ML20050C714

Bounding Analysis Impact Study of NUREG-0630.

ML20050C714
Person / Time
Site:	Midland
Issue date:	04/02/1982
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML20050C713	List: ML20050C712 ML20050C714
References
RTR-NUREG-0630, RTR-NUREG-630 12-11-32424, NUDOCS 8204090281
Download: ML20050C714 (20)

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I BOUNDING ANALYSIS IMPACT STUDY OF NUREG-0630 B&W DOCll4ENT NO.12-1132424, Rev. O I

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1. INTRODUCTION During a postulated large break loss-of-coolant accident (LOCA), the reactor system pressure will drop below the fuel rod internal gas j pressure, which may cause the fuel cladding to swell and rupture. Core behavior af ter rupture will depend on the type of accident, time of rupture, and the resulting flow blockage due to flaring of the cladding.

In particular,10 CFR 50.46 requires that peak cladding temperatures do .

not exceed 2200*F, and that models used to predict degree of swelling and incidence of rupture are conservative. NUREG-0630 (reference 1) establishes correlations for cladding rupture temperature, cladding burst strain, and fuel assembly flow blockage which differ from present B&W model s.

L-This study was undertaken to determine the impact of NUREG-0630 in a worst case analyses for the 177 FA lowered-loop plants at a power level of 2772 MWt. The results can be generally applied to the 177 FA raised-loop design as well as the 205 FA design.

II. CONCLUSIONS Using a bounding analysis impact study of HUREG-0630 results in a projected impact of -0.5 KW/f t on the allowable LOCA limits. This analysis was performed on the 177 FA lowered-loop NSS, but the results are considered to be generally applicable to all B&W NSS. The bounding analysis was done for the most limiting large break, (8.55 f t2 at the PD, CD = 1.0). The LOCA limit was evaluated at the 2 foot core elevation, since previous experience has demonstrated this core elevation to be the most sensitive with respect to the cladding swelling and rupture model phenomena. Other core elevations will experience less inpact.

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I The above analysis was performed with a CRAFT 2 input power peak of 14.5 KW/ft which is the present technical specification LOCA limit for the first 50 EFPD. The THETA 1-B analysis was performed at 13.5 KW/f t and resul ted in a peak cladding emperature of 1734*F. This procedure has been shown to be conservative. A more refined analysis would show acceptable results of about 14.0 KW/f t (-0.5 KW/f t impact) .

III. METHODS A. Base Case The Lase case analysis for this impact study sas an 8.55 f t2 double ended break at the RCP discharge with a CD of 1.0. This break size and location was shown in reference 4 to be limiting. The mast limiting core elevation for inpact from MREG-0630 is at 2 feet. The 2 foot elevation is the most rupture node limited and the NUREG-0630

-I impact is on the ruptured de. Previous studies (reference 3) have also shown the maximum impact of TACO 2 to be at the 2 f oot elevation.

I Therefore, the analysis at the 2 foot elevation with TACO 2 was chosen as the base case for this impact study. The codes used were CRAFT 2, REFLOD 3, THETA 1-B, and TAC 0 2 fuel data. A CRAFT 2 run at 14.5 KW/ft was chosen since this is the present LOCA technical specification limit at the 2 foot elevation for the first 50 EFPD from reference 3.

The comparison of the base case with the analysis herein is presented in Table 1.

The above versions of the THETA 1-B, REFLOO 3 and TACO 2 codes are currently under review by the NRC Staff.

I B. Input Assumptions I The NUREG-0630 rupture temperature as a function of engineering hoop stress correlation with a heating ramp rate of 0 C/s was used. Thi s I ramp rate (Figurc 1) represents a bounding value for rupture data.

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I NUREG-0630 strain vs. temperature data is contained in a fast and a slow ramp rate correlation. The circumferential strain model (Figure 2) used in this analysis bounded the composite of the slow and I the fast ramp models.

I Coolant flow blockage data (Figure 3) is derived from burst strain data, and therefore, also bounded the composite of the slow and fast I ramp models.

I Inputs into the CRAFT 2 code were stress versus rupture temperature data (Figure 1) and blockage based on the reduction in flow area data I (Figure 3). Inputs to the THETA 1-B code were stress versus rupture temperature data (Figure 1) and maximum rod circumferential strain data (Figure 2) .

I The CRAFT 2 results during blowdown and the REFLOD 3 results during refill and reflooding were both input to THETA 1-B.

IV. RESULTS OF ANALYSIS The restits of this analysis are summarized in Table 1 under the NUREG-0630 col umnn. For comparison, a base case analysis using the present evaluation model rupture curves is also provided on the table.

THETA 1-B for the NUREG-0630 case was run at 1 KW/ft below the base case, thereby resulting in lower cladding temperature. However, the rupture times for the two cases were very close since the lower 0 C/sec rupture curve from Figure 1 used in the NUREG-0630 case results in lower cladding temperatures required for rupture in comparison to the evaluation model curve. The maximum local oxidation for NUREG-0630 (1.44%) was lower largely due to the lower PCT. For similar PCT, it is expected that the local oxidation would be higher for NUREG-0630 due to the more conservative strain model (Figure 2) in THETA 1-B. The blockages I I

I calculated by CRAFT 2 were similar since the area reduction models (Figure 3) were not very different at the CRAFT 2 rupture temperature.

The results show that the core is cooled adequately and long term cooling capability is not impaired by the inclusion of NUREG-0630 models.

I V. CONSdVATISitS I The bounding analysis methodology employed herein included several areas of identifiable conservatisms:

1. The THETA 1-B analysis performed at 13.5 KW/f t used CRAFT inputs generated with a heat rate of 14.5 KW/ft. If an iteration were performed between CRAFT and THETA, the result would be an improvement in the allowable KW/f t limit. Based on previous analyses f rom reference 5, Figure 7-1, this method overpredicts peak cladding temperature by 18 F compared to running CRAFT 2 at 13.5 KW/f t.

2. The PCT from the THETA 1-B calculation for 13.5 KW/f t was 1734*F which is 466*F below the 10 CFR 50.46 limit of 2200"F. This temperature difference translates to margin on the LOCA PLHR (KW/f t) limits.

, B&W's calculational method for LOCA limits involves increasing the LOCA limit in each successive THETA 1-B calculation until the maximum l LOCA limit is achieved within the limits of 10 CFR 50.46. Based on l

previous iterations with THETA 1-B (reference 6), the two above conservatisms would be expected to add f rom .20 to .40 KW/ft to the lI calculated LOCA limits.

I 3. This analysis was performed at 2772 MWt total NSS power. The Midiand P1 ant application is for 2452 MW t . This difference in NSS I power translates to margin on the LOCA PLHR (KW/f t) limits.

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4. An inconsistency between the NUREG-0630 input data and the CRAFT 2 internal calculation of plastic strain prior to rupture resulted in a calculated pin pressure just prior to rupture about 30 psi higher than would have otherwise been calculated.

These last two conservatisms have not been quantified, but they are believed to be significant (0.1 - 0.3 KW/f t) based on previous analyses reported in references 4, 5, and 6.

In addition, the conservatisms inherent in the bounding NUREG-0630 models discussed under Section 111-0, " Input", are also operative. Specifically, the ramp rate was assumed to be 0*C/sec for rupture temperature and the strain and blockage models assumed the most 1imiting ramp rate.

I These above conservatisms are more than adequate to justify an impact of only -0.5 KW/f t at the 2 foot core elevation. If the NUREG-0630 models I were completely implemented, rather than bcunded, only a very mir.imal impact would be experienced from NUREG-0630.

I I VI. MVISEDLOCALIMITS I

The impact of NJREG-0630 is seen to be 0.5 kW/f t at the 2 foot elevation.

Since NUREG-0630 models impact the ruptured node, the E and 10 foot elevations will not be affected since these elevations are not rupture I node limi ted. The 4 and 6 foot elevations can be expected to see about one half or less of this impact based on studies performed on the inpact of the TAC 0 2 code. The sensitivity of impact to elevation is expected to be the same for the TACO 2 code and NUREG-0630. The revised Midland LOCA 1imits are shown in Table 2.

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l Table 1 NUREG-0630 LOCA Limit Impact at 2 Foot Core Elevation 8.55 f t2DE-PD, CD = 1.0 TACO Fuel Data, Pin Pressure = 1526 psi.

Base Case NUREG-0630 CRAFT Run C2NTONJ ABHYEND REFLOD 3 Run R2NT00H ABHYETU THETA 1-B Run T2NT08F ABHYAGU CRAFT, KW/ft 14.5 14.5 THETA 1-B LOCA Limit 14.5 13.5 Peak Temperature, F, Unruptured Node / 1916.3/41.0 1659.7/42.0 time, sec.

Peak Temperature, F, Ruptured Node / 2027.25/41.9 1734.26/41.1 time, sec.

Rupture Time, sec. 22.95 22.9 End of Blowdown, sec. 25.0 24.8 End of Adiabatic Heatup, sec. 35.88 35.64 Maximum Local 0xidation, % 2.59 1.44 CRAFT 2 Bic:kage, % 58.6 52.33 I.

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Table 2 MIDLAND PLHR (KW/FT)

I 0-50 EFPD ELEVATION (FT) 2 4 6 8 10 BAW-10103 (KW/FT) 15.5 16.6 18.0 17.0 16.0 TACO-2 -1.0 -0.5 -0.5 0 0 NUREG-0630 'id -0.25 -0.25 0 0 TOTAL 14.0 15.85 17.25 17.0 16.0 50 EFPD-E0L ELEVATION (FT) 2 4 6 8 10 BAW-10103 (KW/FT) 15.5 16.6 18.0 17.0 16.0 TACO-2 0 0 0 0 0 NUREG-0630 -0.5 -0.25 -0.25 0 0 l

TOTAL 15.0 16.35 17.75 17.0 16.0 I

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I REFERENCES

1. D. A. Powers and R. O. Meyer, NUREG-0630 " Cladding Swelling and Rupture Models for LOCA Analysis", April,1980.

2. " Code of Federal Regulations",10 CFR 50.46, January 1,1981.

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3. Design meno 32-1120818 00, "TAFY BOL Pin Pressure Analysis, David M. Waite, 8/12/80.

4. R. C. Jones, J. R. Biller, and B. M. Dunn, "ECCS Analysis of B&W's 177 FA Lowered-Loop NSS", BAW-10103A, Rev.'3, July, 1977.

5. B. M. Dunn, et al, "B&W's ECCS Evaluation Model Report With Specific Application To 177 FA Class Plants With Lowered Loop Arrangement",

BAW-10091, August 1974.

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6. R. J. Lowe, et al, "ECCS Evaluation of B&W's 205 FA NSS", BAW-10102, Rev. 2, I. December 1975.

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