ML20154A141

Safety Evaluation Supporting Amend 106 to License DPR-36

ML20154A141
Person / Time
Site:	Maine Yankee
Issue date:	09/07/1988
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20154A126	List: ML20154A121 ML20154A131 ML20154A141
References
NUDOCS 8809120086
Download: ML20154A141 (3)

Text

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y[fRio 'o,g UNITED STATES NUCLEAR REGULATORY COMMISSION

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOP REGULATION RELATING TO AMENDMENT NO. Ing; TO FACILITY OPERATING LICENSE NO. DPR-36 MAINE YANKEE ATOMIC POWER COMPANY MAINE YANKEE ATOMIC POWEP STATION DOCKET NO. 50-309 INTRODUCTION By letter dated November 7, 1986. Maine Yankee Atomic Power Company (MYAPCo) requested a safety evaluation concerning the adequacy of their offsite power system for the Maina Yankee Atomic Power Station (MYAPS). In December 1982, a low voltage condition occurred while the plant was starting up and was being supplied only by the 115 kV Surowiec offsite power line. Another offsite power line, the 115 V Mason line was out of service at the time. A similar low voltage event occurred in July 1986.

On August 13, 1988 Station auxiliary loads did not automatically transfer from the nomal source (main generator) to the "reserve" source (offsite power) following loss of the nomal source due to a main transformer fault.

This is significant because (even though offsite power was available) unreliable or unstable sources of offsite (preferred) power can result in their unavailability to support equipment responding to transient / accident conditions, and can lead to unnecessary and excessive challenges to the onsite power system (e.g., diesel generators, circuit breakers, load sequencers, engineered safety feature system pumps, etc.)

The emergency buses at MYAPS are supplied from the main generator during nomal plant operation via two separate auxiliary transfomers. During plant  ;

startup, or upon the loss of the main generator the emergency buses are supplied from the 115 kV lines to the Mason and Surowiec substations.

Voltages from these substations are stepped down to 115 kV at the substations and then supplied to the 115 kV switchyward at the Maine Yankee plant. In I the uvent that both 115 kV lines are lost, the emergency buses can be supplied I within six hours by disconnecting the main generator from the main transfomers I and backfeeding the auxiliary t~ansfomer from the 345 kV transmission line.

Furthemore, the plant technical specifications require only one 115 kV line to be operable whenever the reactor coolant system temperature and pressure exceeds 210'F and 400 psig, h912OOG6000907 p ADOCK 05000309 PNU ,

2 By letter dated December 23, 1987, MYAPCo stated, using feasonally adjusted worse case load predications from Central Maine Power (CMP) for the Surowiec line and maximum predicted in-plent loading, that the calculated voltages on the 4.16 kV and 480 V buses drop below acceptable levels. The study reconfirms, however, that the Mason 115 kV line has adequate capacity and capability to provide Maine Yankee's auxiliary power supply requirement. In that letter it was stated:

"To correct the low voltage condition at the plant that could occur if the Surowiec 115 kV line, under peak load conditions, was being utilized to provide the plant's auxiliary power supply requirements,

1. Maine Yankee will install a capacitor bank at the plant's 115 kV switchyard.

"The capacitor bank will provide significant reactive power (VAR) relief for the Surowiec line resulting in satisfactory voltages at Maine Yankee even under the scenario stated above. The capacitor bank is expected to be installed and in service by October 1988.

"Pending the completion of this modification, we will not rely on the Surowiec line to meet the technical specification requirements for offsite power during periods of peak loads."

By letter dated June 29, 1988 MYAPCo proposed a Technical Specification change providing limiting conditions for operation for the electrical power system to prohibit reliance on the 115 kV Surowiec line unless additional justification is provided to the NRC. That restriction would remain in effect until a new capacitor bank is placed in operation.

2.0 8ACKGROUND AND DISCUSSION The existing offsite power system at MYAPS satisfies the reouirements set forth in General Design Criterion 17 when the 115 kV Mason line is one of the lines in service. When the Surowiec line is the only 115 kV in service, there are two situations where under worse case low voltage conditions the capacity of the line has not been demonstrated to be adequate. One is following a unit trip concurrent with or without an accident. Under this condition, it has not been shown that there is sufficient capacity in the line to supply all connected non-safety loads in addition te the emergency loads required to mitigate the consequences of the accident or to shutdown the reactor. The other situation concerns the capacity of the Surowiec line to startup the unit which was demonstrated to be insufficient during the low offsite power voltage event in December 1982. MYAPCo plans to install a capacitor bank to improve the voltage of the Surowiec line. With the capacitor bank operational, it is expected that the Surowiec line will be capable of maintaining adequate voltage while supplying peak auxiliary and safeguards loads under all expected conditions. Since there are conditions under which the Surowiec line may not be adequate, until the capacitor bank is installed, the technical specifications require the licensee not to rely on the Surowiec line to satisfy operability requirements until the capacitor bank becomes operable.

b e 3.0 EVALUATION The proposed change is administrative in nature formally requiring implementation of existing procedures in the technical specifications.

3.1 S,UMMARY The proposed change would modify Technical Specification 3.12. "Station Service power", which provides limiting conditions for operation for the electric power system. The proposed change prohibits reliance on the 115 kV Surowiec line unless additional justification is provided to the NRC. This restriction wculd remain in effect until a new capacitor bank is placed in operation. This justification could be related to the duration of time involved, reduced plant loads, reduced 115 kV line loads or other considerations.

The proposed change will not remov0 or relax any existing requirement related to the probability or consequences of accidents previously considered and does not involve a significant hazards consideration.

The proposed change will not remove or relax any existing requirement needed to provide reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner. The staff, therefore, finds the proposed change acceptable.

4.0 ENVIRONMENTAL CONSIDERATION

This amendment involves changes in the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes in inspection and surveillance requirements. The staff has determined that the amendment involves no significant increase in the amount and no significant change in the types of any effluents that may be released offsite and that there is no significant increase in individual or cumulative occupational radiation exposure. The Comission has previously issued a proposed finding that this amendment involves no significant hazards consideration and there has been no public coment on such finding.

Accordingly, this amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR Section 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of this amendment.

5.0 GENERAL CONCLUSION We have concluded, based on the consideration discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner; and (2) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the comon defense and security or to the health and safety of the public.

Principal Contributor: P. Sears Dated: September 7, 1988 ,