ML20078M720
| ML20078M720 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 11/23/1994 |
| From: | Schnell D UNION ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| ULNRC-3101, NUDOCS 9412050072 | |
| Download: ML20078M720 (11) | |
Text
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'53 November 23, 1994 U.S. Nuclear Regulatory Commission Attn:
Document Control Desk Mail Station P1-137 Washington, DC 20555 Gentlemen:
ULNRC-3101 1
SOCKET NUMBER 50-483 CALLAWAY PLANT 10CFR50.46 THIRTY DAY REPORT-ECCS EVALUATION MODEL REVISIONS
References:
1)
ULNRC-2141 dated 1-19-90 2)
ULNRC-2373 dated 2-28-91 3)
ULNRC-2439 dated 7-19-91 4)
ULNRC-2664 dated 7-16-92 5)
ULNRC-2822 dated 7-15-93 6)
ULNRC-2892 dated 10-22-93 7)
ULNRC-3087 dated 10-19-94 to this letter describes changes to the Westinghouse SBLOCA ECCS Evaluation Model which have been implemented for-Callaway s.uce the last report (Reference 7). provides an ECCS Evaluation Model Margin Assessment which accounts for j
the peak cladding temperature (PCT)
.anges resulting from the resolution of the issues'd, aribed in
- as they apply to Callaway.
References 1-7 above transmitted prior 10CFR50.46 reports; Reference 7 i
noted that this 30-day report would follow.
i
! describes the resolution of those issues which have been implemented for Callaway.
The margin allocations for Callaway to date are identified in Attachment 2.
The large break LOCA table included in Attachment 2 remains unchanged f r sn that submitted in Reference 7 and is enclosed here for completeness'only.
However, based on the criteria and i
reporting requirements of 10CFR50.46 (a) (3) (ii), as clarified in Section 5.1 of WCAP-13451,1the cumulative changes since the last 30-day report, Reference 6, are l
significant for small break LOCA and require a 30-day report.
Since the limiting Callaway small break LOCA k
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U.S. Nuclear Regulatory Commission Page 2 (4-inch break) clad heatup transient has been reanalyzed with the revised SBLOCTA code and methodology, in conjunction with the NOTRUMP thermal-hydraulics analysis of record, no further reanalysis is planned by Union Electric.
This position on reanalysis is also supported by the large margin to the 2200 F regulatory limit.
1 Should you have any questions regarding this letter, please contact us.
very truly yo rs,
}f<
Donald F. Schnell GGY/jdg Attachments i
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T. A.
Baxter, Esq.
Shaw, Pittman, Potts & Trowbridge 2300 N.
- Street, N.W.
Washington, D.C.
20037 M.
H.
Fletcher Professional Nuclear Consulting, Inc.
j 18225-A Flower Hill Way l
Gaithersburg, MD 20879-5334 L.
Robert Greger Chief, Reactor Project Branch 1 U.S.
Nuclear Regulatory Commission Region III 801 Warrenville Road Lisle, IL 60532-4351 Bruce Bartlett callaway Resident Office U.S. Regulatory Commission RR#1 Steedman, MO 65077 L. R. Wharton (2)
Office of Nuclear Reactor Regulation U.S.
Nuclear Regulatory Commission 1 White Flint, North, Mail Stop 13E21 11555 Rockville Pike Rockville, MD 20852 Manager, Electric Department Missouri Public Service Commission P.O.
Box 360 Jefferson City, MO 65102
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ULNRC. 3101 NITACHMENT ONE CHANGES TO THE WESTINGHOUSE ECCS EVALUATION MODELS l
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llACKGROUND 1
10CFR50.46, Appendix K prescribes the acceptable features and required documentation for ECCS Evaluation Models. More specifically, Section 11.3 requires that documentation be in place to verify that sensitivity studies have demonstrated the adequacy of nodalization schemes used in the analysis models. A study was recently undertaken with the Westinghouse small break LOCA Evaluation Model to examine the sensitivity of predicted results to the nodr.lization used for the hot rod model. The results of that study raised concems regarding the adequacy of the standard axial nodalization prescribed for use in the SBLOCTA code for licensing basis analyses. As a result of this concern, Westinghouse investigated this as a Potential Issue per 10 CFR 21.
ISHILE DESCRIPTION The standard rod model (developed in the 1970's) used in perfonning SBLOCTA calculations has 19 axial nodes with a finer distribution in the top elevations.
However, sensitivity studies to justify the number and distribution of these nodes cannot be documented. A series of calculations was perfonned using increasingly finer axial nodalizations than prescribed for the 19 node model and indicated that the standard SBLOCTA 19 node model was not conservative. Nearly all cases demonstrated a significantly non-conservative behavior with respect to PCT. The penalty is attributed to a net increase in single-phase steam enthalpy rise as these nodes uncover sooner and heat up more than coarser nodes panially covered by the mixture 1
level. Thus, it was concluded that a revised model that included a much finer axial nodalization could potentially lead to less favorable results than those predicted in the current analyses, possibly challenging the 10 CFR 50.46 acceptance criteria.
As a result of further investigation into the SBLOCTA code, several additional related issues associated with nodalization and the overall solution of the fluid conservation j
equations were subsequently identified and corrected. As a separate, but related, issue Westinghouse has implemented a revised model for calculating transient fuel rod internal pressure in the SBLOCTA code. Fuel rod pressure is a governing factor in defining the clad creep, burst, and blockage behavior for small break LOCA transients.
i The NRC was infonned of this modeling change per Westinghouse letter NTD-NRC-94-4253, " Revision to the Rod Internal Pressure Model in the Westinghouse SBLOCTA Code (Proprietary)". The letter also infonned the NRC that Westinghouse has validated and instituted the model as a methodology improvement to the small break LOCA model for standard implementation on a forward-fit basis in accordance with WCAP-13451, " Westinghouse Methodology for Implementation of 10 CFR 50.46 Reponing," October,1992.
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TECIINICAL EVALUATION At this time Westinghouse has completed the generic technical evaluation of the fuel rod axial nodalization methodology. A revised standard for rod nodalization has been established which insures an adequate solution to the hot channel calculation by specifying a fine nodalization of 0.25 ft nodes for all elevations that are predicted to uncover during the transient.
Since the improved axial nodalization methodology and revised fuel rod internal pressure model can have significant synergistic effects on the predicted peak clad temperature, the SBLOCTA calculation from the limiting small break LOCA transient has been rerun with the revised code and methodology in order to obtain an accurate estimation of the net effect of these changes on the analysis of record. Several recent code revisions and error corrections of lesser magnitude have also been incorporated in the code version used to conduct this calculation. Those items have t een discussed in prior 10CFR50.46 repons. As a consequence of using the revised code to obtain results for this reanalysis, those items have been addressed in the msults provided (as discussed in Attachment 2). Since this portion of the ECCS Small Break Evaluation Model has already been reanalyzed, Westinghouse believes that no additional reanalysis is necessary to satisfy 10CFR50.46 for those plants that have a significant PCT change as a result of this issue.
Since all of the issues relate to ponions of the SBLOCTA code and/or its associated input methodology, they may be reported as a single, closely-related gmup of changes. presents a revised small break LOCA ECCS Evaluation Model Margin Assessment which contains a compilation of the net effects on PCT.
POWER MARGIN UTILIZATION During the process of reviewing the analysis of record for Callaway as part of addressing the atmve issues, conservatism was noted in the core power axial offset limit assumed in the analysis. The current licensing basis analyses restrict the axial offset to a maximum positive skew of 13% at full power. Additional conservatism had been incorporated into the small break LOCA analysis to provide margin above and beyond the present core design limits (a positive skew of 30% was used in the analysis).
Following consultation with cognizant core design and utility personnel, it was concluded that this margin is not being utilized and could be made available to offset the penalty associated with resolution of the present issues. The revised calculation was therefore performed with an axial offset limit of 20% which supports the same RSAC core design limits as previously supponed by the analysis of record, and therefore there are no changes to plant Technical Specifications from incorporating this revision.
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POWER MARGIN REVISION During the process of reviewing the analysis of record for Callaway as pan of addressing the above issues, conservatism was noted in the core power methodology used for the analysis. The current analysis of record was perfonned with an older methodology which was replaced in 1990. Excessive conservatism is known to exist in the old methodology and, as pan of the recalculation, the core power input was updated to correspond to current standards. The revised calculation conforms to the same RSAC core design limits as previously supponed by the analysis of record, and therefore there are no changes to the plant Technical Specifications from incorporating this revision.
11ESULTS As a result of the SBLOCTA axial nodalization error, the limiting Callaway small break LOCA clad heatup tmnsient calculation was performed. The NOTRUMP thennal-hydraulics analysis from the current analysis of record was used. In addition, the overly conservative axial offset of +30% typically assumed foi older analyses was reduced to +20%. This assumption remains conservative and increases PCT margin.
The renodalization coupled with revised modeling in SBLOCTA resulted in a 278 F benefit.
Because the SBLOCTA calculation was perfonned, it was possible to remove prior penalties previously reponed in Reference 3. These were 37 F for " Fuel Rod Initial Conditions Inconsistencies" and 20"F for "SBLOCA Rod Internal Pressure Assumption" As such, the 1991 LOCA Model Assessments have been reduced from 57 toO F.
Finally, the 15 F " Burst and Blockage / Time in Life" penalty reponed in Reference 7 does not currently apply since the SBLOCTA reanalysis has reduced the PCT to a value N10w the 1700 F threshold for burst.
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ULNRC-3101 ATTACHMENT TWO ECCS EVALUATION MODEL MARGIN ASSESSMENT FOR CALLAWAY t
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2-LARGE BREAK LOCA A.
ANALYSIS OF RECORD PCT = 2014 F B.
1989 LOCA MODEL ASSESSMENTS
+ 10 F (refer to ULNRC-2141 dated 1-19-90) j I
l C.
1990 LOCA MODEL ASSESSMENTS
+0F
]
l (refer to ULNRC-2373 dated 2-28-91) i D.
1991 LOCA MODEL ASSESSMENTS
+ 10 F j
(refer to ULNRC-2439 dated 7-19-91)
E.
1992 LOCA MODEL ASSESSMENTS, MARGIN
+ 29 F ALLOCATIONS, AND SAFETY EVALUATIONS
)
(refer to ULNRC-2664 dated 7-16-92 and ULNRC-28% Jated 10-22-93)
F.
1993 LOCA MODEL ASSESSMENTS 65 F (refer to ULNRC-2822 dated 7-15-93 and ULNRC-2892 dated 10-22-93) l G.
1994 LOCA MODEL ASSESSMENTS -
6F (refer to ULNRC-3087 dated 10-19-94) j i
IL POWER SIIAPE SENSITIVITY MODEL
+0F (PSSM)
(refer to Item 5 of Attachment I to ULNRC-2822 dated 7-15-93)
LICENSING BASIS PCT + MARGIN ALLOCATIO'NS 1992 F
=
ABSOLUTE MAGNITUDE OF MARGIN ALLOCATIONS SINCE LAST 30-DAY REPORT (ULN'RC-2892) 6F
=
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SMALL BREAK LOCA A.
ANALYSIS OF RECORD PCT 1528 F
=
B.
1989 LOCA MODEL ASSESSMENTS
+ 229 F (refer to ULNRC-2141 dated 1-19-90)
C.
1990 LOCA MODEL ASSESSMENTS
+0F (refer to ULNRC-2373 dated 2 28-91)
D.
1991 LOCA MODEL ASSESSMENTS
+0FI (refer to ULNRC-2439 dated 7-19-91)
E.
1992 LOCA MODEL ASSESSMENTS AND
+0F SAFETY EVALUATIONS (refer to ULNRC-2664 dated 7-16-92)
F.
1993 LOCA MODEL ASSESSMENTS 13 F2 (refer to ULNRC-2892 dated 10-22-93)
G.
1993 SAFETY EVALUATIONS
+4F3 (refer to ULNRC-2822 dated 7-15-93)
II.
BURST AND BLOCKAGE /TIMEIN LIFE
+0Fl (This PCT assessment is tmcked separately since it will change depending on future margin allocations.)
I.
1994 LOCA MODEL ASSESSMENTS
- 4F (refer to ULNRC-3087 dated 10-19-94) 3.
CURRENT LOCA MODEL ASSESSMENTS -
NOVEMBER 1994
- 1. AXIAL NODALIZATION, RIP MODEL 278 F4 REVISION, SBLOCTA ERROR CORRECTIONS ANALYSIS (see Attachment 1)
LICENSING BASIS PCT + MARGIN ALLOCATIONS 1466 F
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NOTES:
1.
See Attachment 1. The 1991 assessments have been eliminated as a result of the new SBLOCTA calculation. The Small Break Burst and Blockage penalty is a function of the base PCT plus margin allocations and has been reduced to 0 F since the total PCT has been reduced to a value below that at which burst would occur.
2.
At the January 12, 1994 meeting between Westinghouse and the NRC, Westinghouse agreed to provide to the NRC an addendum to WCAP-10054-P-A describing the SI model used in NOTRUMP including SI to the broken loop.
Addendum 2 to WCAP-10054 has been submitted to NRC. It references the improved condensation model (COSI) described in WCAP-11767 and provides justification for application of this model to small break LOCA calculations. In the interim, Union Electric will track the Peak Cladding Temperature (PCT) change reported in ULNRC-2892 (+150 F/-150 F) as a permanent change to Callaway's calculated PCT.
3.
The +4.0 F Cycle 6 CRUD Deposition penalty will be carried until such time as it is evaluated to no longer apply.
i 4.
Based on the limiting case clad heatup transient reanalysis with axial offset reduced from 30% to 20%.
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