ML20213F653

Safety Evaluation of Rev 2 to Vol 3 of XN-NF-80-19(P), Exxon Nuclear Methodology for Bwrs,Thermex:Thermal Limits Methodology Summary Description. Rept Acceptable for Ref in Licensing Applications

ML20213F653
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Issue date:	11/10/1986
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8611140370
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ENCLOSURE I l

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION  !

RELATING TO EXXON NUCLEAR COMPANY TOPICAL REPORT XN-NF-80-19 (P), VOLUME 3, REVISION 2

" EXXON NUCLEAR METHODOLOGY FOR BOILING WATER REACTORS THERMEX: THERMAL LIMITS METHODOLOGY

SUMMARY

DESCRIPTION"

1.0 INTRODUCTION

In April 1931, the Exxon Nuclear Company submitted for review a BWR methodology report entitled " Exxon Nuclear Methodology for Boiling Water Reactors, THERMEX:

Thermal Limits Methodology Summary Description" (Ref. 1). The original report was revised and resubmitted as Reference 2. The staff review was performed on the revised report. The substantive topics addressed in the report include: a correlation of boiling transition test data, hydraulic characterization of the fuel, evaluation of safety limits, analysis of plant and core abnormal event response, and the statistical convolution of uncertainties. The staff has completed its review of the information and our evaluation follows.

2.0

SUMMARY

AND TECHNICAL EVALUATION The report XN-NF-80-19, Volume 3 describes the Exxon approach to thermal limits analyses and provides references to the more detailed methods comprising the safety analyses. The staff review of XN-NF-80-19 (P), Vol. 3, Rev. 2 includes confirmation that the referenced methodologies have been approved or that the reviews are substantially complete and Safety Evaluation Reports (SERs) are in preparation. Evaluation of the referenced methods is addressed in the associated SERs. Evaluation of the methodology addresses its applicability to both ENC 8x8 and 9x9 reload fuel types. ,

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The THERMEX thermal limits methodology refers to a series of related analyses to establish an operating limit minimum critical power ratio (OLMCPR). The OLMCPR is determined from two calculated values, the Safety Limit MCPR (SLMCPR) and the limiting transient delta-CPR. The overall methodology is comprised of four major segments, which are (1) reactor core hydraulic methodology, (2) XN-3 critical power correlation, (3) plant transient simulation methodology and (4) critical power methodology. The individual components of the THERMEX methodology and their current review status are as follows:

In Section 2 of the subject Topical Report, the ENC pressure drop methodology report, XN-NF-79-59 is referenced. The report addresses part of the calculational procedure used by Exxon Nuclear Company to determine the assembly pressure drop that will be used to calculate assembly flows for a BWR core.

The XN-NF-79-59 report has been approved (Ref. 3) by the NRC staff. Reference 3 identifies the constitutive relationships to determine void fraction and two-phase pressure losses which are in turn used as input to the calculation of the assembly pressure drop by the XCOBRA computer code. XCOBRA is a steady state version of the XCOBRA-T code which is described in a later section of this Safety Evaluation. Comparisons of CHFR calculations at steady-state conditions as predicted by the XCOBRA and XCOBRA-T codes were made by ENC and the results were reported ir. the XCOBRA-T Topical Report (Ref. 7). The CHFR results determined with the two codes showed similar axial distributions and very close agreement. Based on the similarity of the computational models of the two codes and NRC approval of the XCOBRA-T code (Ref. 13), we find the use of the steady-state code acceptable in this context.

In Section 3, the use of the XN-3 Critical Power Correlation is cited as the mechanism for the prediction of onset of transition boiling (critical heat flux). The basis for the use of XN-3 is addressed by ENC for both the 8x8 and 9x9 Exxon fuel types. Staff approval of the use of the XN-3 correlation is l provided in References 4 and 5 for the 8x8 and 9x9 fuel respectively.

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In Section 4, COTRANSA (Ref. 6), PTSBWR3 (Ref. 6) and XCOBRA-T (Ref. 7) are identified as the plant transient simulation codes used in the THERMEX methodology. Reference 6 has been approved by the staff in Reference 8. More recently, ENC has modified the hot channel model in the COTRANSA methodology and submitted a licensing topical report on the modification (Ref. 14). The revisions have not been formally reviewed by the staff. Therefore, only the unmodified version of COTRANSA identified in Reference 6 or the acceptable alternate use of XCOBRA-T is approved at this time for use in the THERMEX methodology. The staff review of the XCOBRA-T Licensing Topical Report has been completed and the staff SER has been issued (Ref.13).

In the generation of the limiting transient delta-CPR in the THERMEX methodology, consideration is given to the statistical convolution of uncertainties associated with the calculation of the thermal margin. The statistical uncertainty analysis methodology is described on a generic basis in Reference 9. The staff SER on this subject is included in Reference 9.

Section 5 of the THERMEX Topical Report discusses the Exxon critical power methodology for boiling water reactors. This methodology is described in Reference 10. The staff SER on this subject is included in Reference 10.

The THERMEX analytical process requires the input of certain external data, a portion of which may be plant dependent. The specific input sources are described in Sections 1.3 and 6.0 of the THERMEX Topical Report. It is expected that the primary use of the THERMEX methodology will be in the analysis of plant reloads. Therefore, although the methodology is approved on a generic basis, the safety limit MCPR (SLMCPR) and operating limit MCPR (0LMCPR) values will be dependent on the plant-specific input parameters. The values of SLMCPR and OLMCPR incorporated into plant Technical Specifications thus need tc, be evaluated for each reload cycle.

ENC states in Section 6 of the THERMEX Topical Report that the THERMEX '

methodology is tailored for compatibility with ENC's core monitoring technique

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4 (POWERPLEX) which is described in Reference 12. Other systems may be used provided that the associated power distribution uncertainties are identified and appropriate operating parameters compatible with the ENC transient safety analyses are monitored. Whatever monitoring system is used should be specifically identified in plant submittals referencing the THERMEX methodology.

3.0 EVALUATION FINDINGS We have completed our review of the ENC Thermal Limits Methodology Report, THERMEX, as described in the ENC Topical Report XN-NF-80-19, Volume 3, Revision 2 (Ref. 2). The report is an updated version of an earlier report (Ref.1) submitted at the request of the staff so that it applies to both ENC 8x8 and 9x9 fuel types and incorporates the use of the XCOBRA-T transient delta-CPR methodology. Based on our confirmation of the acceptable status of the referenced components comprising the THERMEX methodology, we find the Topical Report XN-NF-80-19, Volume 3, Revision 2 acceptable for referencing in nuclear plant licensing applications.

As noted in Section 2.0 of this Safety Evaluation, the hot channel modifications to the COTRANSA code have not been reviewed by the staff and are not approved for use in the calculation of delta-CPRs, However, the results of our review of Reference 14 related to the hot channel methodology will become a part of the overall approval of THERMEX. Our Safety Evaluation of Reference 14 will address its applicability for in orporation into the THERMEX methodology.

Also as noted in Section 2.0 of this Safety Evaluation, the specific core monitoring system employed should be identified in a licensing submittal referencing the subject Topical Report.

REFERENCES

1. XN-NF-80-19, Vol. 3, Rev. 1, Exxon Nuclear Methodology for Boiling Water Reactors - THERMEX: Thermal Limits Methodology Summary Description, April 1981.

2. XN-NF-80-19, Vol. 3, Rev. 2, Exxon Nuclear Methodology for Boiling Water Reactors - THERMEX: Thermal Limits Methodology Sumary Description, September 1986.

3. XN-NF-79-59(P)(A) " Methodology for Calculation of Pressure Drop in BWR Fuel Assemblies" with SER enclosed. Issue date November 17, 1983.

4. Letter, H. Bernard (NRC) to G. F. Owsley (ENC), July 22, 1982, " Acceptance for Referencing of Topical Report XN-NF-512, Revision 1.

5. Letter, C. O. Thomas (NRC) to J. C. Chandler (ENC), February 1,1985,

" Acceptance for Referencing of Licensing Topical Report XN-NF-734, Confirmation of the XN-3 Critical Power Correlation for 9x9 Fuel Assemblies."

6. XN-NF-79-71(P), Rev. 2, " Exxon Nuclear Plant Transient Methodology for Boiling Water Reactors," November 1981.

7. XN-NF-84-105(P), Vol. 1, "XCOBRA-T: A Computer Code for BWR Transient Thermal Hydraulic Core Analysis," May 31, 1985.

8. Letter, H. N. Berkow (NRC) to J. C. Chandler (ENC) " Acceptance for Referencing of Licensing Topical Report XN-NF-79-71, Revision 2, Supplements 1 through 3: Exxon Nuclear Plant Transient Methodology for Boiling Water Reactors" dated December 16, 1985. i

9. XN-NF-81-22(P)(A), " Generic Statistical Uncertainty Analysis Methodology,"

November 1983.

10. XN-NF-524(A), Revision 1, " Exxon Nuclear Critical Power Methodology for Boiling Water Reactors," November 1983.

11. XN-NF-80-19(A) Volume 1, Supplements 1 and 2 " Exxon Nuclear Methodology for Boiling Water Reactors - Neutronics Methods for Design and Analysis,"

March 1983. ,

12. XN-NF-80-19(A), Supplement 2, " Exxon Nuclear Methodology for Boiling Water Reactors - Neutronics Methods for Design and Analysis" March 1983.

13. Letter, G. Lainas (NRC) to G. N. Ward (ENC), " Acceptance for Referencing of Licensing Topical Report XN-NF-105(P), "XCLBRA: A Computer Code for BWR Transient Thermal Hydraulic Core Analysis," dated October 27, 1986.

14. XN-NF-86-113(P), "COTRANSA Updated Hot Channel Model," August 1986.

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