ML20214E421
| ML20214E421 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 09/18/1984 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20213F005 | List: |
| References | |
| NUDOCS 8603260181 | |
| Download: ML20214E421 (6) | |
Text
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ATTACHMENT Limerick Generating Station, Units 1 and 2 Docket Nos. 50-352/353:
Materials Engineering Branch Materials Application Section Fracture Prevention of Containment Pressure Boundary Our safety evaluation review assessed the ferritic materials-in the Limerick Generating Station Units 1 & 2 containment system that constitute the con-tainment pressure boundary to determine if the material fracture toughness is in compliance with the requirements of General Design Criterion 51,
" Fracture Prevention of Containment Pressure Boundary."
GDC 51' requires that under operating, maintenance, testing and postulated accident conditions, (1) the ferritic materials of the containment pressure boundary behave in a nonbrittle manner and (2) the probability of rapidly propagating fracture is minimized. '
The Limerick Generating Station containment system is a reinforced concrete g
structure with a thin steel liner on the inside surface which serves as a leaktight membrane.
The ferritic materials of the containment pressure boundary which were considered in our assessment are those which have been applied in the fabrication of the drywell head, equipaent hatch, personnel locks, penetrations and fluid system components,~ including the valves required to isolate the system. These components are the parts of the con-tainment system which are not backed by concrete and must sustain loads during the performance of the containment function under the conditions cited by GDC 51.
We have determined that the fracture toughness requirements contained in ASME Code editions and addenda typical of those used in the design of the Limerick Generating Station containment may not ensure full compliance with GDC 51 for all areas of the containment pressure boundary.
As a result, we have elected to apply in our licensing reviews of ferritic containment 0603260181 86 h 2 PDR ADOCK PDR P
-s pressure boundary materials, the criteria for Class 2 components identi-fied in the Sumer 1977 Addenda of Section III of the ASME Code. Because
,the fracture toughness criteria that have been applied in construction typically differ in Code classification and Code edition and addenda, we have chosen the criteria in the Sumer 1977 Addenda of Section III L
of the Code to provide-a uniform review consistent with the safety function of the containment pressure boundary materials. Therefore, we reviewed the materials of the components of the Limerick Generating l
Station containment pressure boundary according to the fracture toughness requirements of the Sumer 1977 Addenda of Section III for Class 2 components.
Considered in our review were components of the containment system which are load bearing and provide a pressure boundary in the performance of the containment function under operating, maintenance, testing and postu-lated accident conditions as addressed in GDC 51. These components are the drywell head, equipmeat hatch, personnel airlocks, penetrations and elements of specific containment penetrating systems.
Our assessment of the fracture toughness of materials of the Limerick Station containment pressure boundary was based on the metallurgical characterization of these materials and fracture toughness data presented in'NUREG-0577, " Potential for Low Fracture Toughness and Lamellar Tearing-on PWR Steam Generator and Reactor Coolant Pump Supports," USNRC, October 1979, for coment, and ASME Code Section III, Sumer 1977 Addenda, Subsec-tion NC.
The metallurgical characterization of these materials, with respect to their fracture toughness, was developed from a review of how these materials were fabricated and what thermal history they experienced during fabrication. The metallurgical characterization of these materials, when correlated with the data presented in NUREG-0577 above and the Sumer 1977 Addenda of the ASME Code Section III, provided the technical basis for our evaluation of compliance with GDC 51.
Based on our review of the available fracture toughness data and materials l
fabrication histories, and the use of correlations between metallurgical characteristics and material fracture toughness, we conclude, with one condition,that the ferritic materials of the components of the Limerick Generating Station containment pressure boundary meet the' fracture toughness requirements that are specified for Class 2 components by the 1977 Addenda of Section III of the ASME Code. Compliance with~these Code requirements provides reasonable assurance that mate.-ials of the Limerick Generating Station reactor containment pressure boundary will behave in a nonbrittle manner, that the probability of rapidly propagating fracture will be mini-mized and that the requirements of GDC 51 are satisfied. The condition relates to the'feedwater check valves (1F074 A&B and 2F074 A&B) which is addressed below.
l Limerick 1 & 2 Feedwater Check Valves (IF074 A/B:2F074 A/B)
Our review identified 24" feedwater check valves IF074 A&B and 2F074 A&B as parts of the reactor containment pressure boundary. The cast bodies of these valves are known to contain shrinkage flaws which have been known to propa-gate in service. Because of the presence of these flaws and the uncertainty,
related to their propagation in service, we were unable to conclude, relative-i to fracture toughriess, that sufficient margin of safety existed under the limiting environmental condition to be experienced by this valve, viz.,
1180 psi at 42*F postulated for HPCI, as identified by the applicant, when this valve is called upon to serve as a containment pressure boundary.
In accordance with our review practice, conformance with GDC 51 is assured when the lowest service temperature is 30*F above the NDT of the material.
In this case, therefore, if we could be assured that the NDT was at or below 12*F, we would consider the component to be acceptable.
The applicant submitted in support of his position regarding the acceptability of these valves, Bechtel Tech Report No. 1183-05EV, Revision 2, dated May 1984,
a s
. and titled " Acceptability of Class 1 24-inch Feedwater Check Valves."
We have reviewed the report within the context of compliance with GDC 51 requirements.
Although the applicant has submitted Charpy data that he feels supports an NDT of 10 F or below, we have reservations about the NDT-Charpy relationship for this material, and also have concerns that tests on separately cast " keel blocks" may not conservatively represent the proper-ties of the actual castings.
Using NUREG-0577 recommendations and con-sidering the guidance given in Table NC-2311(a)-1 of the Summer 1977 Addenda Boiler and Pressure Vessel Code, we have concluded that it is reasonable and conservative to assume that these valve body castings have an NDT mf 30*F, and therefore do not meet our basic criterion.
The applicant performed a fracture mechanics calculation that concluded that wide margins against failure by brittle fracture exist. We have some reservations regarding the assumptions used to determine the critical K7g of the material.
The approach was to determine the K by a correlation IC
\\
with Charpy energy values.
The correlation method used has not received universal acceptance.
Because we were not satisfied with the applicant's fracture mechanics analysis, we performed our own independent calculations.
Instead of a correlation with Charpy values, we chose to use the method recommended in Appendix G of Section III of the Code, suppi?mented by additional calculations using Section XI.
In these methods, the fracture toughness of the material is given by K -temperature curves, and KIC-temperature IR curves, that are indexed to the RT of the material.
The RT is NDT NDT basically the same as NDT for material with normal Charpy properties.
We used the stress levels furnished by the applicant for our analysis as these appeared to be somewhat more conservative than our own calculations indicated. We also chose to use the assumed flaw size selected by the
- s the applicant, one inch deep by 3.5 inches long.
This size, in our opinion, bounds the dimensions of acceptable shrinkage (severity level 2) with margin for postulated growth during operation. The results of our calculations are as follows.
For the Section III Appendix G calculations, the KIR (1 wer bound estimate of dynamic or arrest toughness) at RTNDT+12*Fis41Ksi[I~n.
The applied K, including both membrane and bending stress, is calculated to be 24.3 y
Ksi\\in.
This indicates a margin of 41 divided by 24.3, or a factor of 1,7 against failure under dynamic conditions at 1180 psi and 42F.
Appendix G requires a margin of a factor of two on pressure stress, but only a factor,
of one on bending stress (assuming the bending stress is not caused by pressure).
In this case, it was stated by the applicant that some of the bending stress could be pressure-related, so we assume that a factor of two should be-provided for the combined stresses. Applying a factor of two on the membrane stress of 10.4 psi and a factor of one on the bending stress g.
of 10.2 psi, the total K is 38.3, which is less than the K of 41, so the y
IR Code recommended margins would be met if bending stress were independent of pressure.
Although these calculations indicated that the valve body would come very close to meeting the Code Appendix G criteria, we recognized a'possible non-conservatism in the approach.
The Code states that the KIR cune may be used for steel with minimum specified yield strengths of 50 Ksi and less.
As the minimum yield strength for the valve body material is 35 Ksi, the use of the K curve is acceptable to the Code.
Nevertheless, the principle IR l
underlying the development of the K curve and the temperature correlation IR l
with NDT implies that the K values should be reduced by the ratio of the IR yield strength of the. material of concern and that of the steels used to l
develop the correlation.
Because all of the steels used to establish the l
K curve had minimum yield strengths of 50 Ksi, we performed calculations IR in which the assumed K at 12F was reduced by the ratio of 35 divided by IR 50.
In this case, the K valueis28.7Ksidin,'whereasthecalculated IR
K was 24.3 KsiNT,providingamarginofonly1.2insteadofthefactor g
of two specified in Appendix G.
We also performed a more realistic calculation using the lower bound KIC values from Section XI of the Code. This assumes the more probable quasi-NDT + 12*F is 69 Ksi 6, giving static loading condition.
The K at RT IC a margin of a factor of 2.8 against failure.
Reducing the K by the ratio IC of the yield strength (as discussed above) still results in a margin of a factor of 2.0.
We have concluded.from these calculations that although the valve body may not quite. meet the Appendix G requirements, even our most conservative approach still shows some safety margin.
Adequate margins against failure (at least a factor of two) will exist under the most. probable loading con-ditions.
These conclusions assume that the flaw size assumed will not be exceeded significantly.
However, service experience on similar castings has dis-closed that normal, acceptable shrinkage may be extended by cracking during service. We, therefore, recommended to the applicant that these
~
valves be inspected for surface cracks on the inside and outside surfaces at the first refueling outage and at other times when the valve is dis-assembled for maintenance.
The applicant has committed to including this augmented inspection by methods acceptable to the staff, which will be determined during staff's review of the inservice inspection program.
We have concluded that the results of our evaluation and the augmented 1
inservice inspection program for these valves will provide reasonable assurance of compliance with the requirements of GDC 51, contingent on confirmation by the augmented ISI that the shrinkage flaws existing in toe valve bodies on entering service have not propagated to either of the surfaces.
Should the augmented ISI disclose that these flaws have propa-gated to either of the surfaces, the valves then are to be replaced by the licensee.
-s PHILADELPHIA ELECTRIC COMPANY 2301 M ARKET STREET P.O. BOX 8699 PHILADELPHI A. PA.19101 JOHN 5.KEMPER o - n r. $.
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84 00I17 P069 Mr. A. Schencer, Chief Docket No.:
50-352 LicensingpBranch No. 2 U. S. NuclhSf Regulatory Ccmnission Washington, D.C. 20555
Subject:
Limerick Generating Station, Unit 1 Request for Additional.Inforration 'frcrn NRC Materials Engineering Branch (MIEB)
SER Confinnatory Issue No. 22
References:
(1) Istter, J. S. Ke:per to A. Schencer, dated 9/7/84.
(2) 10/1/84 Telecon between W. Harelton (h"AC/MIEB),
R. E. Martin (NPC/DC), and H. D. Honan and T. C. Hinkle (PERO).
(3) Ietter, J. S. Ke:per to A. Schwencer dated Septerrber 6,1984.
File:
GOVT l-1 (NRC)
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Dear Mr. Schencer:
This letter modifies our reference (1) letter in order to confirm comitments made during the reference (2) telecon.
PECO will provide an augmented inservice inspection program for the Limerick Unit 1 outboard feedwater check valves (lF074A and 1F074B valves). This inspection program will be inplerented at the first refueling cutage. It will include inspection of the entire surface of the valve bodies, both internal and external, by surface or other examination technique acceptable to the staff. The inspection program will, after appropriate surface preparation, be of sufficient sensitivity to detect a minimum crack length of 3 inches. This is equivalent to the conservative crack size assu&hTin'~the fracture mechanics analysis provided via the reference (3) letter. Inspections subsequent to the first refueling outage will be in accordance with the Unit 1 ISI program.
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The aegr:enteiinspection program will be further described in the ISI program for Limerick Unit 1.
The ISI program will be submitted for NBC evaluation 6 months after ccrmercial cperation as regaired by SER confi natory issue 12.
We trust this information is sufficient to close conficatorf issue no. 22.
Sincerely, fi 5 bLl-s JHA/gra/10028402 cc: See Attached Service List b
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2301 MARKET STREET pr P.O. BOX 8699 9
84 g 29 P5 37 PHILADELPHIA. PA.19101 JOHN S. KEMPER
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.t Mr. A..Schwencer, Chie f Docket No.:
50-352 Licensing Branch No. 2 U. S. Nuclear Pegulatory Camtissicn Washington, D.C. 20555
Subject:
Limerick Generating Station, Unit 1 Pequest for Additional Infomation frcm NRC Materials Engineering Branch (MTEB)
SER Ccnficatory Issue No. 22
References:
(1)
Letter, J. S. Karper to A. Schwencer, dated 10/4/84.
(2) Telecen between A. Sc. encer (NRC) and 5
J. T. Pchb (PECO), 10/12/84.
File:
GOVI l-1 (NRC)
Dear Mr. Schwencer:
This letter Inodifies cur reference (1) letter in order to confim ccmitments trade during the reference (2) telecen.
PECO will provide an augmented inservice inspelon program for the Limerick Unit 1 cutboard feedwater check valves (lF074A and 1F074B valves). This inspectica program will be inplecented at the first refueling cutage. It will include inspection of the entire surface of the valve bodies, both internal and external, by surface or other examination technique acceptable to the staff.
The augmented inspection program will be further described in the ISI p%um for Linerick Unit 1.
The ISI program will be subnitted for NRC evaluatien 6 mcnths after ccmercial cperatien as required by SER confimatory issue 12.
We trust this infomation is sufficient to _close confinatory issue no. 22.
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