ML20248E778

Annual Rept on Abb CE ECCS Performance Evaluation Models

ML20248E778
Person / Time
Site:	Waterford
Issue date:	06/01/1998
From:	ENTERGY OPERATIONS, INC.
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Download: ML20248E778 (15)

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1 Annual Report on ABB CE ECCS Performance Evaluation Models i I

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ABSTRACT This report describes. changes and errors in the ABB Combustion Engineering .

evaluation models for ECCS performance analysis in 1997 per the requirements of 10CFR50.46. With the exception of changes made to facility / fuel inputs, there were four sets of reportable changes or errors in the evaluation models or application of the

. models in 1997:

1. All of the ECCS performance analysis codes were converted to HP workstations operating under the HP/UX operating system. This had no effect on the peak cladding temperature (PCT).

2. A change in the implementation of the methodology for small break LOCA (SBLOCA) ECCS performance analysis was made to improve the analysis l process which would introduce a 3*F cumulative change in PCT for SBLOCA r when the new process is used for the analysis of record.

L 3. An error in the implementation of the boric acid concentration calculation with flushing flow for both large and small break LOCA was found and corrected. This affected the post-LOCA boric acid concentration but is unlikely to affect the time to initiate combined hot-side / cold-side injection for large break LOCA (LBLOCA). It had no effect on the boric acid precipitation 4 conclusions for SBLOCA. Hence, there is no effect on core coolability or PCT.

4. . An error in the decay heat energy redistribution factor used for large break

. LOCA ECCS performance analysis was found and corrected. This required a reduction in the peak linear heat generation rate to mainisin the applicability of the analyses of record and meet the ECCS acceptance criteria.

The sum of the absolute magnitude of the PCT changes for large break LOCA from all previous reports, except for the effect of changes to plant inp its and the decay heat energy redistribution factor (ERF) error, has been less than 1Y The effect of the ERF error for Waterford is 36*F. The combined effect of changed facility / fuel inputs to the LBLOCA analysis since the last analysis submitted for NRC approval is 12'F. Thus, the total LBLOCA PCT impact for Waterford is 49*F.

The sum of the absolute magnitude of the PCT changes for SBLOCA from all previous reports was less than 3*F in 1997. No change occurred in the PCT for post-LOCA long term cooling. Per the criteria of 10CFR50.46, no action beyond this annual repod is required.

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TABLE OF CONTENTS 1.0 l N T R O D U C TI O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 ABB CE CODES USED FOR ECCS EVALUATION ............................... 3 3.0 EVALUATION MODEL CHANGES AND ERROR CORRECTIONS 3.1 C od e C o nve rs i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2 Automation of Data Transfer for Small Break LOCA Analysis......... 5 3.3 Post-LOCA Boric Acid Concentration .............................................. 5 3.4 Decay Heat Energy Redistribution Factor Error ..... ........................ 7

4. 0 C O N C L U S I O N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

5. 0 R E F E R E N C E 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 l

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1.0 , INTRODUCTION This report addresses the NRC requirement to report changes or errors in ECCS 4 performance evaluation models. The ECCS Acceptance Criteria, Reference 1, gives reporting requirements and actions required when errors are corrected or changes are made in an evaluation model or in the application of a model for an operating licensee or construction permittee of a nuclear power plant.

The action requirements in 10CFR50.46(a)(3) are:

1. Each applicant for or holder of an operating license or construction permit shall estimate the effect of any change to or error in an acceptable evaluation model or in the application of such a model to determine if the change or error is significant. For this purpose, a significant change or error is one which results in a calculated peak fuel cladding temperature (PCT) different by more than 50 F from the temperature calculated for the limiting transient using the last acceptable model, or is a cumulation of changes and errors such that the sum of the absolute magnitudes of the respective temperature changes is greater than 50 F.

2. For each change to or error discovered in an acceptable evaluation model or in the application of such a model that affects the temperature calculation, the applicant or licensee shall report the nature of the change or error and its estimated effect on the limiting ECCS analysis to the Commission at least annually as specified in 10CFR50.4.

3. If the change or error is significant, the applicant or licensee shall provide a report within 30 days and include with the report a proposed schedule for providing a reanalysis or taking other action as may be needed to .

show compliance with 10CFR50.46 requirements. This schedule may be developed using an integrated scheduling system previously approved for the facility by the NRC. For those facilities not using an NRC approved

, integrated scheduling system, a schedule will be established by the NRC staff within 60 days of receipt of the proposed schedule.

4. Any change or error correction that results in a calculated ECCS performance that does not conform to the criteria set forth in paragraph (b) of 10CFR50.46 is a reportable event as described in 10CFR50.55(e),

50.72 and 50.73. The affected applicant or licensee shall propose ,

immediate steps to demonstrate compliance or bring plant design or i operation into compliance with 10CFR50.46 requirements. 1 L

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This report documents all of the corrected errors and/or changes made to the presently licensed ABB CE ECCS performance evaluation models for 1997. This document is provided to satisfy the reporting requirements of the second item above. ABB CE reports for earlier years are given in References 2-10.

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2.0 ' ABB'CE CODES USED FOR ECCS EVALUATION ABB CE uses several digital computer codes for ECCS performance analysis that are described in topical reports, are licensed by the NRC, and are covered by the provisions of 10CFR50.46. The computer codes used for LBLOCA calculations are CEFLASH-4A, COMPERC-II, HCROSS, PARCH, STRIKIN-il, and COMZlRC.

CEFLASH-4AS is used in conjunction with COMPERC-II, STRIKIN-il, and PARCH for SBLOCA calculations. The codes for post-LOCA long term cooling analysis are BORON, CEPAC, NATFLOW, and CELDA.

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EVALUATION MODEL CHANGES AND ERROR CORRECTIONS This section discusses error corrections and model changes to the ABB CE ECCS performance evaluation models which may affect the calculated PCT.

3.1 Code Conversion All of the NRC licensed codes and related utility programs used for ECCS performance analysis were converted from operation on Hewlett Packard (HP) Apollo workstations (WS) operating under the Domain operating system (OS) to HP WS operating under the HP/UX operating system. The coding changes to maintain numeric precision and comply with FORTRAN programming conventions described in the 1992 code conversion (Reference 6) were preserved. A few changes to the FORTRAN coding were necessary to accommodate differences in FORTRAN compiler implementations and replace non-standard coding with standard coding. Some new edits were added to facilitate use of the codes. The code version numbers and implementation dates were adjusted to maintain unique identification of the new code versions. Except for one new procedure for SBLOCA described in Section 3.2, no changes were made to the models or their implementation.

The same procedures used to validate the converted codes in the 1992 conversion (Reference 6) were used in this conversion. Direct comparison of all results from runs on the HP Apollo computers with those from the new HP computers were made with an acceptance criterion of 0.01% when feasible. For some codes, such as those that l

compute their own time step lengths, a direct comparison of all output is not possible due to differences in the times at which results are reported. In such cases, comparison of key parameters passed on to subsequent steps in the analysis, objective parameters such as PCT and peak cladding oxidation, or graphical comparison of the time history of important parameters were used to validate the converted codes.

3.1.1 Largs Break LOCA The only changes made to the ECCS performance analysis codes used for LBLOCA analysis are those required to make them function on the new computers, add new edits, and provide unique version identification. No changes to the models used for LBLOCA analysis or their implementation were made. The overall impact of the conversion from the HP Apollo WS Domain OS to the HP/UX OS was no change in PCT. A precision of < 0.01 F was used for comparison of the printed results.

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3.1.2, Small Break LOCA One change was made to the ECCS performance analysis codes used for SBLOCA.

This change was beyond those required to make the codes function on the new coraputers, add new edits, and provide unique version identification. The change and its impact on PCT, including the impact of code conversion is discussed in Section 3.2.

3.1.3 Post-LOCA Long Term Cooling The only changes made to the ECCS performance analysis codes used for post-LOCA long term cooling analysis are those required to make them function on the new computers, add new edits, and provide unique version identification. No changes to the models or their implementation were made. The overall impact of the conversion was l no significant change to any result. Hence, there was no effect on PCT due to conversion of the post-LOCA long term cooling codes.

3.2 Automation of Data Transfer for Small Break LOCA Analysis As part of the code conversion process, one change was made to the ECCS performance analysis codes used for SBLOCA. This change was beyond those required to make the codes function on the new computers, add new edits, and provide unicue version identification. An option was added to transfer the core boundary conditions computed by the CEFLASH-4AS blowdown hydraulics computer code to the l hot rod heatup code PARCH via a file created by CEFLASH-4AS and read by PARCH.

Previously, the process involved a manual transfer of up to 50 values for each boundary condition from CEFLASH-4AS to PARCH. The two codes were revised to transfer 2000 evenly spaced values in time during the transient for core pressure, coolant mixture level, coolant mass, and power from CEFLASH-4AS to PARCH in a disk file. The revised process for data transfer provides a more accurate and efficient method of transferring the data. In a sample calculation, this change produced a 2.4 F reduction in PCT during the transient.

3.3 Post-LOCA Boric Acid Concentration An error in the calculation of the boric acid concentration for post-LOCA long term cooling was found in the BORON code for LBLOCA and in the CELDA code for SBLOCA. The error affects the boric acid concentration in the reactor core and the sump when the hot side injection flow rate exceeds the boil-off rate in the core. It does not affect the results before recirculation between the sump and reactor vessel as described below.

Once recirculation between the sump and reactor vessel begins, after the boric acid storage or makeup tanks (BASTS) and refueling water tank (RWT) empty, the BORON 5

code calculates the boric acid concentration in the reactor vessel and sump from the following differential equations, pg. C-10 in Reference 11:

dBCoR/ dt =

(WouT x BSUMP - Win x BCoR) / Va

=

dBSUMP/dt (Win x BCoR - Wourx BSUMP) / VSGAL where Bcoa boric acid concentration in core (wt%)

BSUMP boric acid Concentration in sump (Wt%)

Va volume of liquid water in reactor vessel (gallons)

V SGAL Volume of Water in sump (gallons)

Win flow rate into sump (gpm)

Wour flow rate from sump (gpm)

The CELDA code finds the boric acid concentrations using the same methodology.

In the original implementation of the BORON and CELDA codes, the flow rate into the sump and out of the sump were found, respectively, as W in = WFLuSH Wour = Wsoit where Wooit boil-off rate (gpm)

WFLuSH flushing flow rate (gpm)

The flow rate out of the sump into the core should include the flushing flow rate as well as the boil-off rate or Wour = Wooit + WFLUSH This change was implemented in the BORON and CELDA codes. The effect of i correcting the codes is discussed below.

3.3.1 Effect of Error for LBLOCA Boric Acid Concentration I

The BORON code finds the boric acid concentration in the core after a LBLOCA. The effect of the error in Wour depends on the timing of hot side injection and the

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magnitude of the flow. Two examples are used to illustrate this point. l I'

The first example initiates hot side injection with a flow rate sufficient to exceed the boil-off rate at the time hot side injection begins. The boric acid concentration in the core 6

for this case is shown in Figure 1 for the erroneous and corrected calculation. The corre'ction lias no effect on the maximum boric acid concentration although it shows a slower rate of decrease than the erroneous implementation of the boric acid concentration model.

The second example initiates hot side injection with a flow rate less than tne boil-off rate at the time injection begins. Figure 2 shows that the core boric acid concentration continues to rise until nearly four hours with injection starting at three hours. The corrected maximum boric acid concentration is about 0.22 wt% higher and occurs about one-tenth of an hour later. Consistent with the results of the first example, the rate of decrease is slower with the corrected model.

Results similar to those reported in Figures 1 and 2 are expected for all post-LOCA boric acid precipitation analyses performed by ABB CE although the magnitude of the results would differ from plant to plant. The effect of the error shown in Figure 2 is small compared to the difference between the maximum calculated boric acid concentration for ABB CE plants and the associated solubility limit.

3.3.2 Effect of Error for SBLOCA Boric Acid Concentration CELDA includes a copy of the BORON coding as a subroutine with revisions to link it to the overall CELDA hydraulics calculation. The BORON subroutine is run at the end of the CELDA calculation to evaluate boric acid concentration using flushing flows calculated by CELDA. The corrected code gives a maximum boric acid concentration which is approximately 0.1 wt% higher than the erroneous implementation of the boric acid concentration model. This increase is not significant in comparison to the margin of the boric acid solubility limit for the SBLOCA boric acid concentration analysis.

3.4 Decay rieat Energy Redistribution Factor Error An error in the decay heat energy redistribution factor (ERF) for voided conditions used in the LBLOCA analysis was discovered and corrected in 1997 (References 12 and 13).

The ERF is defined as the ratio of the fission energy deposited in the hot fuel rod divided by the energy generated in the hot rod. Typical values range from 0.92 to 0.98 depending on the hot rod pin-to-box ratio.

As described in the ABB CE LBLOCA evaluation model topical report (Reference 14),

the ERF used in the LBLOCA evaluation model includes the effect of moderator voiding. Because of fuel design changes made in the 1970's which produced flatter power distributions with the hot rod no longer adjacent to a guide tube, ABB CE calculated new values for the ERF for both fission and decay heat power. Erroneously, the calculations did not include the effect of moderator voiding on the recalculated

! values for the decay beat ERFs.

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The magnitude of the error in ERF is dependent on the pin-to-box ratio and pin lattice as shown in the table below. An approximation of the effect on PCT can be made based on an observed sensitivity of about 40 F in PCT per 1% change in ERF for a plant like Waterford with a 16X16 lattice where the limiting PCT occurred during the reflood phase of the event.

16X16 Fuel Assembly Pin / Box ERF Chanae (%) PCT Chanae ( F) 1.03 1.0 40 1.10 0.7 28 The above table shows the results of a sensitivity study addressed in ABB CE Infobulletin 97-04 (Reference 12). Based on the limiting pin-to-box ratio used in the Waterford LBLOCA analysis, a change in PCT of 36 F is estimated. To compensate for the ERF error, the Peak Linear Heat Rate (PLHR) was reduced by 0.2 kW/ft. This reduction in PLHR ensured that the PCT would remain less than the current FSAR PCT of 2177 F. In accordance with 10CFR50.59, the effects of a 0.2 kW/ft reduction in PLHR were evaluated and documented in a written safety evaluation (Reference 15) to support related changes to the Waterford Core Operating Limits Report (COLR).

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Figure 1 Initial Hot-Side Injection Rate Exceeds Boil-off Rate Corrected Version Erroneous Version I

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l 4.0 , CONCLUSIONS All of the codes used in the ABB CE ECCS performance evaluation model were converted to HP workstations operating under the HP/l)X operating system and one l change in the application of the evaluation model for SBLOCA occurred in 1997. In l addition, two errors were identified -- one for post-LOCA long term cooling and one for l LBLOCA.

l l The change in computers used to run the ABB CE ECCS performance analysis l computer codes produced no changes in the PCT for LBLOCA, SBLOCA, or post-LOCA long term cooling. Excluding plant specific effects due to the error in ERF, the sum of the absolute magnitude of the changes in PCT calculated using the ABB CE ECCS evaluation models, including those from previous annual reports (References 2 -

10) has been less than 1 F for LBLOCA. Based on an ABB CE sensitivity study, the increase in PCT due to the ERF error was determined to be 36 F for Waterford. A 0.2 kW/ft reduction in the PLHR was used to ensure that the PCT would remain less than the current FSAR PCT of 2177 F. Furthermore, the combined effect of changed facility / fuel inputs to the LBLOCA analysis resulted in a 12 F change in PCT. Thus, the total LBLOCA PCT impact for Waterford is 49 F.

The change in the application of the SBLOCA model combined with the effects from previous annual reports (References 2 - 10) produced a cumulative change in PCT for SBLOCA of less than 3 F. However, the application of a changed plant input to the SBLOCA model resulted in a PCT which exceeded the 2200 F criteria of 10CFR50.46(b)(1). This condition was reported in Licensee Event Report (LER) 97-033-00 and a reanalysis using the new Supplement 2 version of the ABB CE SBLOCA model was su'o mitted for review on April 30,1998, in W3F1-98-0090. As a result of the new methodology used to obtain the SBLOCA results, a request to change Technical Specification 6.9.1.11.1 which lists references to the analytical methods used to determine the core operating limits will be submitted within 90 days of the issuance of the "A" revision of the CE topical report (July 29,1998).

An error was discovered and corrected in the computer codes that calculate the post-LOCA boric acid concentration in the core. The error has minimal or no impact on the margin between the maximum calculated boric acid concentration for ABB CE plants and the associated solubility limit. Correction of the error in boric acid concentration has no effect on the PCT.

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5.0 , REF,ERENCES

1. " Acceptance Criteria for Emergency Core Cooling Systems for Light Water Nuclear Power Reactors," Code of Federal Regulations, Title 10, Part 50, Section 50.46.

2. " Annual Report on C-E ECCS Codes and Methods for 10 CFR50.46," CENPD-279, April,1989.

3. " Annual Report on C-E ECCS Codes and Methods for 10 CFR50.46," CENPD-279, Supplement 1, February,1990.

4. " Annual Report on C-E ECCS Codes and Methods for 10 CFR50.46," CENPD-279, Supplement 2, April,1991.

5. '" Annual Report on C-E ECCS Codes and Methods for 10 CFR50.46," CENPD-279, Supplement 3, April,1992.

6. " Annual Report on C-E ECCS Codes and Methods fo- 10 CFR50.46,11 CENPD-279, Supplement 4, April,1993.

7. " Annual Report on C-E ECCS Codes and Methods for 10 CFR50.46," CENPD-279, Supplement 5, February,1994.

8. " Annual Report on ABB C-E ECCS Performance Evaluation Models," CENPD-279, Supplement 6, February,1995.

9. "Annue: Report on ABB C-E ECCS Performance Evaluation Models," CENPD-279, Supplement 7, February,1996.

10. " Annual Report on ABB C-E ECCS Performance Evaluation Models," CENPD-279, Supplement 8, February,1997.

11. " Post-LOCA Long Term Cooling Evaluation Model," CENPD-254-P-A, June 1980.

12. " Potential Error in the Energy Redistribution Factor Used in LOCA Analysis,"

Combustion Information Bulletin 97-04, Rev. 01, July 11,1997,

13. " Report Pursuant to 10 CFR 21- Regarding Error in the Energy Redistribution Factor Used in LOCA Analysis," LD-97-024, August 14,1997.

14. " Calculative Methods for the C-E Large Break LOCA Evaluation Model,"

CENPD132P, August 1974.

, 15.- Waterford 10CFR50.59 Safety Evaluation, "COLR Cycle 9, Revision 2 (Safety Evaluation Number 98-012)."

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