ML20133D336

Forwards Description of ATWS Prevention & Mitigation Sys Proposed for Installation,In Response to Generic Ltr 85-06. Completion of Installation Expected by Startup from Cycle 9/10 Refueling Outage in Spring 1987

ML20133D336
Person / Time
Site:	Maine Yankee
Issue date:	07/31/1985
From:	Whittier G Maine Yankee
To:	Butcher E Office of Nuclear Reactor Regulation
References
6134L-FWS, GDW-85-208, GL-85-06, GL-85-6, MN-85-141, NUDOCS 8508070307
Download: ML20133D336 (6)

Text

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.I EDISON DRIVE MAIRE HARHEE ATOMWPOWERCOMPARUe ruausra, unlue oasse (207) 623-3521 O July 31, 1985 MN-85-141 GDW-85-208 Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Washington, D. C. 20555 Attention: Mr. Edward J. Butcher, Jr.

Acting Branch Chief Operating Reactors Branch No. 3 Division of Licensing

References:

(a) License No. DPR-36 (Docket No. 50-309)

(b) USNRC Letter to MYAPCo dated April 16, 1985 - Quality Assurance Guidance for ATWS Equipment which is not Safety Related (Generic Letter 85-06)

Subject:

Maine Yankee Concept for ATWS Prevention and Mitigation Gentlemen:

This letter is submitted in accordance with the requirements of 10 CFR 50.62.

The attachment to this letter describes the ATWS prevention a d mitigation system Maine Yankee proposes to install. Our present plan calls for completion of the installation by the time of plant start up from the Cycle 9/10 refueling outage in the spring of 1987.

We believe that this submittal fulfills the reporting requirements of 10 CFR 50.62. However, if you need additional information, do not hesitate to contact us.

Very truly yours, MAINE YANKEE ATOMIC POWER CorPANY bhV G. D. Whittier, Manager Nuclear Engineering & Licensing GDW/bjp

Enclosure:

cc: Dr. Thomas E. Murley 00 Mr. Cornelius F. Holden L dg i

8508070307 850731 i PDR ADOCK 05000309 P PDR L 6134L-FWS

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, M AINE VANKEE ATOMIC POWER COMPANY ATTACHMENT Conceptual Description ATWS Prevention and Mitigation System Maine Yankee Atomic Power Company BACKGROUND The Nuclear Regulatory Commission has amended its regulations tc require improvements in the design and operation of light-water-cooled nuclear power plants to reduce the likelihood of-failure of the Reactor Protection System to shut down the reactor following anticipated transients. An anticipated transient without scram (ATWS) is an expected operational transient (such as loss of feedwater, loss of condenser vacuum, or loss of off-site power to the reactor), which is accompanied by a failure of the Reactor Trip System to shut down the reactor. The Reactor Trip System consists of those power sources, sensors, initiation circuits, logic matrices, bypasses, interlocks, racks, panels, control boards, and actuated devices that are required to initiate reactor shutdown; this includes circuit breakers, control rods, and control rod drive mechanisms.

DESIGN CONSIDERATIONS Considerations regarding the ATWS System and equipment criteria include quality, diversity, redundancy, seismic qualification, environmental qualification, testability, physical separation, reliability, and the application of the Class lE criteria.

Quality assurance guidance for non-safety related portions of the ATWS prevention and mitigation system was issued in Generic Letter 85-06.

1. Toaddressthismatter,MaineYankeewilladdthosecomponentsc['

these systems which are non-safety related to Appendix C of our Quality Assurance Program as "Other Items Requiring Quality Assurance".

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, MQlME YONKEE ATOMIC POWER COMPANY The additional equipment required by the ATWS rule to implement diversity does not have to meet all of the stringent requirements normally applied to safety-related equipment. The equipment required is for the purpose of reducing the probability of unacceptable consequences following anticipated operational occurrences. Since the combination of an anticipated operational occurrence, failure of the Reactor Trip System, and a seismic event or an event which results in significant plant physical damage has a low probability, seismic qualification and physical separation criteria need not be applied. In view of the redundancy provided in the existing Reactor Trip System, the equipment required by the ATWS rule does not have to be redundant within itself.

The ATWS rule requires diversity from those portions of the existing Reactor Trip System in plants where only minimal diversity is currently provided. The logic circuits and actuation devices in the existing Maine Yankee Reactor Trip System measure a variety of plant parameters and utilize a variety of sensor types. Common cause failures in the diverse sensors of existing Reactor Trip Systems are considered sufficiently unlikely that additional sensor diversity is not necessary. Even though sensor diversity is not necessary, it is desirable that sensors in the existing Reactor Trip System not be used to provide the signals for the diverse equipment required by the ATWS rule.

l l Use of the same sensor in the Reactor Trip System and the alternate trip equipment would result in interconnections between the two systems that are difficult to analyze and which could increase the potential for common cause failures affecting botn systems. Since sensors for the equipment required by the ATWS rule do not have to be safety-related, there should be considerable flexibility for using existing sensors without using Reactor Trip System sensors. However, there may be some cases where use of non-safety-related equipment would result in increased risk to safety systems, or where it would not be cost effective to use sensors separate from those in the existing Reactor Trip System.

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, MQlNE VAP3KEE AVOMIC POWER COMPANY This is particularly the case where not using sensors in the existing Reactor Trip System would result in the need to install a new sensor connected to the Reactor Coolant System. This could result in significant radiation doses to the personnel making the modification. Another case would be where l Installation of additional containment penetrations would be required. In cases where existing Protection System sensors are used to provide signals to diverse equipment, particular emphasis should be placed on the design of the method used to isolate the signal from the existing Protection System to minimize the potential for adverse electrical interactions.

The equipment required by the ATWS rule must be installed such that it does not degrade the existing Protection System. This is to be accomplished by making the diverse equipment electrically independent to the extent practicable from the existing Protection System and by ensuring that the existing Protection System will continue to meet all applicable safety-related criteria after installation of the diverse equipment.

DESIGN CONCEPT - ALTERNATE REACTOR TRIP Maine Yankee has selected reactor coolant pressure for its diverse Reactor Trip System parameter. Existing pressure transmitters, which are connected to the Reactor Coolant System and which are not part of the Reactor Trip System, l will be utilized to provide the signal to the diverse Trip System. These

transmitters, while not part of the Reactor Trip System, were purchased and

) installed as Class lE equipment. In that these transmitters are not part of

{ the Reactor Trip System, their usage in the ATWS System insures that the f existing Protection System will continue to meet all applicable safety-related criteria. Additionally, since these transmitters have the proper QA documentation for safety-related applications, their use will minimize the risk of safety system challenges and inadvertent reactor scrams, i

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, MAINE VANKEE ATOMIC POWER COMPANY The output signals from these transmitters will be utilized in a two-out-of-three logic matrix to develop a high Reactor Coolant System pressure trip signal. This signal will actuate existing Control Element Drive System motor-generator set output contactors, between the motor-generators and the reactor trip breakers. This will interrupt the motor-generator set output power to the control rod drives. A high reactor coolant pressure signal, set between the existing RPS high pressure trip and below the lowest pressurizer safety valve setpoint, will open these contactors, interrupt power to the control rod drives and initiate a reactor scram. This alternate trip system will be designed so that no single failure will result in an inadvertent reactor scram. On-line testability will also be designed into the system, which will allow testing of all equipment except the final actuated contactors.

DESIGN CONCEPT - ATWS MITIGATION The ATWS Mitigation System will initiate a turbine-generator trip and start the Emergency Feedwater System. Steam generator level has been selected as t'ie initiating parameter for this Mitigation System. Presently, there are four safety-related level transmittters on each of the three steam generators whica provide input to the Reactor Trip System. These twelve steam generator level instrument loops also provide signals through electrical isolators to initiate the Emergency Feedwater System in the event of a low water level in any one steam generator. These isolated auxiliary feedwater initiation signals will be used to trip the turbine-generator on a low steam generator water level. Although the ATWS Mitigation System will use the same transmitters that provide input to the Reactor Trip System, the electrical isolution provided in the existing systems minimizes the potential for adverse electrical interactions, will be implemented such that it does not degrade the existing Protection System, and will allow the existing Reactor Trip System to continue to meet all applicable safety-related criteria.

On-line testability will be designed into the system to allow testing of all equipment except the final actuated equipment.

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1 MAINE YANKEE ATOMIC POWER COMPANY e

SUMMARY

In summary, the diverse Reactor Trip System will utilize safety-related pressure transmitters that are not part of the Reactor Trip System, will be electrically independent of the Reactor Trip System, will invoke the physical separation requirements consistant with the Maine Yankee FSAR, will incorporate the QA guidance issued pursuant to the ATWS rule, will provide for testability, and will be operable during a loss of off-site power by using on-site power sources. The system will not be redundant within itself.

2. The diverse Reactor Trip System will be completed prior to startup from the Cycle 9/10 refueling outage.

The ATWS Mitigation System will be implemented such that the existing Protection System will continue to meet all applicable safety-related criteria, will utilize sensors which are presently part of the Reactor Trip System, will utilize the existing Energency Feedwater Initiation System which is electrically isolated from the Reactor Trip System and which minimizes the potential for common cause failures, will provide physical separation consistent with the Maine Yankee FSAR, will incorporate the QA guidance issued with the ATWS rule, will provide for testability, and will be operable during a loss of off-site power by using on-site power sources.

3. The ATWS Mitigation System will be completed prior to the startup from the Cycle 9/10 refueling outage.

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