ML25196A172
| ML25196A172 | |
| Person / Time | |
|---|---|
| Issue date: | 02/16/1977 |
| From: | Bender M Advisory Committee on Reactor Safeguards |
| To: | Rowden M NRC/Chairman |
| References | |
| Download: ML25196A172 (1) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, D. C. 20555 February 16, 1977 Honorable Marcus A. Rowden Chairman U. s. Nuclear Regulatory Corrmission Washington, D.C. 20555
SUBJECT:
REPORT ON THE EXXON NUCLEAR COMPANY WREM-BASED GENERIC PWR-ECCS EVALUATION MODEL UPDATE, ENC-WREM-II
Dear Mr. Rowden:
At its 202nd meeting, February 10-12, 1977, the Advisory Committee on Reactor Safeguards reviewed the modifications to the Exxon Nuclear Company Water Reactor Evaluation Model which provides a generic eval-uation model {ENC-WREM-II) for the analysis of the emergency core cooling systems {ECCS) for pressurized water reactors without upper head injection and having a low containment pressure such as that for an ice-condenser pressure-suppression system. '!his matter was considered at the 201st ACRS meeting, held in Washington, D.C., on January 6-8, 1977, and at ECCS Subcommittee meetings held on July 21-22, 1976, in Richland, Washington, and December 21, 1976, in Wash-ington, D.C.
During its review, the Committee had the benefit of discussions with representatives of the Exxon Nuclear Company and the NRC Staff. 'lbe Committee also had the benefit of the documents listed.
'!he previous Exxon PWR Evaluation Model utilizing WREM was approved by the Corrmittee in its September 15, 1975, report. For ENC-WREM-II, the principal modifications involve the evaluation of flow blockages, reflood heat transfer coefficients, and the hot wall delay time.
'Ihe Committee concurs with the NRC Staff on the approval of these modifi-cations.
Use of the generic model will require review by the NRC Staff of plant specific items for case-by-case applications. '!he Committee believes that, with the requirements set forth by the NRC Staff, the Exxon Nuclear Company Evaluation Model Upda~e, ENC-WREM-II, will conform to Appendix K to 10 CFR 50.
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Honorable Marcus February 16, 1977 Drs. s. H. Bush and M. w. carbon did not participate in this review.
Sincerely yours, M. Bender Chairman ADDITIONAL COMMENTS BY MEMBERS DAVID OKRENT AND MIL'ION PLESSET The ENC-WREM-II ECCS Evaluation Model is intended for application to the D.C. Cook Unit 1 on which the ACRS reported on January 14, 1977. It is proposed that this reactor be operated with a maximum Fq of only 1.95.
We believe that, particularly for such an application, the proposed modifications in the ECCS evaluation model must be treated with some reserve. Reflood heat transfer coefficients should be more reliably determined, particularly under conditions of low pressure and low re-flood rates. Also, further studies, both analytical and experimental, are needed to justify that conservatism exists under all conditions for assumption of a large reduction in flow blockage for a narrow range of fuel element gas pressure.
We believe that further studies, including sensitivity studies and statistical analysis of flow blockage, heat transfer coefficients, and the effects of significant steam generator leakage, should be made before this particular evaluation model is accepted for application to a low pressure containment.
REFERENCES:
- 1. Exxon Nuclear Company, "XCOBRA-IIIC: A Computer Code to Determine the Distribution of Coolant During Steady-State and Transient Core Operation," XN-75-21, April 1, 1975.
- 2. U.S. Nuclear Regulatory Comnission, "WREM:
Water Reactor Evaluation Model," NUREX;-75/056, Revision 1, May 1975.
- 3. Exxon Nuclear Company, "Exxon Nuclear Company WREM-Based Generic PWR ECCS Evaluation Model," XN-75-41, Vol. I, Vol. II, and Appendices A, B, C, and D, Vol. III (Revision 2), and Supplements 1-7, July-November 1975.
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Honorable Marcus February 16, 1977
- 4. Exxon Nuclear Company, "Definition and Justification of Exxon Nuclear Company DNB Correlation for PWR's," XN-75-48, October 6, 1975.
- 5. Exxon Nuclear Company, "Flow Blockage Model for I.OCA. Analysis,"
XN-75-G(A), January 1976.
- 6. Exxon Nuclear Company, "Exxon Nuclear ~
WREM-Based Generic PWR ECCS Evaluation Model Update ENC--wREM-II," XN-76-27, July 1976, Supplement 1, September 1976, and Supplement 2, November 1976.
- 7. U.S. Nuclear Regulatory Conmission, "Revision 1 to Safety Evaluation Report on the Exxon Nuclear Company WREM-Based Generic PWR-ECCS Evaluation Model Update ENC-WREM-II for Conformance to Requirements of Appendix K to 10 CFR 50 by the Office of Nuclear Reactor Regu-lation," January 5, 1977.
- 8. Exxon Nuclear Company, "Flow Blockage and Exposure Sensitivity Study for D.C. COok unit 1 Reload Fuel Using ENC WREM-II Model," XN-76-51 Supplement 1, January 1977.
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