ML19269D187
| ML19269D187 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 03/01/1979 |
| From: | Counsil W NORTHEAST UTILITIES |
| To: | Reid R Office of Nuclear Reactor Regulation |
| References | |
| TAC-11522, NUDOCS 7903070243 | |
| Download: ML19269D187 (3) | |
Text
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IMHtTHI!AST UTII.ITil!S PO Box 270
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- HARTFORD. CONNECTICUT 06101 (203) 666-6911
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l" March 1, 1979 Docket No. 50-336 Director of Nuclear Reactor Regulation Attn:
Mr. R. Reid, Chief Operating Reactors Branch #4 U.
S. Nuclear Regulatory Commission Washington, D. C.
20555
References:
(1)
D. C. Switzer letter to G. Lear dated September 22, 1977.
(2)
G. Lear letter to D. C. Switzer dated October 12, 1977.
(3)
D. C. Switzer letter to R. Reid dated March 13, 1978.
Gentlemen:
Millstone Nuclear Power Station, Unit No. 2 Enclosure Building Design In Reference (1), Northeast Nuclear Energy Company (NNECO) advised the NRC Staf f that the ten-inch suction line to Fan F-55 was not designed and supported seis-mically. This situation was reported as a 24-hour Reportable Occurrence, on the basis of a postulated coincident safe shutdown earthquake (SSE) and design basis accident (DBA). Under such a circumstance, failure of this line could prevent the enclosure building filtration system (EBFS) from maintaining a 0.25 inch negative pressure in the EBF region.
In Reference (2), the NRC Staff concurred with the assessment and the proposed corrective action, that of qualification of the line as seismic Category I prior to Cycle 2 operation.
In Reference (3), NNECO indicated that two additional non-seismically supported lines in the EBF region were discovered.
All modifications were completed prior to the start of Cycle 2 operation, as reported.
The purpose of this letter is to advise the Staff of recent investigations in this area, and report NNECO's conclusions.
NNECO has determined, based on further review, that the existence of nor. -seis-mically designed and supported lines which penetrate the enclosure building is acceptable. This situation is, in fact, the original design basis as reported in the FSAR and approved by the Staff.
The response to FSAR Questions 6.9 and 6.16.4, provided in Amendments 39 and 16, respectively, discusses various non-seismic Category 1 penetrations through the EBF region. This configuration was, and continues to be, an acceptable design basis. As stated previously, the basis for the Reportable Occurrence of Reference (1) was a postulated coincident SSE and DBA, or DBA followed by an SSE.
Although such a postulate was clearly a safe method to evaluate the adequacy of the design of Millstone Unit No. 2, it was also excessively conservative. Having recently identified certain addi-tional non-seismic penetrations, NNECO has further reviewed and evaluated the original design basis as well as ccrrent regulatory guidance in this area, and concluded that the current configuration is acceptable as presented below.
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A thorough review of the Millstone Unit No. 2 FSAR Sections 5.3.3.1.3, 5.3. 3.1.4, 5. 3.3. 2, and 5.3. 3. 2.1 reveals that although the enclosure building is designed to mitigate the consequences of a DBA and to maintain its structural integrity during and af ter i seismic event, simultaneous occurrence of the two events was not the design basis for the enclosure building.
The Staf f's Safety Evaluation Report, dated May 10, 1974, states in Section 3.9 that "an appropriate combination of loads likely to occur" was considered in the design of the enclosure building.
The NRC Staf f did not require considera-tion of a simultaneous loss-of-coolant accident and seismic event for the enclosure building when the design was approved.
As stated in General Design Criterion 2 of 10CFR50, Appendix A, only " appropriate combinations of the ef fects of normal and accident conditions with the ef fects of the natural phenomena" need be considered.
For example, it is clear that tornado events were not required to be combined with either loss-of-coolant events or seismic events due to the low likelihood of simultaneous occurrence.
Appendix 5.B of the FSAR states that seismic events and loss-of-coolant inci-dent were considered together, but only for the purposes of assuring conservatism in specifying appropriate load combinations used in design equations of the con-tainment structure. These two events were not combined for the purposes of conducting a consequence evaluation regarding performance of the EBFS.
This logical approach of not combining remote probability events to evaluate consequences is evident in a number of regulatory documents. For example, Regulatory Guide 1.117, " Tornado Design Classification" states that "it is not necessary to maintain the functional capability of all Seismic Category I structures, systems, and components because the probability of the joint occurrence of a low-probability event (loss-of-coolant accident with DBT or smaller tornado, or earthquake with DBT or small tornado) is suf ficiently small".
NNECO fully realizes that SSE and LOCA loads have long been combined by means of factored loads for the containment structure and reactor coolant piping system analysis. As indicated by the NRC Director of the Division of Systems Safety, Dr. R. J. Mattson, at the June 2, 1978, ACRS meeting, the reason for this conservative load combination is not clear to the NRC Staf f at the present time.
The NRC is presently developing a methodology for es-tablishing the appropriate margins in the containment structure and reactor coolant piping system design to replace the arbitrary combination of loads from two remote probability, independent events (NUREG-0484, " Methodology for Combining Dynamic Responses").
Io summarize, the enclosure building structure is designed to retain structural integrity subsequent to a seismic event.
However, the EBFS is not designed to be functional subsequent to an SSE.
The hypothetical situation of coincident LOCA and SSE is beyond the scope of the original Millstone Unit No. 2 licensing basis.
Further, such a postulate has insufficient technical basis, and, therefore, cannot be considered credible.
g..- 4 NNECO is not docketing this letter to request Staff review, but rather to clarify the misleading and excessively conservative interpretations we pre-sented in References (1) and (3). We trust the above information is sufficient to clarify our position.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY I it }{J)
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W. G. Counsil Vice President