ML20215C597
| ML20215C597 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe, Maine Yankee, 05000000 |
| Issue date: | 12/05/1986 |
| From: | Papanic G YANKEE ATOMIC ELECTRIC CO. |
| To: | Mckenna E Office of Nuclear Reactor Regulation |
| References | |
| FYR-86-118, MN-86-150, NUDOCS 8612150262 | |
| Download: ML20215C597 (3) | |
Text
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Telephone (617) 8724100 TWx 110-380u?619 YANKEE ATOMIC ELECTRIC COMPANY e
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.Y 1671 Worcester Road. Framingham, Massachusetts 01701
, YANKEE
~~
December 5, 1986 FYR 86-118 MN 86-150 United States Nuclear Regulatory Commission Washington, DC 20555 Attention:
Ms. Eileen M. McKenna Project Manager Project Directorate #1 Division of PWR Licensing-A
References:
(a) License No. DPR-3 (Docket No. 50-29)
(b) License No. DPR-36 (Docket No. 50-309)
(c) Letter, YAEC to USNRC, FVY 83-4, dated January 14, 1986 (d) Letter, YAEC to USNRC, MN 86-127 or FYR 86-098, dated October 16, 1986 (e) Letter, YAEC to USNRC, MN 86-138 or FYR 86-107, dated November 4, 1986 (f) Letter USNRC to YAEC, NYR 86-225, dated October 3, 1986
Subject:
RELAP5YA
Dear Ms. McKenna:
In Reference (c), Yankee Atomic Electric Company (YAEC) submitted RELAPSYA, a computer program for light water reactor system thermal-hydraulic analysis, to the NRC for review and licensing approval. References (d) and (e) provided our responses to 25 questions requiring resolution from the NRC review of RELAPSYA. These responses include answers to the three questions on the application of RELAP5YA for PWR Small Break Loss of Coolant Accident (LOCA) analysis transmitted by you in Reference (f).
In the format of these submittals the three questions / responses correspond to Items 2.2, 2.5, and 2.3, respectively.
Based on the information furnished to you to date, we continue to conclude that RELAPSYA provides a conservative method, conforming with regulatory requirements, for the analysis of BWR LOCAs across the entire break spectrum.
However, our recent PWR Small Break (SB) LOCA investigations have identified the need to modify our PWR SBLOCA approach to better represent observed and expected core liquid inventory behavior.
Reference (e) included documentation of the RELAPSYA simulation of Small Break Test S-LH-1 in the Semiscale Mod 2-c facility. This test was chosen mainly because it simulated a PWR SBLOCA in which core uncovery was observed.
Previous tests selected for RELAPSYA assessment (LOFT L3-1 and LOFT L3-6) did not exhibit this behavior.
In this simulation, RELAP5YA underpredicted the
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i United States Nuclear Regulatory Commission December 5, 1986 Attention:.Ms. Eileen M. McKenna Page 2 core liquid inventory.- This results from the conservative calculation of Emergency Core Cooling (ECC) bypass phenomena and its subsequent impact on core cooling. The RELAP5YA model for the S-LH-1 test represented the downcomer by a one-dimensional set of vertical volumes connected near the top to the intact and broken loops.
ECC injected into the intact loop flowed into an upper downcomer volume, bypassed the lower downcomer volumes, and preferentially flowed to the break in the broken loop. This prevented the lower vessel and core regions.from refilling properly and subsequently cooling the bundle beyond 600 seconds.
It also decreases the depressurization rate and adversely affects accumulator injection.
Our concern is limited to applications of RELAPSYA for PWR SBLOCA analysis and not for BWR applications.
The overly conservative calculation of core liquid inventory for this-test could affect the utility of subsequent applications of RELAP5YA for PWR SBLOCA analysis. Hence, we are proceeding to explore this issue further and modify our method where necessary. We believe we can accomplish this without adversely affecting the overall NRC and YAEC schedules for review and licensing approval of planned RELAP5YA PWR small break applications. This is further discussed below.
We have embarked on a two-stage program to address these items in our method for PWR SBLOCA analysis.
Stage 1 of the program consists of additional assessment of RELAP5YA against the LOFT LS-1 test. This test was chosen because the LOFT facility is more repreuentative of PWR geometry than the Semiscale facility, especially with regard to ECC bypass phenomena.
Additionally, LOFT Test LS-1 was the smallest break size investigated in the LOFT facility for which core uncovery was observed.
Stage 2 of our program consists of two parts. Part A investigates and assesses improvements to certain interphase drag models in RELAPSYA. Part B will address how the multi-dimensional aspects of PWR ECC bypass phenomena may be handled by a more appropriate nodalization of the downcomer region.
All the tasks associated with this program are proceeding in parallel.
We will be prepared to discuss these results with you in late January of 1987. Upon satisfactory resolution of these PWR SBLOCA issues, we will submit documentation of the revised PWR SBLOCA method for your review by March 31, 1987. Contingent upon a 3-month formal review period, we will submit the results of our PWR SBLOCA analyses using RELAPSYA by the end of 1987.
We believe this schedule is consistent with previous commitments for completion of analyses and trust that it is satisfactory to you.
Implementation of PWR SBLOCA model modifications, if required, will be designed such that BWR LOCA response predictions are unaffected. Thus, we feel the schedule for the Safety Evaluation Report on RELAPSYA for BWR applications should remain the same. For PWR SBLOCA, the model and applications changes being investigated do not affect the major elements of RELAPSYA currently under NRC review.
If you have any questions regarding the
,e, United States Nuclear Regulatory Consission December 5, 1986 Attention:
Ms. Eileen M. McKenna Page 3 schedule or program for modifying and applying our PWR SBLOCA analysis method, please contact us.
Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY Il M
George P n c, c.
Senior roject Engin er Licensing CP/jmk
.