ML20151T419
| ML20151T419 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 08/05/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20151T408 | List: |
| References | |
| NUDOCS 8808160356 | |
| Download: ML20151T419 (5) | |
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/ 4 UNITED STATES 8 1 NUCLEAR REGULATORY COMMISSION
'g t WASHINGTON, D. C. 20655
'+,.....,o SAFETYEVALUATIONBYTHEOFFICEOFNUrpARREACTORREGULATION ON ELIMINATION OF POSTULATED PRIP;r LOOP' PIPE RUPTURES AS A DESIGN BASIS.
PORTLAND GENERAL ELECTRIC COMPANY TROJAN NUCLEAR PLANT DOCKET NO. 50-344 INTRODUCTION By letters dated October 25, 1985 and October Portland General Electric Company (PGE) submitted a "leak-before-break"31, 1986,(LBB) evaluation of the reactor coolant loop piping for the Trojan Nuclear Plant._ The licensee proposed to apply the LBB methodology to eliminate the dynamic effects of postulated primary loop pipe ruptures from the design basis for Trojan, as permitted by the revised General Design Criterion 4 (GDC-4) of Appendix A to 10 CFR 50.
PGE submitted the technical basis for the elimination of primary loop pipe ruptures for Trojan in Impe11 report 01-0300-1395, Revisions 0 and 1 (References 1 and E). By letter dated March 14, 1988, PGE submitted the final technical basis in Westinghouse report WCAP-11699 (Reference 3) in response to the staff's request for additional information. PGE also referenced Westinghouse reports WCAP-10456 (Reference 4) and WCAP-10931, Revision 1 (Reference 5), which have been reviewed previously by the staff as discussed in References 6 and 7, respectively.
The revised GDC-4 is based on the development of advanced fracture mechanics technology using the LBB concept. On October 27, 1987, a final rule was published (52FR41288),effectiveNovember 27, 1987, amending GDC-4 of Appendix A to TU CFR Part 50. The revised GDC-4 allows the use of analyses to eliminate from the design basis the dynamic effects of postulated pipe ruptures in high energy piping in nuclear power units. The new technology ,
reflects an engineering advance which allows simultaneously an increase in safety, reduced worker radiation exposures, and lower construction and 1 maintenance costs. Implementation permits the removal of pipe whip restraints and jet impingement barriers as well as other related changes in operating plants, plants under construction, and future plant designs.
Containment design and emergency core cooling requirements are not influenced by this modification. The acceptable technical procedures and criteria are defined in NUREG-1061, Volume 3 (Reference 8).
Using the criteria in Reference 8, the staff has reviewed and evaluated the licensee's submittals for compliance with the revised GDC-4. This Safety Evaluation provides the staff's findings.
8808160356 880805 PDR ADOCK 05000344 p PDC
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l TROJAN PRIMARY LOOP PIPING The Trojan primary loop piping consists of 34-inch, 36-inch, and 32-inch nominal diameter hot leg, cross-over leg, and cold leg, respectively. The piping material in the primary loops is cast stainless steel (SA-351 CF8A piping and SA-351 CF8M fittings). The piping is centrifugally cast and the fittings are statically cast. The welding processes used were sub-merged arc (SAW), shielded metal arc (SMAW), and gas tungsten arc (GTAW).
EVALUATION CRITERIA The staff's criteria for evaluation of compliance with the revised GDC-4 are discussed in Chapter 5.0 of Reference 8 and are as follows:
(1) The loading conditions should include the static forces and moments (pressure, deadweight, and thermal expansion) due to normal operation, and the forces and moments associated with the safe shutdown earthquake (SSE). These forces and moments should be located where the highest stresses, coincident with the poorest material properties, are induced for base materials, weldments, and safe ends.
(2) For the piping run/ systems under evaluation, all pertinent information which demonstrates that degradation or failure of the piping resulting from stress corrosion cracking, fatigue, or water hammer are not likely, should be provided. Relevant operating history should be cited, which includes system opera-tional procedures; system or component modification; water chemistry parameters, limits, and controls; and resistance of material to various forms of stress corrosion and performance under cyclic loadings.
(3) The materials data provided should include types of materials and materials specifications used for base metal, weldments, and safe erties including the fracture mechanics ends; parameterthe caterials prop (J) resistance (J-R) curve used in the "J-integral" analyses; and long-term effects such as thermal aging and other limitations to valid data (e.g., J maximum, and maximum crack growth).
(4) A through-wall flaw should be postulated at the highest stressed l locations determined from criterion (1) above. The size of the '
flaw should be large enough so that the leakage is assured of detection with at least a factor of 10 using the minimum installed ,
leak detection capability when the pipe is subjected to normal l operational loads. I (5) It should be demonstrated that the postulated leakage flaw is stable under normal plus SSE loads for long periods of time; that is, crack growth, if any, is minimal during an earthquake. The
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l margin, in terms of applied loads, should be at least 1.4 and I should be determined by a flaw stability analysis, i.e., that !
the leakage-size flaw will not experience unstable crack growth 1 even if larger loads (larger than design loads) are applied. 1 However, the final rule permits a reduction af the margin of ;
1.4 to 1.0 if the individual normal and seismic (pressure, l deadweight, thermal expansion, SSE, and seismic anchor motion) loads are summed absolutely. This analysis should demonstrate that crack growth is stable and the final flaw size is limited, such that a double-ended pipe break will not occur.
(6) The flaw size should be determined by comparing the leakage-size flaw to the critical-size flaw. Under normal plus SSE loads, it should be demonstrated that there is a margin of at least 2 between the leakage-size flaw and the critical-size flaw to account for the uncertainties inherent in the analyses and leakage detection capability. A limit-load analysis may suffice for this purpose; however, an elastic-plastic fracture mechanics (tearing instability) analysis is preferable.
EVALUATION We have evaluated the information presented in References 1 through 3 for compliance with the revised GDC-4. Furthermore, we performed independent an elastic-plastic fracture mechanics flaw stability procedure computations developed using(Reference 9).
by the staff On the basis of our review, we find the Trojan primary loop piping in compliance with the revised GDC-4. The following paragraphs in this section present our evaluation.
(1) Normal operating loads, including pressure, deadweight, and thermal expansion, were used to determine leak rate and leakage-size flaws. The flaw stability analyses performed to assess margins against pipe rupture at postulated faulted load conditions were based on normal plus SSE loads. In the stability analysis, two load combination methods were considered. The individual normal load components were summed algebraically and the seismic loads were then added absolutely. The individual normal and seismic loads were also sumed absolutely. In the leak rate analysis, the individual normal load components were sumed algebraically. Leak-before-break evaluations were . .
performed for the limiting location in the piping. I (2) For Westinghouse facilities, there is no history of cracking failure in reactor coolant system (RCS) primary loop piping.
The RCS primary loop has an operating history which demonstrates its ini4 rent stability. This includes a low susceptibility to .
j cracking failure from stress corrosion the effects cracking), water of corrosion hamer, (e.g.,(intergranular or fatigue low and '
high cycle). This operating history totals over 400 reactor-years l t
.' including 5 plants each having over 15 years of operation and 15 other plants each with over 10 years of operation.
(3) The material tensile and fracture toughness properties were provided in Reference 3. Because there are cast stainless steel piping (and fitting) and associated welds in the Trojan i
primary loop, the thermal aging toughness properties of cast stainless steel materials were estimated according to procedures in References 4 and 5. The material tensile properties were f estimated using generic procedures. For flaw stability evaluations, the lower-bound stress-strain properties were used. For leakage rate evaluations, the average stress-strain ;
properties were used. l l
{ (4) Trojan has RCS pressure boundary leak detection systems which l l are consistent with the guidelines of Regulatory Guide 1.45 l such that a leakage of one gallon per minute (gpm) can be l
detected. The calculated leak rate through the postulated I flaw is large relative to the staff's required sensitivity of l I the plant's leak detection systems; the margin is at least a l factor of 10 on leakage and is consistent with the guidelines I of Reference 8.
l l (5) In the flaw stability analyses, the staff evaluated the margin I in terms of load for the leakage-size flaw under normal plus i I
SSE loads according to the two load combination methods as )
l discussed in item (1) above. The staff's calculations indicated l
- the margin exceeded 1.4, when the individual normal load !
l components were summed algebraically and the seismic loads were then added absolutely. Similarly, the margin exceeded 1.0 when the individual normal and seismic loads were summed absolutely.
The margin is consistent with the guidelines of Reference 8 and the final rule. l
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(6) Similar to item (5) above, the margin between the leakage-size flaw and the critical-size flaw was also evaluated in the flaw stability analyses. The staff's calculations indicated the margin in terms of flaw size exceeded 2 for the two load combination methods considered. The margin is consistent with j j
the guidelines of Reference 8. '
CONCLUSIONS We have reviewed the subrnitted information and have performed independent flaw stability computations. On the basis of this review, we conclude that the Trojan primary loop piping cornplies with the revised GDC-4 according to the criteria in NUREG-1061, Volume 3 (Reference 8). Thus, the probability or likelihood of large pipe breaks occurring in the primary coolant system loops of Trojan is sufficiently low such that dynamic effects associated with postulated pipe breaks need not be a design basis.
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,o REFERENCES (1)ImpellReport 01-0300-1395, Revision 0, "Leak-Before-Break Evaluation of the Reactor Coolant Loop", June 1985.
(2) Impell Report 01-0300-1395, Revision 1, "Leak-Before-Break Evaluation of the Reactor Coolant Loop", October 1986.
(3) Westinghouse Report WCAP-11699, "Technical Justification for Eliminating large Primary Loop Pipe Rupture as the Structural Design Basis for the Trojan Plant", February 1988, Westinghouse Proprietary Class 2.
(4) Westinghouse Report WCAP-10456, "The Effects of Thermal Aging on the Structural Integrity of Cast Stainless Steel Piping for Westinghouse Nuclear Steam Supply Systems", November 1983, Westinghouse Proprietary Class 2.
(5) Westinghouse Report WCAP-10931, Revision 1, "Toughness Criteria for Thermally Aged Cast Stainless Steel", July 1986, Westinghouse Proprietary Class 2.
(6) Letter from B. J. Youngblood of NRC to M. D. Spence of Texas Utilities Generating Company dated August 28, 1984.
(7) Letter from D. C. Dilanni of NRC to D. M. Musolf of Northern States Power Company dated December 22, 1986.
(8) NUREG 10#1. Volume 3, "Report of the U. S. Nuclear Regulatory Cormission Piping Review Committee, Evaluation of Potential for Pipe Breaks",
November 1984.
(9) NUREG/CR-4572, "NRC Leak-Before-Break (LBB.NRC) Analysis Method for Circumferential1y Through-Wall Cracked Pipes Under Axial Plus Bending Loads", May 1986.
PRINCIPAL CONTRIBUTOR:
S. Lee 4
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