L-PI-15-013, Supplement to 10 CFR 50.55a Requests (RR) 1-RR-5-5 and 2-RR-5-5 (TACs MF4839 and MF4840) Associated with Prairie Island Nuclear Generating Plant (PINGP) Fifth Ten-Year Interval Inservice Inspection (ISI) Program
| ML15036A253 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 02/04/2015 |
| From: | Davison K Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-PI-15-013, TAC MF4839, TAC MF4840 | |
| Download: ML15036A253 (11) | |
Text
Xcel Energy FEB 0 4 2015 US Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Prairie Island Nuclear Generating Plant Units 1 and 2 Dockets 50-282 and 50-306 Renewed License Nos. DPR-42 and DPR-60 L-PI-15-013 10 CFR 50.55a Supplement to 10 CFR 50.55a Requests (RR) 1-RR-5-5 and 2-RR-5-5 (TAGs MF4839 and MF4840) Associated with Prairie Island Nuclear Generating Plant (PINGP) Fifth Ten-Year lntervallnservice Inspection (lSI) Program By "L-PI-14-066, Supplement to License Amendment Request (LAR) to Revise Technical Specification (TS) 3.5.3, ECCS ([[system" contains a listed "[" character as part of the property label and has therefore been classified as invalid.) - Shutdown|letter dated September 15, 2014]] (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14258A073), Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), submitted for NRC approval 10 CFR 50.55a Requests associated with the fifth ten-year interval for the Prairie Island Nuclear Generating Plant (PINGP), Units 1 and 2, lnservice Inspection (lSI) Program. By letter dated October 20, 2014 (ML14293A458), NSPM provided supplemental information for 10 CFR 50.55a Requests 1-RR-5-5 and 2-RR-5-5. By email dated December 18, 2014 (ML14356A073), the NRC requested additional information (RAis) to complete the review of these requests. The enclosure to this letter provides the responses to the NRC Staff RAis for 1-RR-5-5 and 2-RR-5-5.
If there are any questions or if additional information is needed, please contact Mr. Dale Vincent, P.E., at 651-267-1736.
Summary of Commitments This letter contains no new commitments and no revisions to existing commitments.
Kevin Davison Site Vice President, Prairie Island Nuclear Generating Plant Northern States Power Company-Minnesota Enclosures (1) 1717 Wakonade Drive East
- Welch, Minnesota 55089-9642 Telephone: 651.388.1121
Document Control Desk Page 2 cc:
Administrator, Region Ill, USNRC Project Manager, PINGP, USNRC Resident Inspector, PINGP, USNRC
Enclosure Supplement to 10 CFR 50.55a Requests (RR) 1-RR-5-5 and 2-RR-5-5 Associated with Prairie Island Nuclear Generating Plant (PINGP) Fifth Ten-Year Interval lnservice Inspection (lSI) Program By "L-PI-14-066, Supplement to License Amendment Request (LAR) to Revise Technical Specification (TS) 3.5.3, ECCS ([[system" contains a listed "[" character as part of the property label and has therefore been classified as invalid.) - Shutdown|letter dated September 15, 2014]] (Agencywide Documents Access and Management System (ADAMS) Accession No. ML14258A073), Northern States Power Company, a Minnesota corporation, doing business as Xcel Energy (hereafter "NSPM"), submitted for NRC approval 10 CFR 50.55a Requests associated with the fifth ten-year interval for the Prairie Island Nuclear Generating Plant (PINGP), Units 1 and 2, lnservice Inspection (lSI) Program. By letter dated October 20, 2014 (ML14293A458), NSPM provided supplemental information for 10 CFR 50.55a Requests 1-RR-5-5 and 2-RR-5-5. By email dated December 18, 2014 (ML14356A073), the NRC requested additional information (RAis) to complete the review of these requests. Responses to these RAis are provided below.
NRC RAI RR-5-1 (a):
Section 5 of the relief requests proposes a corrosion rate determination as follows:
Pressure pads will be used only where the piping is predicted to retain full structural integrity until the end of the next refueling outage assuming a corrosion rate two times the corrosion rate measured at that location. If corrosion rate measurement is not practical, pressure pads will be used only where the piping is predicted to retain full structural integrity assuming a corrosion rate four times the estimated maximum corrosion rate for the system based on the same degradation mechanism as at the location.
Although similar in nature, the licensee proposed corrosion rate determination shown above is slightly different from a recent NRC safety evaluation approving a relief request for Exelon fleet (ADAMS Accession Number ML14175B593). Exelon proposed and NRC accepted the following corrosion rate determination:
The corrosion rate is two times the maximum rate observed at the location of the repair for the duration of the repair or between inspections. If the degradation rate at the repaired location is unknown, Exelon will apply four times the maximum degradation rate observed for that pipe system or a similar pipe system at the same plant site for the same degradation mechanism. If both the degradation rate at the repaired location and the cause of the degradation are not conclusively determined, Exelon will apply four times the maximum degradation rate observed for all degradation mechanisms observed for that pipe system or a similar pipe system at the same plant site.
Page 1 of3 0CFR50.55a Request 1/2-RR-5-5 Supplement (a)
Discuss whether the licensee would accept NRC-approved corrosion rate determination for Exelon as stated above. If not, provide reasons.
NSPM response:
Yes, NSPM accepts the NRC-approved corrosion rate determination for Exelon as stated below:
The corrosion rate is two times the maximum rate observed at the location of the repair for the duration of the repair or between inspections. If the degradation rate at the repaired location is unknown, NSPM will apply four times the maximum degradation rate observed for that pipe system or a similar pipe system at the same plant site for the same degradation mechanism. If both the degradation rate at the repaired location and the cause of the degradation are not conclusively determined, NSPM will apply four times the maximum degradation rate observed for all degradation mechanisms observed for that pipe system or a similar pipe system at the same plant site.
As a clarification, also note that NSPM intended to ask for relief to use Code Case N-789-1 in the "L-PI-14-066, Supplement to License Amendment Request (LAR) to Revise Technical Specification (TS) 3.5.3, ECCS ([[system" contains a listed "[" character as part of the property label and has therefore been classified as invalid.) - Shutdown|letter dated September 15, 2014]] (ML14258A073). NSPM will use the corrosion rate determination as described in Section 3.1 "Types of Reinforcing Pads" Paragraph (1) in Code Case N-789-1, which is in accordance with the NRC approved acceptance stated above.
NRC RAI RR-5-1(b):
The NRC staff noted that the licensee discussed only the pressure pad in the above proposed corrosion rate determination. Discuss the corrosion rate determination for the structural pads.
NSPM Response:
NSPM intended to ask for relief to use Code Case N-789-1 in the "L-PI-14-066, Supplement to License Amendment Request (LAR) to Revise Technical Specification (TS) 3.5.3, ECCS ([[system" contains a listed "[" character as part of the property label and has therefore been classified as invalid.) - Shutdown|letter dated September 15, 2014]] (ML14258A073). In accordance with Code Case N-789-1, both structural and pressure pads will be monitored as described in Code Case N-789-1, Section 8,"1nservice Monitoring", paragraphs (a) through (e) below:
(a) Upon completion of the repair, inspections shall be performed for structural pads, using ultrasonic or direct thickness measurement, to record the thickness of the plate, the thickness at the attachment welds, including the underlying base metal, and to the extent examinable in a 3 in. (75 mm) wide band, surrounding the repair, as a baseline for subsequent monitoring of the repair.
Page 2 of3
Enclosure 1 OCFR50.55a Request 112-RR-5-5 Supplement (b) The Owner shall prepare a plan for additional thickness monitoring for structural pads using ultrasonic or direct thickness measurement to verify that minimum design thicknesses, as required by the Construction Code or Section Ill, are maintained until the next refueling outage. The monitoring shall be monthly for the first quarter and the subsequent frequency shall be based on the results of the monitoring activities, but at least quarterly.
Provisions shall be made for access to structural pads on buried piping during operation to accomplish these examinations.
(c) Areas containing pressure pads shall be monitored monthly for evidence of leakage. If the areas containing pressure pads are not accessible for direct observation, monitoring shall be accomplished by observation of surrounding areas or ground surface areas above pressure pads on buried piping; or leakage collection systems, if available, shall be monitored.
(d) If the results of the monitoring program identify leakage or indicate that the structural margins required by 3 will not be maintained until the next refueling outage, additional repair/replacement activities not prohibited by 1 (d) shall be performed prior to encroaching upon the design limits.
(e) All reinforcing pads, regardless of when installed, shall be removed no later than the end of the next refueling outage.
Revision 1 of Code Case N-789 addressed the concerns identified in the letter to Exelon Generation dated May 10, 2012 (ML12121A637). The discussion on page 8 of this letter describes the acceptability of using inservice monitoring to verify and provide reasonable assurance that the structural integrity and leakage integrity will be maintained during the one-cycle of application.
Code Case N-789-1 is provided for reference as Attachment 1 to this Enclosure.
Page 3 of3 to 0CFR50.55a Request 1/2-RR-5-5 Supplement ASME Code Case N-789-1 "Alternative Requirements for Pad Reinforcement of Class 2 and 3 Moderate-Energy Carbon Steel Piping for Raw Water Service,Section XI, Division 1" 5 Pages Follow
CASE CASES OF ASME BOILER AND PRESSURE VESSEL CODE N-789-1 Approval Date: November 13, 2013 Code Cases will remain available for use until annulled by the applicable Standards Committee.
Case N-789-1 Alternative Requirements for Pad Reinforcement of Class 2 and 3 Moderate-Energy Carbon Steel Piping for Raw Water Service Section XI, Division 1 Inquiry: As an alternative to replacement or internal weld repair in accordance with IWA-4400, what require-ments may be applied for wall reinforcement of Class 2 and 3 moderate-energy carbon steel raw water1 piping systems that have experienced internal wall thinning from localized erosion, corrosion, and cavitation or pitting?
Reply: It is the opinion of the Committee that, in lieu of meeting IWA-4400, areas of Class 2 and 3 moderate-energy [i.e., less than or equal to 200°F (93°C) and less than or equal to 275 psig (1.9 MPa) maximum op-erating conditions] carbon steel raw water piping experi-encing internal wall thinning from localized erosion, corrosion, and cavitation or pitting may have the wall re-inforced by applying reinforcing pads to the outside sur-face of the piping in accordance with the following requirements. Excluded from these provisions are condi-tions involving flow-accelerated corrosion (FAC),
corrosion-assisted cracking, or any other form of cracking.
1 GENERAL REQUIREMENTS
{a) Application of the reinforcing pad shall be per-formed in accordance with a Repair /Replacement Plan sa-tisfying the requirements of IWA-4150.
(b) The design, materials, and installation shall meet the requirements of the Construction Code and IWA-4000, ex-cept as stated in this Case.
(c) If the minimum required thickness of reinforcing pad necessary to satisfy the requirements of 3 is greater than the nominal thickness for the size and schedule of the piping, this Case shall not be used.
(d) Additional reinforcement or repair is not permitted on top of an existing reinforcing pad.
(e) Reinforcing pads, including those installed during a refueling outage, shall not remain in service beyond the end of the next refueling outage.
W This Case may only be applied to piping not required to be ultrasonically examined for inservice inspection.
2 INITIAL EVALUATION (a) The material beneath the surface to which the rein-forcing pad is to be applied and the adjacent area shall be ultrasonically measured to establish the existing wall thickness and the extent and configuration of degradation to be corrected by the reinforcing pad.
(b) The cause and rate of degradation shall be deter-mined. If the cause is determined to be flow-accelerated corrosion (FAC), corrosion-assisted cracking, or any other form of cracking, this Case shall not apply. The extent and rate of degradation in the piping shall be evaluated to en-sure that there are no other unacceptable locations within the surrounding area that could affect the integrity of the repaired piping. The dimensions of the surrounding area to be evaluated shall be determined by the Owner, consid-ering the type of degradation present.
(c) The effects of the repair on the piping and any re-maining degradation shall be evaluated in accordance with IWA-4311.
3 DESIGN 3.1 TYPES OF REINFORCING PADS (a) Reinforcing pads may be used for leak prevention only (pressure pad), or for leak prevention plus structural reinforcement of thinned areas including areas that do, or are expected to, penetrate the piping wall (structural pad).
(1) Pressure pads are designed to retain pressure, and may be used only where the piping is predicted to re-tain full structural integrity until the next refueling outage assuming a corrosion rate of either 2 times the actual mea-sured corrosion rate in that location, or 4 times the esti-mated maximum corrosion rate for the system.
1 Raw water is defined as water such as from a river, lake, or well or brackish/salt water; used in plant equipment, area coolers, and heat exchangers. In many plants it is referred to as "Service Water,"
The Committee's function is to establish rules of safety, relating only to pressure integrity, governing the construction ofboilers, pressure vessels, transport tanks and nuclear components, and inservice inspection for pressure integrity of nuclear components and transport tanks, and to interpret these rules when questions arise regarding their intent This Code does not address other safety issues relating to the construction of boilers, pressure vessels, transport tanks and nuclear components, and the inservice inspection of nuclear components and transport tanks. The user of the Code should refer to other pertinent codes, standards, Jaws, regulations or other relevant documents.
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CASE (continued)
N-789-1 CASES OF ASME BOILER AND PRESSURE VESSEL CODE
{2) Structural pads are designed for pressure plus structural reinforcement and may be used where the pip-ing is predicted not to retain full structural integrity until the next refueling outage.
3.2 GENERAL DESIGN REQUIREMENTS-PRESSURE AND STRUCTURAL PADS (a) The design of reinforcing pads shall be in accor-dance with the applicable requirements of the Construc-tion Code or Section III (NC-3100, ND-3100 and NC-3600, ND-3600 including Appendix II).
(b) The reinforcing pad shall be sized to encompass the unacceptable area with the attachment welds located on adjacent base material of sufficient thickness to accommo-date the design stresses.
(c) The plate for the reinforcing pad shall be rolled or otherwise formed to fit the contour of the piping to achieve proper weld fit-up.
(d) The thickness of the reinforcing pad shall be suffi-cil=h"lt to maintain required thickness until the next refuel-in~ outage.
(e) The tensile strengths of the plate and weld filler me-ta(:for the reinforcing pad shall be at least that specified fot the base metal to which it is applied.
(/) The predicted maximum degradation of the rein-fo~ced piping until the next refueling outage shall be in-cluded in the design. The predicted degradation of the piping shall be based on in-situ inspection of, and estab-lished data for, similar base metals in similar environ-ments. If the reinforcing pad is predicted to become exposed to the raw water, the predicted degradation of the reinforcing pad shall be based upon established data for base metals or weld metals with similar chemical com-position to that used for the reinforcing pad.
(g) Material for reinforcing pads shall be ferritic, with welds of compatible weld filler metal.
(h) The following factors shall be included, as applic-able, in the design and application of the pad:
(1) shrinkage effects, if any, on the piping (2) stress concentrations caused by installation of the reinforcing pad or resulting from existing and predicted piping internal surface configuration
{3) effects of welding on any interior coating (4) added weight of the pad with respect to any de-sign analyses that could be affected (i) If flexibility analysis was required by the original Construction Code, the effect of the reinforcing pad shall be reconciled with the original analysis. For rectangular-shaped reinforcing pads on piping designed to NC-2650, ND-3650 and aligned parallel or perpendicular to the axis of the piping, unless a lower stress intensification factor
[SIF or (i)] is established, an SIF (i) of 2.1 shall be applied for reinforcing pads on straight pipe and adjacent welds.
Also, a stress multiplier of 1.7 shall be applied to the SIF (i) for standard elbows, and an SIF (i) of 2.1 shall be 2 Design thickness as prescribed by the Construction Code.
applied for tees and branch connections when the toe of the attachment weld is not less than 2.5~Rtnom from any branch reinforcement in Figure 1.
(j) Corners of reinforcing pad plates shall be rounded with radii not less than the reinforcing pad thickness, and the toes of attachment welds at the corners shall have 1 in. (25 mm) minimum radius.
(k) The distance between toes of attachment welds and other attachments or branch reinforcement (Figure 1 and
- 2) shall not be less than the following equation:
d = 2.5~Rtnom where d = minimum distance between toes of fillet welds of adjacent fillet welded attachments R = the outer radius of the piping t n 0 m = nominal thickness of the piping
(/) When permitted by the design, suitable gasket mate-rial may be applied inside the pad to prevent moisture during welding (see Figures 1 and 2).
3.3 SPECIFIC DESIGN REQUIREMENTS-PRESSURE PADS Pressure pads shall meet the requirements of 3.2, Figure 2, and the following:
(a) Fillet-welded pressure pads shall be designed to withstand the membrane strain of the piping in accor-dance with the requirements of the Code specified in 3.2(a) such that the following criteria are satisfied:
(1) The allowable membrane stress is not exceeded in the piping or the pad.
{2) The strain in the pad does not result in fillet weld stresses exceeding allowable stresses for such welds.
(b) Design as a reinforced opening in accordance with the Construction Code shall satisf'y (a).
(c) As an alternative to (a), pressure pads may be de-signed as structural pads in accordance with 3.4 or as pre-qualified designs in accordance with 3.5.
3.4 SPECIFIC DESIGN REQUIREMENTS -
STRUCTURAL PADS Structural pads shall meet the requirements of 3.2, Figure 1, and the following:
(a) Unless otherwise established by analysis in accor-dance with the requirements of 3.2(a), structural pads shall be attached by partial penetration attachment welds (see Figure 1) that extend for a distance of at least s in each direction beyond the area predicted, by the next re-fueling outage, to infringe upon the required thickness.2 s :2: 0.75,./Rtnom NC-SUPP. 4 2 (N-789-1)
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CASE (continued)
CASES OF ASME BOILER AND PRESSURE VESSEL CODE N-789-1 Figure 1 Structural Pad d
Appropriate gasket material C= predicted circumferential growth of degradation L =predicted axial growth t
Minimum dimensions of plate _)
bevel optional
=predicted maximum dimensions of degradation below minimum required thickness R== outer radius of the component s
1 in. (25 mm) minimum tnom nominal wall thickness of the component (b) The thickness of the partial penetration attachment welds shall equal the thickness of the pad and the edges of the welds shall be tapered to the piping surface at a max-imum angle ("a" in Figure 1) of 45 deg.
(c) Final configuration of the structural pad including attachment welds shall permit the examinations and eva-luations required herein, including any required preser-vice or inservice examinations of encompassed or adjacent welds.
Required thickness treq
~---d---~1 Limit of branch reinforcement d ~ 2.5 ~ Rtnom (d) Except for the tapered edges, the structural pad plate and attachment welds shall have a uniform thickness.
3.5 PREQUALIFIED DESIGN Application of structural pads on straight pipe, standard elbows, and associated welds shall be exempt from the re-quirements of 3.2(a), provided all of the following condi-tions are satisfied.
(a) All other requirements of 3.1, 3.2, and 3.4 are satisfied.
(b) The axial length of structural pad plus width of par-tial penetration attachment welds shall not exceed the greater of 6 in. (150 mm) or the outside diameter of the piping.
3 (N-789-1)
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CASE (continued)
N-789-1 CASES OF ASME BOILER AND PRESSURE VESSEL CODE Figure 2 Pressure Pad D
~
Appropriate gasket material if required tnom t
+
.41
?
k--L,C Minimum dimensions of plate __j(
~
~I Predicted growth of degradation below minimum wall thickness based on 2 X or 4 X estimated corrosion rate.
treq Required
+/-
thickness t*
=predicted extent of degradation below minimum wall thickness based on 2 X the measured corrosion rate at the location or 4 X the maximum estimated corrosion rate for the system.
{c) The finished structural pad shall be circular, oval, or rectangular in shape.
{1) The maximum dimension compensated by a circu-lar structural pad shall not exceed two-thirds of the nom-inal outside diameter of the piping.
{2) Rectangular structural pads shall be aligned par-allel with or perpendicular to the axis of the piping.
{3) For oval structural pads, the end radii shall not be less than 0.75~Rtn0m, and the axis of the structural pad shall be aligned parallel with or perpendicular to the axis of the piping.
4 WATER-BACKED APPLICATIONS
{a) Attachment welds on water backed piping shall be applied using the SMAW process with low-hydrogen electrodes.
{b) When welding a reinforcing pad to a leaking area, precautions shall be taken to prevent welding on wet sur-faces, such as installation of a gasket or sealant beneath the pad.
{c) For piping materials other than P-No. 1, Group 1, the surface examination required in 6 shall be performed no sooner than 48 hr after completion of welding.
5 INSTALLATION
{a) The base material in the area to be welded shall be cleaned to bare metal.
{b) Weld metal shall be deposited using a groove -
welding procedure qualified in accordance with Section IX and the Construction Code.
{c) Provisions for venting during the final closure weld, or for pressurizing for leak-testing, shall be included, if necessary.
NC-SUPP. 4 4 (N-789-1)
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CASE (continued)
CASES OF ASME BOILER AND PRESSURE VESSEL CODE N-789-1 (d) The surface of the attachment weld shall be pre-pared, if necessary, by machining or grinding to permit performance of surface and volumetric examinations re-quired by 6. For ultrasonic examination, a surface finish of 250 RMS or better is required.
6 EXAMINATION (a) The completed attachment weld shall be examined using the liquid penetrant or magnetic particle method and shall satisfy the surface examination acceptance cri-teria for welds of the Construction Code or Section III (NC-5300, ND-5300).
(b) Except for the tapered edges, partial penetration at-tachment welds, including the piping base metal upon which they are applied, shall be ultrasonically measured to verify acceptable wall thickness.
(c) Partial penetration attachment welds shall be volu-metrically examined when full penetration girth welds in the piping are required by the Construction Code to be vo-lumetrically examined. Where configuration does not per-mit meaningful volumetric examination, the first layer, each 'lz in. (13 mm) thickness of weld deposit, and the fi-nal surface shall be examined in accordance with (a) in lieu of volumetric examination.
(d) If volumetric examination is required, the full vo-lume of the attachment weld, excluding the tapered edges, but including the volume of base metal required for the in-tended life of the reinforcing pad, shall be examined in ac-cordance with the Construction Code or Section III, using either the ultrasonic or radiographic method, and shall, to the depth at the surface of the piping, satisfy the accep-tance criteria for weldments of the Construction Code or Section III (NC-5320, ND-5320 or NC-5330, ND-5330).
Any volume of the piping beneath the reinforcing pad that is credited in the design shall satisfy the volumetric accep-tance criteria of Section III (NC-5320, ND-5320 or NC-5330, ND-5330), as applicable.
7 PRESSURE TESTING In lieu ofiWA-4540, a system leakage test of the repair/
replacement activity shall be performed in accordance with IWA-5000 prior to, or as part of, returning to service.
Reinforcing pads attached to piping that has not been breached shall be equipped with pressure taps for perfor-mance of pressure testing.
8 INSERVICE MONITORING (a) Upon completion of the repair, inspections shall be performed for structural pads, using ultrasonic or direct thickness measurement, to record the thickness of the plate, the thickness at the attachment welds, including the underlying base metal, and to the extent examinable in a 3 in. (75 mm) wide band, surrounding the repair, as a baseline for subsequent monitoring of the repair.
(b) The Owner shall prepare a plan for additional thick-ness monitoring for structural pads using ultrasonic or di-rect thickness measurement to verify that minimum design thicknesses, as required by the Construction Code or Section III, are maintained until the next refueling out-age. The monitoring shall be monthly for the first quarter and the subsequent frequency shall be based on the re-sults of the monitoring activities, but at least quarterly.
Provisions shall be made for access to structural pads on buried piping during operation to accomplish these examinations.
(c) Areas containing pressure pads shall be monitored monthly for evidence of leakage. If the areas containing pressure pads are not accessible for direct observation, monitoring shall be accomplished by observation of sur-rounding areas or ground surface areas above pressure pads on buried piping; or leakage collection systems, if available, shall be monitored.
(d) If the results of the monitoring program identify leakage or indicate that the structural margins required by 3 will not be maintained until the next refueling outage, additional repair/replacement activities not prohibited by l(d) shall be performed prior to encroaching upon the de-sign limits.
(e) All reinforcing pads, regardless of when installed, shall be removed no later than the end of the next refuel-ing outage.
5 (N-789-1)
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