.
Heat Affected Zone Characteristics The HAZ in the as-welded state contains a narrow zone which may exhibit little or no austenite with a resultant reduction in corrosion resistance and toughness.
PSE&G plans to visually inspect a sample of the weld repairs.
performed on the CD4MCu service water pump castings that have not been PWHT within the first 18 months after installation to establish corrosion trends.
The toughness that can be expected is shown in the foundry's welding procedure qualification records. These records show values of 27-29 ft/lb at +28°F when PWHT was not performed following weld repairs. This toughness exceeds the 1989 and 1995 editions of the ASME III. Class 3. requirement of 20 ft/lb average for the CD4MCu yield strength range.
Effects on the Desired Material Properties The desirable properties of the CD4MCu material for this application are:
- 1) wear resistance (i.e.. abrasion resistance against the silt in the service water). and 2) corrosion resistance from the brackish service water.
The wear characteristics are not expected to change as a result of repair without PWHT.
since the as-welded strength and hardness are similar to the base metal.
However. the corrosion properties may be diminished following weld repairs without PWHT.
PSE&G stated that. according to the literature, it is reasonable to expect that the principal degradation mechanism (i.e. corrosion) following any weld repairs without subsequent PWHT would first manifest in the localized area of the HAZ as pitting corrosion due to the brackish water chemistry. The dual-phase microstructure. ferrite plus austenite. and possible predominately ferrite microstructure in a HAZ without PWHT would preclude Intergranular Stress Corrosion Cracking (IGSCC) which is expected in fully austenitic stainless steels that have been sensitized. Weld repairs without PWHT should have little effect on erosion due to particulate flo~.
since the as-welded tensile strength which is related to wear is not reduced.
In this postulated condition. the effect is expected to be a localized attack rather than a large general surface area reduction of the CD4MCu pump casting.
PSE&G stated that the experience so far with this material as far as general corrosion resistance has been good. This material is currently installed in the Salem service water pumps and the Hope Creek service water pumps.
These are submerged vertical pumps located in a partial saline (e.g. brackish) environment containing silt. The exterior pump surfaces are likely to corrode due to the river water chemistry and the lower flow conditions. whereas the pump internals exposed to higher velocity flows are more likely to erode from the silt and particulates in the water. A visual inspection of the Salem CD4MCu service water pump castings after 18 months of service showed that the CD4MCu material demonstrated good corrosion/wear resistance to both expected degradation mechanisms.
The pumps are in low energy systems that operate at ambient (river water) temperatures. Consequently, given the service conditions and expected degradation mechanisms. there is no evidence to suggest any reduction of structural integrity that would lead to a catastrophic consequence.
2.3 Licensee's Proposed Alternative To ensure that wear and corrosion resistance remain sufficient. PSE&G proposes to visually inspect a sample of weld repairs that are performed without PWHT within the first 18 months of the repair to establish corrosion trend information.
PSE&G would perform these visual inspections directly on the surface areas of the completed weld repairs. i.e.. either on the exterior or interior of the pump casting. These inspections would be conducted with or without pump disassembly, depending on the actual location of the weld repairs and the accessibility to the pump interior without disassembly (e.g.. using mirrors. optical equipment. etc.). Based upon the initial visual examination results and the establishment of degradation trends. if any, subsequent examinations may be continued. reduced in frequency, or discontinued.
2.4
NRC Staff Evaluation
The staff finds that PSE&G's proposed alternative provides an acceptable level of quality and safety.
PSE&G's test data showed that properly performed weld repairs of this material without PWHT will provide adequate strength and toughness without compromising structural integrity. The data showed that the structural properties of the weld in the as-welded condition meet or exceed the base metal tensile and yield strength requirements required by the ASME Code.
Consequently, the structural integrity of the castings made from this material would not be compromised by the pe(forming of onsite or offsite weld repairs without solution annealing PWHT.
Other important properties in this application are wear resistance to silt and corrosion resistance to brackish service water.
The staff finds that the wear properties are not expected to change as a result of repair without PWHT.
because the as-welded strength and hardness are similar to that of the base metal.
In addition. the staff finds that the corrosion properties may be diminished after weld repairs without PWHT.
Heat treating after welding is desirable for corrosion resistance but can affect the dimensional tolerances. Of the alternatives. maintaining the dimensional tolerances with some loss of corrosion resistance by not postweld heat treating would be preferable to the possibility of having to replace the casting. However. the loss of corrosion resistance would be localized, because a narrow band. the HAZ. would be the area affected. General corrosion is not at issue: a visual inspection of the Salem CD4MCu service water pump castings after 18 months of service showed that they had good corrosion/wear resistance.
PSE&G's proposal to visually inspect a sample of weld repairs that are performed without PWHT within the first 18 months of the repair should reveal any degradation early enough to take action if needed.
3.0 CONCLUSION
The NRC staff concludes that weld repair of SA-351 material without PWHT constitutes a reasonable. safe alternative to the requirements of the 1989 Edition of the ASME Code.and our safety evalu9tion of May 5. 1995. The use of the alternative is authorized pursuant to 10 CFR 50.55a(a)(3)(i) as the alternative would provide an acceptable level of quality and safety.
Principal Contributor:
M. Banic Date:
August 7, 1 9 9 7
c.,..
' 4.0 REFERENCE Safety evaluation by the Office of Nuclear Reactor Regulation - Request to Weld SA-351 Grade CD4MCu Material. Salem Nuclear Generating Station and Hope Creek Generating Station. enclosed with letter dated May 5, 1995. to Leon R.
Eliason. PSE&G. from Leonard N. Olshan. USNRC.