ML20140E578

Modifies Approach to Testing of Diverse Reactor Trip & Turbine Trip for ATWS Sys Per Testability of ATWS Prevention & Mitigation Sys.Diverse Reactor Trip & Mod of Engineered Safeguard Circuit Not Needed for Testing

ML20140E578
Person / Time
Site:	Maine Yankee
Issue date:	03/24/1986
From:	Whittier G Maine Yankee
To:	Thadani A Office of Nuclear Reactor Regulation
References
7112L-GDW, GDW-86-55, MN-86-35, NUDOCS 8603280059
Download: ML20140E578 (2)

Text

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EolSON ORIVE MilRE HARHEE ATOMICPOWERCOMPARSe AUGUSTA, MAINE 04336 g

UCT March 24,1986 (207) 623 3521 g MN-86-35 GDW-86-55 Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Washington, D. C. 20555 Attention: Mr. Ashok C. Thadani, Director PWR Project Directorate #8 Division of Licensing

References:

(a) License No. [PR-36 (Docket No. 50-309)

(b) MYAPCo Letter to USNRC, dated July 31, 1985 - Maine Yankee Concept for ATWS Prevention and Mitigation (bN-85-141)

(c) MYAPCo Letter to USNRC dated December 19, 1985 - Schedule for Modification of ATWS Prevention and Mitigation (MN-85-212)

Subject:

Testability of ATWS Prevention and Mitigation System Gentlemen:

Maine Yankee provided the Staff with a conceptual design for the ATWS prevention and mitigation system required by 10 CFR 50.62 for review, Reference (b). As we later noted, Reference (c), we will not finalize the design until we have your concurrence.

However, we have begun to pursue some of the major design features of our proposed systems which has led to new insights regarding on-line testability.

Therefore, this letter is submitted to modify Maine Yankee Atomic Power Company's approach to the testing of the diverse reactor trip and the turbine trip for the ATWS System.

Reactor Trip In Reference (b), Maine Yankee proposed the installation of a diverse reactor trip using reactor coolant pressure. This trip would use existing pressure transmitters connected to the Reactor Coolant System. These transmitters are not part of the Reactor Protective System. We also stated that the diverse reactor trip would be designed so that testing at power would be possible.

To provide testability at power, we are proposing a scheme where the function of the diverse reactor trip will not be available during testing.

Maine Yankee feels that this approach is justified because the probability of an ATWS event is very low and the time required for the test is short. An additional consideration is the extreme complexity of the circuitry necessary to provide a diverse trip while testing. In our judgement, this added complexity would have an adverse effect on the system reliability and could result in unnecessary trips. During the test period an annunciator will indicate to the operator that the trip system is in test.

1 7112L-GDW B603200059 060324 g PDR ADOCK 05000309 P PDR

( MAINE YANKEE ATOMIC POWER COMPANY United States Nuclear Regulatory Commission Page Two Attention: Mr. Ashok C. Thadani, Director MN-86-35 Turbine Trip The turbine trip circuit proposed in Reference (b) makes use of an existing low steam generator level logic circuit to initiate the trip.

Reference (b) also stated that the turbine trip would be testable while the plant was on line.

To allow testing of the turbine trip circuit at power, an existing engineered safeguard circuit would require modification to provide for testing of the turbine trip. Maine Yankee believes that modifications to the engineered safeguard circuit should not be made and consequently testing at power would not be feasible for the following reasons:

The engineered safeguard circuit was not designed to be tested while the plant is at power. To redesign this safety circuit to allow testing at power would reduce the reliability of a safety circuit without a substantial increase in protection for the plant.

The reduction in reliability of the engineered safeguard circuit would also cause a reduction in reliability of the ATWS turbine trip. This increases the likelihood of inadvertent turbine trips and reactor trips.

o In the case that the turbine fails to trip on a low steam generator level event and the Reactor Protective System fails to operate, there remains three methods to scram the reactor:

1. The operator would actuate the manual reactor trip by pressing the two buttons on the main control board. If this should fall to operate, then,

2. The operator would trip the control rod drive motor-generator output contactors by pressing the "ATWS" trip button on the main control board.

3. The diverse reactor trip proposed in Reference (b) would also trip the control rod drive motor-generator output contactors.

'In our opinion, sufficient protection from an ATWS event at Maine Yankee will be provided without on-line testing of the turbine trip and the reduction in reliability necessary to provide on-line testing would not be justified.

Very truly yours, MAINE YANKEE ATOMIC POWER COW ANY t

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G. D. Whittier, Manager Nuclear Engineering & Licensing GOW/bjp cc: Dr. Thomas E. Murley Mr. Cors elius F. Holden Mr. Pat Sears