ML19345E918
| ML19345E918 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 02/04/1981 |
| From: | Nichols T SOUTH CAROLINA ELECTRIC & GAS CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8102060417 | |
| Download: ML19345E918 (3) | |
Text
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Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Subject:
Virgil C. Summer Nuclear Station Docket No. 50/395 Load Sequencer
Dear Mr. Denton:
South Carolina Electric and Gas Company (SCE&G) and the NRC Staff are currently discussing open items in the Safety Evaluation Report (SER) for the Virgil C. Summer Nuclear Station seeking to find mutually satis-factory resolution of the items.
There is one issue, however that has been identified where SCE&G cannot proceed without further clarification from the Staff.
The item in question concerns the design and reliability of load sequencers which sequence Engineered Safety Features (ESF) components onto the engineered safety feature buses.
One sequencer is dedicated to each safety related bus. A sequencer would be used following the loss of offsite power, a safety injection signal or a combination of these conditions.
The Staff has voiced a concern that if both onsite and offsite power is availcble, the sequencer could possibly prohibit the availability of both sources of power to ESF components. To miti-gate this concern the NRC Staff has proposed that-SCE&G provide:
1.
A detailed description of the load sequencer.
2.
A reliability study for the load sequencer.
3.
A sneak circuit analysis (SCA) of the load sequencer.to determine if there are failure modes in the sequencer that would prohibit the availability of an onsite and offsite power.
~
With respect to the first item, the Summer FSAR in Chapter 7 presents the design basis for the. sequencer and describes the qualifications and on going testing of the device.
SCE&G is currently expanding the discussion f
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a Mr. Harold R. Denton February 4, 1981 Page Two of the sequencer in Section 8.3 of the FSAR.
However, as is the case for all of the equipment discussed in the FSAR, detailed design specifi-cations and schematics are not provided within that document. Detailed descriptions are available at SCE&G. With regard to the second item, a reliability study of the sequencer was provided to Mr. Om Chopra of the NRC Staff in January 1981.
With regard to the third item, SCESG requires further clarification of what the NRC Staff is requesting. While SCA has been in existance (in varying forms) for several years, the application of this technique has not been routine for nuclear power plant systems. More specifically, none of the NRC regulations or regulatory guides or Standard Review Plans or IEEE Standards for design of nuclear power plant systems discusses SCA. It appears that the existing application of SCA in aerospece industries follows no standard technique; thus the results of an SCA do not have a universal con-notation as is the case for a single failure analysis.
In any event, a pre-requisite of an SCA for a component or system would be a definition of the bounds of the study.
A final concern with this item is that a sequencer does not make ti.e decision as to whether offsite or onsite power is supplied to an ESF bus.
This decision is made external to a sequencer by the undervoltage relays on an ESF bus and the control logic for the diesel generator breaker.
The failure to load a conponent or group of components controlled by a sequencer would require the active failure of a component in the sequencer.
Such an active failure or the failure of the devices providing inputs to a sequencer-of components is outside the bounds of an SCA.
The preoperational-testing of each sequencer has been performed at the site under live conditions.
Initial testing of the sequencer consisted of operating each input and output device to insure that the proper action took place. After insuring that the external connections were correct, various combinations of inputs were initiated, 'and it was verified that the proper outputs occurred with correct timing. This testing included starting of loads.
The final test is the integrated safeguards test which is a ecmprehensive test of the complete engineered safeguard system including the load sequencer.
In this test, loss of offsite ' power and ' safety injection'vith and without loss'of offsite power will be simulated and the reaction'of the complete system will be verified.
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Mr. Harold R. Denton February 4, 1981 Page Three Based on the above we conclude that no further analyses of the sequencer are required to justify the integrity of the design and net allocation of this device.
Very truly yours, T. C. Nichols, Jr.
JAW:TCN:rh cc:
V. C. Sununer G. H. Fischer T. C. Nichols, Jr.
E. H. Crews, Jr.
O. W. Dixon, Jr.
d V. A. Williams, Jr.
O. S. Bradham a
D. A. Nauman R. B. Clary A. R. Koon A. A. Smith.
J. B. Knotts, Jr.
J. L. Skolds B. A. Bursey H. N. Cyrus hTCF/thitaker File e
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