ML20059G993
| ML20059G993 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 11/03/1993 |
| From: | Link B WISCONSIN ELECTRIC POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| CON-NRC-93-117 VPNPD-93-188, NUDOCS 9311090234 | |
| Download: ML20059G993 (7) | |
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Wisconsin Electnc POWER COMPANY 231 W Mchgon. PO Box 2046 Miwohe. VA 53201-2046 (414)221 2345 VPNPD-93 188 NRC 117 November 3, 1993 Document Control Desk U.
S.
NUCLEAR REGULATORY COMMISSION Mail Station P1-137 Washington, DC 20555 Gentlemen:
DOCKET 50-266 AND 50-301 ECCS EVALUATION MODEL CHANGES, 10 CFR 50.46 POINT BEACH NUCLEAR PLANT. UNITS 1 AND 2 As required by Title 10 of the Code of Federal Regulations Part 50.46(a) (3) (ii), Wisconsin Electric Power Company (Licensee) is submitting this report of changes to, and errors discovered in, emergency core cooling system (ECCS) evaluation models for Point Beach Nuclear Plant, Units 1 and 2.
This letter provides a summary of ECCS evaluation model changes and errors identified since the last report dated February 23, 1993.
Model changes include changes to the small break loss of coolant accident (LOCA) model considered to be reportable under the 30 day reporting requirement in 10 CFR
- 50. 46 (a) (3) (ii).
A summary of the changes is provided below with additional details and peak cladding temperature (PCT) margin summary sheets in the attachment.
Small Break LOCA Evaluation Model The effect of safety injection (SI) in the broken loop of the reactor coolant system (RCS) during a small break LOCA modeled by NOTRUMP has been assessed a PCT penalty of 150*F.
Broken loop SI spilling to containment is an assumption historically judged to be conservative due to the loss of SI inventory.
Delivery of SI to the cold leg has been found to be a penalty in the NOTRUMP evaluation model because competition between steam leaving and SI entering the break increases the RCS pressure.
Higher RCS pressure reduces the amount of SI delivered by the centrifugal SI pumps which increases the PCT.
The Westinghouse Owners Group is reviewing this issue and possible development of a generic program for resolution.
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The effect of an improved condensation model in NOTRUMP has been assessed a PCT benefit of 150*F.
Condensation reduces the RCS l
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4 Document Control Desk November 3, 1993 Page 2 pressure and increases the amount of SI delivered by the centrifugal SI pumps.
A comparison with test data shows that the PCT benefit from the improved condensation model more than offsets the PCT penalty due to SI in the broken loop.
The Westinghouse owners Group is reviewing this issue and possible development of a generic program for resolution.
Drift flux flow regime errors in NOTRUMP have been assessed a PCT benefit of 13*F.
Errors in the coding of the drift flux flow regime map caused discontinuities in the map under certain circumstances.
Although the change is not considered to be significant (<50*F) it is being reported at this time for completeness.
Large Break LOCA Evaluation Model No changes have been made to, and no errors have been discovered in, the large break LOCA evaluation model since the last report.
Please contact us if you have any questions about this information.
Sincerely,
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Bob Link Vice President Nuclear Power Attachment cc:
NRC Resident Inspector NRC Regional Administrator Subscribed and sworn to before me this 3i day of ther ber 1993.
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ty, ruter No ary Public, State of Wisconsin j
JMy Commission expires
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SI IN BROKEN LOOP AND IMPROVED CONDENSATION MODEL ISSUE DESCRIPTION Westinghouse emergency core cooling system (ECCS) evaluation models are considered to be composed of several features which include underlying assumptions. Westinghouse recently completed an evaluation of a potential issue concerning the modeling of Safety Injection (SI) flow into the broken RCS loop for small break loss of coolant accident (SBLOCA).
Westinghouse previously assumed that SI to the broken RCS loop would result in a lower calculated PCT and, therefore, modeled the ECCS broken loop branch line to spill the SI to the contamment sump. The basis for this assumption included consideration for the effect of back pressure on the spilling ECCS line for cold leg br-As, which would see a higher back pressure for SI connected to the broken RCS loop when compared to spilling against contamment back pressure. Spilling to the higher RCS pressure would increase SI to the intact loops, which is a benefit for PCT. The effect on intact loop SI flow rates as well as the assumption that some of the SI to the broken loop would aid in RCS/ Core recovery resulted in the Westinghouse ECCS model assumption that SI to the broken loop was a benefit. However, when SI is modeled to enter into the broken loop, a significant PCT penalty is calculated by the NOTRUMP small break evaluation model (approximately 150 degrees F for a typical Westinghouse 3-loop design).
TECIDTICAL EVALUATION An analysis by Westinghouse indicates that the penalty (as described above) occurs as a result of competition between the steam venting out the break and the SI to the broken loop, which also exits through tne break. The competition between the steam and the SI results in higher RCS pressures for the identical core steammg rates. Since the ECCS uses centrifugal pumps, higher RCS pressure results in lower delivered SI flow rates to the intact RCS loops, leading to the calculated PCT penalty. This penalty is somewhat aggravated by the use of the Moody two-phase break flow model, which is a thermal equilibrium model being used to model a clearly nonequilibrium process. However, the penalty is large enough such that a change to a nonequilibrium break flow model would not be expected to offset the break flow RCS pressure interaction seen when SI is assumed to enter into the broken loop.
However, when a newer conservative model based on prototypic test is used which modeled the 1
configuration of the SI piping to the RCS cold leg in a Westinghouse designed PWR, a net PCT benefit is calculated. Improved condensation of the loop steam in the intact loops results in lower RCS pressure and larger SI flow rates. The increase in SI flow rates, due to Icwer RCS pressure, leads to the lower calculated PCT. Thus, the negative effects of SI into the broken loop can be offset by an improved SI condensation model in the intact RCS loops.
The improved condensation model is based on data obtained from the COSI test facility. The COSI test facility is a 1/100 scale representation of the cold leg and SI injection ports in a Westinghouse designed PWR, 'a ce COSI tests demonstrated that the current NOTRUMP condensation model under-predicted condensation in the intact loops during SI and thus is a conservative model. Use of the improved condensation model has demonstrated that the current NOTRUMP small break LOCA analyses without the improved condensation model and no SI into the broken loop is more conservative (higher calculated PCT) than a case which includes SI into the broken loop and the improved condensation model.
n Additionally, the effects of SI in the broken loop have been determir,ed to not change RCP trip symptoms developed in response to US-NRC Generic Letters83-10C and 8512 or SI termination criteria found in the Westinghouse Owners Group Emergency Response GuideGes.
MSESDfEST OF SAFETY SIGNTFICANCE The COSI tests demonstrated that the current NOTRUMP condensation model under-predicted condensation in the intact loops during SI and thus is a conservative model. Furthermore, recent evaluations have shown that the current NOTRUMP small break LOCA analyses without the improved condensation model and no SI inv;-J hroken loop is more conservative (higher calculated PCT) than a case which includes SI into the bc4 s o and the improved condensation model. Based on these evaluations, Westinghouse determineo
. this issue does not involve a Substantial Safety Hazard as defined in 10 CFR Part 21. Reanalyses are not necessary since current NOTRUMP based small break LOCA analyses have a conservatively calculated PCT and, therefore, remain valid.
NOTRUMP DRIFT FLUX FLOW REGIME MAP ERRORS Backcround Errors were discovered in both WCAP-10079-P-A and related coding in NOTRUMP SUBROUTINE l
DFCORRS where the improved TRAC-P1 vertical flow regime map is evaluated. In Evaluation Model i
applications, this model is only used during counter-current flow conditions in vertical flow links. The affected equation in WCAP-10079-P-A is Equation G-65 which previously allowed for unbounded values of the parameter C contrary to the intent of the original source of this equation. This allowed a discontinuity to exist in the flow regime map under some circumstances. This was corrected by placing 1
an upper limit of 1.3926 on the parameter C., as reasoned from the discussion in the original source.
As stated, this correction returned NOTRUMP to consistency with the original source for the affected equation.
i Further investigation of the DFCORRS uncovered an additional closely related logic error which led to discontinuities under certain other circumstances. This error was also corrected and returned the coding to consistency with WCAP-10079-P-A.
This was determined to be a Non-discretionary Change as described in Section 4.1.2 of WCAP-13451 and was corrected in accordance with Section 4.1.3 of WCAP-13451.
Affected Evaluation Models 1985 Small Break LOCA Evaluation Model Eltimated Effect
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Representative plant calculations indicated PCT effects ranging from -13 degrees to -55 degrees. For the purposes of tracking PCT, the minimum benefit of -13 degrees has been assigned to these changes.
When considering reportability under 10 CFR 50.46(a)(3)(i), however, it has been demonstrated that the effect of these changes may exceed 50 degrees F. Westinghouse, therefore, recommends that these changes be considered significant with respect to 10 CFR 50.46(a)(3)(i) requirements.
1 l
Small BIrak Peak Clad Tempsrature Malcin Utilization Revision Date: 8/93 l
Plant Name: Point Beach Units 1 and 2 Eval.Model: NOTRUMP-Fuel:14x14 OFA Utility Name: Wisconsin Electric Power Company FQ=2.5 FAH = 1.7 SGTP=25%
Clad Temperature A. ANALYSIS OF RECORD (7/88)
PCT =
809 'F j
1 B., PRIOR PERMANENT ECCS MODEL ASSESSMENTS APCT=
37'F C.10 CFR 50.59 SAFETY EVALUATIONS APCT=
25 'F D.199310 CFR 50.46 MODEL ASSESSMENTS APCT=
0 'F (Permanent Assessment of PCTMargin)
.i
- 1. Effectof Siin Broken Loop APCT=
150 'F l
- 2. Effect of Improved Condensation Model APCT= - 150 'F
- 3. Drift Flux Flow Regime Errors APCT=
- 13 'F i
E. TEMPORARY ECCS MODEL ISSUES
- 1. None APCT=
0 *F F. OTHER MARGIN ALLOCATIONS
- 1. None APCT=
0 'F LICENSING BASIS PCT + MARGIN ALLOCATIONS PCr=
858 'F i
l
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' Large Break Peak Clad Temperature Martin Utilization Revision Date: 8/93 Plant Name: Point Beach Units 1 and 2 Eval.Model: WCff Fuel: 14x14 OFA Utility Name: Wisconsin Electric Power Company FO= 2.5 FAH= 1.7 SGTP=25%
-i Clad Temperature Notes A. ANALYSIS OF RECORD (2/91)
PCT =
2028 *F B PRIOR PERMANENT ECCS MODEL ASSESSMENTS APCT=
-76 *F 1
C.10 CFR 50.59 SAFETY EVALUATIONS APCT=
0 ";
D.199310 CFR 50.46 MODEL ASSESSMENTS (Permanent Assessment of PCTMargin)
- 1. None APCT=
0 *F E. TEMPORARY ECCS MODELISSUES
- 1. Nane APCT=
0 *F F. OTHER MARGIN ALLOCATIONS
- 1. None APCT=
0 *F LICENSING BASIS PCT + MARGIN ALLOCATIONS PCT =
1952 *F Notes:
- 1. No ECCS Analysis PCTmargin is allocated for, but 5% of the ECCS Analysis SGTP margin has been allocated to offset SG Tube / Seismic etfects.
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