ML12082A036
| ML12082A036 | |
| Person / Time | |
|---|---|
| Site: | Boiling Water Reactor Owners Group |
| Issue date: | 04/04/2012 |
| From: | Joe Golla Licensing Processes Branch (DPR) |
| To: | Schiffley F Exelon Generation Co |
| Golla J | |
| References | |
| BWROG-TP-11-023, Rev 0, TAC ME7650 | |
| Download: ML12082A036 (7) | |
Text
April 4, 2012 Mr. Frederick Schiffley BWROG Chairman Exelon Generation Co., LLC Cornerstone II at Cantera 4300 Winfield Road Warrenville, IL 60555
SUBJECT:
REQUEST FOR ADDITIONAL INFORMATION RE: BOILING WATER REACTOR OWNERS GROUP LICENSING TOPICAL REPORT BWROG-TP-11-023, REVISION 0, LINEAR ELASTIC FRACTURE MECHANICS EVALUATION OF GENERAL ELECTRIC BOILING WATER REACTOR WATER LEVEL INSTRUMENT NOZZLES FOR PRESSURE-TEMPERATURE CURVE EVALUATIONS (TAC NO. ME7650)
Dear Mr. Schiffley:
By letter dated November 17, 2011, the Boiling Water Reactor Owners Group (BWROG) submitted for U.S. Nuclear Regulatory Commission (NRC) staff review licensing topical report (LTR) BWROG-TP-11-023, Revision 0, Linear Elastic Fracture Mechanics Evaluation of General Electric Boiling Water Reactor Water Level Instrument Nozzles for Pressure-Temperature Curve Evaluations. Upon review of the information provided, the NRC staff has determined that additional information is needed to complete the review. On January 23, 2012, Lucas Martins, Project Manager for the BWROG, and I agreed that the NRC staff will receive your response to the enclosed Request for Additional Information (RAI) questions by June 13, 2012. If you have any questions regarding the enclosed RAI questions, please contact me at 301-415-1002.
Sincerely,
/RA/
Joseph A. Golla, Project Manager Licensing Processes Branch Division of Policy and Rulemaking Office of Nuclear Reactor Regulation Project No. 691
Enclosure:
RAI questions cc w/encl: See next page
ML12082A036 NRR-106 OFFICE PLPB/PM PLPB/LA EVIB/BC PLPB/BC PLPB/PM NAME JGolla DBaxely SRosenberg JJolicoeur JGolla DATE 3/23/2012 3/22/2012 4/3/2012 4/3/2012 4/4/2012 REQUEST FOR ADDITIONAL INFORMATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION LICENSING TOPICAL REPORT BWROG-TP-11-023, REVISION 0 LINEAR ELASTIC FRACTURE MECHANICS EVALUATION OF GENERAL ELECTRIC BOILING WATER REACTOR WATER LEVEL INSTRUMENT NOZZLES FOR PRESSURE-TEMPERATURE CURVE EVALUATIONS BOILING WATER REACTORS OWNERS GROUP PROJECT NO. 691
RAI-1
Section 2.2 of the Licensing Topical Report (LTR) BWROG-TP-11-023, Revision 0, discussed model geometry, materials, and heat transfer coefficients used in the stress analyses. The U.S.
Nuclear Regulatory Commission (NRC) staff has the following questions:
It is stated that, [s]ince there is no specific material specification identified for the CS
[carbon steel] nozzle inserts from Table 2-1 of Reference [13], SA-541 Class 1 is assumed. Depending on the material properties used, this assumption could change the calculated stresses significantly. Please estimate the ranges of key material properties for CS nozzle inserts and assess the impact on the LTR results and conclusion, considering that each property could be at either end of its range.
It is stated that, [t]he weld material is always assumed as Alloy 600 for the different nozzle material cases. Justify use of Alloy 600 for the variety of instrument nozzles covered by this LTR and assess the impact of this assumption on the results summarized in Figures 6-4 and 6-5 and on the conclusion presented in Section 9.0 of this LTR due to the fact that the real weld material may not be Alloy 600 material.
In the thermal analysis, the heat transfer coefficient is assumed as 500 Btu/hr-ft2-°F for the inside surface of the vessel and nozzle and 0.2 Btu/hr-ft2-°F for the outside surface of the vessel and nozzle. Please justify the use of these coefficients in this application and cite NRC safety evaluations accepting use of these coefficients in similar applications.
ENCLOSURE
RAI-2
It is stated in Section 2.3.3 of the LTR that, [t]herefore, pipe reaction loads are evaluated in order to assess the effect of piping loads on the stress intensity factor [KI] calculated at the instrument nozzle. In the finite element method (FEM) model such as that in Figure 4-1, only the reactor pressure vessel (RPV) and the instrument nozzle are modeled. Please confirm that the piping loads are from the piping attached to the safe-end of the instrument nozzle only and not from the various large piping (not shown in Figure 4-1) attached to the RPV.
RAI-3
It is stated in Section 2.4 of the LTR that, [a] stress path, with the orientation shown in Figure 2-1, is chosen for extracting hoop stress results from the FEA [finite element analysis].
Please demonstrate that selecting the 45 ° line as the stress path to represent the opening stresses of the entire postulated crack face in the FEA and then using the 45 ° line stresses as input to the subsequent boundary integral equation/influence function (BIE/IF) solution can produce reliable, conservative results.
RAI-4
It is stated in Section 3.0 of the LTR regarding Assumption 3 that, [t]herefore, applying the weaker base metal properties instead of weld material properties is typically considered conservative. Please note that by doing so, the thermal properties of the base metal might also be used in the analyses instead of the weld material thermal properties. If this is the case, please assess the impact of the difference in thermal properties on the thermal analysis results under the thermal transient shown in Figure 2-3, noting, for instance, that Table 2-3 for the base metal and table 2-5 for the assumed weld material indicate a 3 to 1 difference in their thermal conductivities.
RAI-5
It is stated in Section 4.2 that, [t]he nodes on the free end of the safe end are coupled in the nozzle axial direction to ensure equal axial displacement of the end of the nozzle and RPV in response to the membrane load so as to simulate the effects of the attached piping and closed end of the RPV. This requires clarification. Usually, the moments and the forces from the existing piping system analysis are applied to the free end of the nozzle safe end in the FEM analysis, in addition to the membrane loads mentioned in the quote. Is the purpose of coupling to make the nozzle free end nodes move as a plane? Elaborate on why coupling the nodes on the free end of the safe end in the nozzle axial direction best simulates the real situation.
RAI-6
It is stated in Section 6.1 that, Figure 6-2 illustrates the circumferential stress distribution for one of the un-cracked models for the pressure load case. Since the FEM model represents a quarter of the RPV and the instrument nozzle assembly, the radial crack can be placed at any location along the 90 ° circumference of the nozzle attached end, depending on the stress values.
Under the stress distribution of Figure 6-2, what is the crack plane orientation (e.g., defined by degrees from the edge of the 90 ° FEM model) which would give the highest KIs? Discuss the effort spent in finding this critical crack plane orientation, considering that the critical crack plane orientation for the pressure load case may not be the critical crack plane orientation for the thermal load case and the critical crack plane orientations may not be the same for the 38 FEM models studied in the LTR.
RAI-7
It is stated in Section 6.1 that, the stress solution exhibits a large elastic pseudo-stress adjacent to the geometric discontinuity. This pseudo-stress may be ignored since the real structure would exhibit local yielding Please clarify that (1) no adjustment of stresses is made as long as the pseudo-stresses are below the yield strength of the material and (2) ignoring the pseudo-stress means adjusting the pseudo-stress to the yield strength of the material.
RAI-8
It is stated in Section 6.2 that, Paths are defined for a case where the RPV clad is excluded in the path definition and for a case where the path includes the RPV clad. Figure 6-7 shows these two paths.
Please confirm that in both cases, the FEM model has considered RPV cladding, and the paths shown in Figure 6-7 simply represent two different paths for extracting stresses.
It appears that the Path excluding clad shown in Figure 6-7 will pass the gap between the RPV bore and the nozzle, resulting in hoop stress discontinuity. Please confirm that this is the case in your analysis.
It appears that the Path including clad shown in Figure 6-7 actually did not pass RPV cladding. Instead, it passes the nozzle inner end. Please confirm.
RAI-9
The lower part of Figure 6-9 shows a plot of thermal ramp KI for the finite element-linear elastic fracture mechanics (FE-LEFM) and the BIE/IF analyses versus the crack front location. Please explain why in the thermal case, the KI value increases rapidly when the crack front moves from 25 ° to 90 ° while the KI values are about the same at these two locations in the pressure case (see Figure 6-5).
RAI-10
It is stated in Section 7.0 that, [t]he BIE/IF solution is shown to not bound the maximum KI along the crack front obtained from the FE LEFM analysis. For pressure-temperature (P-T) curve analysis, a conservative 1/4 thickness flaw is assumed; a real flaw does not exist.
Consequently, the inherent conservatism in assuming a 1/4 thickness flaw is considered sufficient such that requiring use of the maximum KI along the entire crack front is considered to be excessively conservative.
Since a real 1/4 thickness flaw does not exist for almost all plants, applying the P-T limits methodology based on this assumption is equivalent to upholding the same level of margin of fracture toughness versus the applied KI for all beltline materials, including the instrument nozzle material, of all plants. Using the maximum applied KI along the crack front in the P-T limit analysis for the instrument nozzle is technically correct and maintains the same margin for the RPV materials of all plants. Please assess the practicality of applying a factor of 1.1 to the applied pressure KI values based on the BIE/IF and 1.4 to the applied thermal KI values based on the BIE/IF to bound the applied KI values and address (1) whether these factors will always make the instrument nozzle limiting for the P-T limit curves and (2) whether plant operation will be severely limited if the instrument nozzle becomes limiting.
RAI-11
Section 8.0 proposed generic equations for estimating applied KI due to pressure (Equation 8-1) and applied KI due to 100 °F/hr thermal ramp load (Equation 8-2). These equations are based on best-estimate linear fit of calculated results for a variety of instrument nozzles. Using the best-estimate fit will allow approximately half of the number of instrument nozzles having safety margins less than that required by Title 10 of the Code of Federal Regulations Part 50 Appendix G. The bounding line, instead of the best-estimate line, may be more appropriate.
Therefore, another factor of 1.1 may be needed. After combining this factor with the factors mentioned in RAI-10 to account for the maximum applied KI along the crack front, we have a combined factor of 1.5 for the applied pressure KI values based on the BIE/IF and 1.2 to the applied thermal KI values based on the BIE/IF. Please address (1) whether these combined factors will always make the instrument nozzle limiting for the P-T limit curves and (2) whether these combined factors will alter your response to RAI-10 regarding plant operation due to instrument nozzle being limiting.
cc:
BWROG Chairman Frederick P. Schiffley Exelon Generation Co., LLC Cornerstone II at Cantera 4300 Winfield Road Warrenville, IL 60555 frederick.schiffley@exeloncorp.com BWROG Project Manager Lucas Martins GE-Hitachi Nuclear Energy PO Box 780 M/C F-12 3901 Castle Hayne Road Wilmington, NC 28402 Lucas.martins@ge.com BWROG Program Manager Craig J. Nichols GE-Hitachi Nuclear Energy PO Box 780 M/C F-12 3901 Castle Hayne Road Wilmington, NC 28402 craig.nichols@ge.com GEH Senior Vice President Jerald G. Head Senior Vice President, Regulatory Affairs GE-Hitachi Nuclear Energy PO Box 780 M/C A-18 Wilmington, NC 28401 jerald.head@ge.com