ML20206Q060

Topical Rept Evaluation of Addendum 3 to Rev 1 to WCAP 9561, Thimble Modeling in Westinghouse ECCS Evaluation Model. Changes Meet Requirements of 10CFR50.46 & 10CFR50,App K & Acceptable

ML20206Q060
Person / Time
Site:	Point Beach
Issue date:	08/25/1986
From:	NRC
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Shared Package
ML17342B405	List: ML20206Q060
References
TASK-2.K.3.30, TASK-2.K.3.31, TASK-TM TAC-45853, TAC-45854, TAC-48192, TAC-48193, TAC-62166, NUDOCS 8609040001
Download: ML20206Q060 (5)

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o SAFETY EVALUATION ON CHANGES IN THE 1981 WESTINGHOUSE ECCS EVALUATION MODEL WITH BART Introduction I In a meeting held on June 23, 1986, Westinghouse met with representatives of the staff to discuss changes in their large break ECCS Evaluation Models.

These changes were necessitated by a modeling; change in the WREFLOOD code and an input error to the BART code. Westinghouse estimated that inclusion of the modeling change and correction of the input error could result in an increased  !

peak clad temperature of up to 120*F for ECCS analyses that were performed with the 1981 Evaluation Model with BART. The corrections and remedial actions are described in Topical Report WCAP 9561 Addendum 3, Revision 1. " Thimble Modeling in Westinghouse ECCS Evaluation Model," which was submitted in a letter dated July 24, 1986 (Reference 1).

The model change in the WREFLOOD code was required because the water volume which flows into the control rod guide thimbles during the core reflooding period following a large break LOCA was previously neglected. This model change was found to have the effect of increasing the calculated peak cladding

temperature by up to 20*F. The BART change is an error in input which caused l systematically low values of hot assembly bundle power to be used by the i code. This error was found to have the effect of increasing peak cladding temperature by approximately 100 F. To offset these changes, Westinghouse will include a portion of the heat transfer model calculated by BART in the peak claddirig temperature calculation of the LOCTA code. The combined effect of the thimbles on flooding rate and the error in the hot assembly power was determined to be offset by the benefit of including a portion of the heat transfer model. A net benefit was obtained for plants with high peak cladding temperatures approaching 2200*F and a small penalty was obtained for plants with lower peak cladding temperatures.

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Thimble Filling This issue affects the following Westinghouse ECCS Evaluation Models: 1) 1978;

2) 1981; and 3) 1981 with BART. This safety evaluation applies only to the 1981 model with BART. Westinghouse does not intend to correct the 1978 or 1981 models since they will not be used in the future. The control rod guide thimbles are hollow tubes within the fuel rod bundles which replace 25 fuel rods in 17x17 fuel assemblies and 21 fuel rods in 15x15 fuel assemblies.

Control rods are operated within 1/3 of the thimbles. The thimbles may also contain burnable poison rods or in-core neutron detectors. The thimbles contain small flow holes at the bottom to allow water to escape during control rod insertion.

Durtng a large LOCA, the fluid in the thimbles will flash to steam. During the reflooding period, the flow holes will allow water to reenter. In evaluating core reflooding, thimble refilling was not considered by Westinghouse in the WREFLOOD code. This tends to be nonconservative since water which would otherwise enter the coolant channels w7uld instead flow into the thimbles.

Although nonconservative, this simplifying assumption was considered to be reasonable in view of the relatively restricted flow into the thimbles compared to flow through the core and considering the effect of the thimble hole plugging devices. During analysis of the effect of removal of the thimble plugging devices, it was found that plug clearances were sufficiently large and core flow was sufficiently low during reflood that the assumption of no flow in the thimbles warranted reconsideration.

Subsequent work by Westinghouse assessed the effect of the assumption of flow through the thimbles during the reflood phase - a phase in which core flow rates are significantly lower than during blowdown and during which refill of the thimbles would tend to be at the same rate as that of the core. These i subsequent studies indicated that a modeling assumption of flow in the thimbles during reflood would result in a slightly higher peak clad temperature calculation than would result from the assumption of no flow through the thimbles. For the 1981 Westinghouse ECCS Evaluation Model with BART, thimble

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refilling has now been included in a conservative manner in which all thimbles are assumed to be empty at the beginning of the reflooding period and fill at the same rate as the core. This is accomplished by including the total thimble volume in an existing model of the WREFLOOD code which has been approved by the staff (Reference 2). The effect of this conservative modeling of thimble filling during reflood is small (10 to 20 degrees F) because the more significant phemonena of liquid entrainment in the core and steam binding in the coolant loops would be unchanged.

Hot Assembly Bundle Power The hot assembly bundle power error in BART resulted from a confusion between similar input requirements for BART which is utilized to calculate heat trans-fer coefficients and LOCTA which is utilized to calculate peak cladding temperatures. The LOCTA code evaluates the thermal behavior of a single pin and the fluid conditions in the adjacent hydraulic channel. The hydraulic channel is defined by the hydraulic diameter which is a function of the wetted perimeter. The wetted perimeter includes both heated rod surfaces and unheated thimble surfaces. To account for the effect of the unheated surfaces in computing channel enthalpy rise, the total number of fuel pins plus thimbles is input to LOCTA and utilized to calculate a total surface area and an average heat flux for use in the coolant enthalpy rise calculation.

The BART code evaluates an entire fuel bundle including ,the thimbles. In cal-culating the coolant enthalpy rise the BART code correctly utilizes a heated diamciir in defining the coolant channel adjacent to a fuel rod. Only the fuel

~ rod perimeter is utilized to derive the heated diameter and not the thimble perimeter. The use of an average heat flux for both rods and thimbles is therefore not required in BART. Only the number of rods should be input rather than both rods and thimbles as in LOCTA. The same input was utilized for both codes however. This produced an under-prediction in enthalpy rise in BART and caused an over-prediction of the fuel rod heat transfer coefficient to be cal-culated and transferred to LOCTA. The higher heat transfer coefficient caused LOCTA to calculate a peak cladding temperature that was too low by

approximately 100*F. The error is corrected by inputting the number of fuel rods into BART rather than the number of rods plus thimbles. The staff concludes the correction is acceptable.

Heat transfer from the fuel rod surface to the fluid is a combination of con-vection and radiation. Evaluation of data from the FLECHT reflooding heat transfer experiments has shown that radiation represents a significant fraction of the total heat flux. Radiation heat transfer is a function of the fuel rod surface temperature to the fourth power and increases rapidly at elevated tem-peratures. The radiation models in the current BART code were reviewed and approved by the staff as discussed in Reference 2. In transferring the fuel rod heat transfer coefficients from BART to LOCTA for calculation of peak clad-ding temperature, current Westinghouse ECCS evaluations have not included a portion of the radiation heat transfer coefficient. Westinghouse deleted this heat transfer mode because it was thought to have only a small effect on fuel rod cooling and its deletion would provide additional conservatism in their ECCS Evaluation Model with BART. As discussed in Reference 1, the effect of including this portion of radiation heat transfer compensates for the iden-tified hot assembly bundle power and control rod thimble changes in the 1981 ECCS Evaluation Model with BART. Since the staff previously approved the radiation heat transfer model (Reference 2), incorporation of this portion at this time is acceptable.

Conclusion As stated above, the NRC staff concludes that the changes to the 1981 Westinghouse ECCS Evaluation Model with BART, as described in Reference 1, meet the requirements of 10 CFR 50.46 and Appendix K to 10 CFR 50 and are, therefore, acceptable.

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References

1. Westinghouse letter, NS-NRC-86-3147, from E. P. Rahe to J. Lyons, NRC,

" Review of WCAP-9561-P, Addendum 3, Revision 1," July 24,1986.

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2. .NRC letter, from C. O. Thomas to E. P. Rahe, Westinghouse Electric Cor-poration, " Acceptance for Referencing of Licensing Topical Report WCAP-9561, BART A-1: A Computer Code for Best Estimate Analysis of Reflood Transients," December 21, 1983.

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