ML19281A273

Provides Requested Details of Limit Switch Operation & Potential Failure Modes.Continued Plant Operation Is Justified Since Environmentally Unqualified Limit Switches in Use Are in fail-safe Circuitry

ML19281A273
Person / Time
Site:	Vermont Yankee
Issue date:	02/28/1979
From:	Johnson W VERMONT YANKEE NUCLEAR POWER CORP.
To:	Office of Nuclear Reactor Regulation
References
WVY-79-23, NUDOCS 7903090269
Download: ML19281A273 (2)

Text

n VERhiONT Y AN KEE NUCLEAR POWER CORPOR ATION SEVENTY SEVEN OROVE STRCCT B.3.2.1 RuTLao, VERMONT 05701 R EPLY 70.

ENGINEERING OFFICE TURNPIKE ROAD WESTBORO. M ass ACHUSETTS 01581 T ELEPHC'4 E e 17 3 66 9011 k'VY 79-23 February 28, 1979 Director, Division of Operating Reactors Office of Nuclear Reactor Regulation United States Nuclear Regulatory Comm' sion k'ashington, D. C . 20555

Reference:

(1) License No. DPR-28 (Docket No. 50-271)

(2) USNRC letter to VY'!PC dated February 8,1979 (Bulletin 79-01)

(3) VYNPC letter to USTRC dated February 14, 1979 (24 Hour Notification)

Subject:

Environmental Qualification of Class lE Equipment, 14 Day Report Reference (2) requested that Vermont Yankee deteraire if certain types of stem mounted limit switches are used and located inside the containment. In reply, there are 4 MSIV's, each with 4 limit switches. Of this total of 16 limit switches, only 7 switches are unqualified. Reference (3) reported that these 7 switches are in use and provided justification for continued plant opera-tion. In accordance with Reference (2), the following details of the limit switch operation and potential failure modes are provided:

thin Steam Isolation Valve Position Indication One Namco Model SL-3C-llL limit switch is located on each of the four inboard main steam isolaticn valves to provide valve position indica-tion. There is no postulated failure mode of these switches that could interfere with the operation of the Emergency Core Cooling System, the Primary Containment Isolation System, or the Reactor Protection System.

The worst potential failure is a short to ground the switch. This could cause a fuse to blow in this circuitry resulting in loss of position indication of the in-board MSIV's. The same blown fuse would de-energize the MSlV DC pilot solenoid valve. A DC solenoid, in conjunction with an AC so]enoid, is required to de-energize in order to close the main steam isolation valve. Therefore, loss of power to the DC solenoid is a fail safe condition.

In the event that position indication of the in-board main steam isolation valves is lost during the Loss of Coolant Accident, Control Room personnel would survey other parameters to insure that containment isolation has been achieved. Other indicators of an isolated condition may be:

Ob303('o %

Director, Division of Operating Reactors March 2, 1979 Office of Nuclear Reactor Regulation Page 2 a) Closed outboard MSIV's b) No main steam flow c) Low main steam line pressure d) Closed turbine stop and bypass valves Reactor Protection Inrut - MSIV 10% closed Three Namco Model SL-3cM-7L li=it switches provide an input to the Reactor Protection System. Two of these switches are located on MSlV-80A and one on MSIV-80D. There is no postulated failure code of these switches that could interfere with the operation of the Etergency Core Cooling System, the Pr mary Containeent System, or the Reactor Protection System. The worst potential failure would cause a fuse te blow in this circuitry re-sulting in the de-energizaticn of a scram sensor relay. Due to system redundancy, this failure would neither cause nor prevent a scram from occurring.

The Reactor Protection System input is required only to anticipate the pressure and flux transient which would occur if these main steam isolation valveo should close during an operational event. This function is not required to cope with a Loss of Coolant Accident. Separate environ-centally qualified sensors prcvide these inputs to the Reactor Protection System. The closure of the redundant isolation valves outside the contain-ment provide the identical anticipatory signals to the Reactor Protection System.

In summary, continued plant operation is completely justified since the few environmentally unqualified limit switches that are presently in use inside the primary containment are in fail safe circuitry. As described above, there are no potential consequences from failure of these switches. However, we recognize that Control Room personnel require as cuch accurate information as possible in order to efficiently perform plant operations. Based on that, we will replace the environmentally unqualified switches inside the containtent during the next scheduled plant outage which is presently planned to commence in mid-March, 1979.

If you require further information, do not hesitate to call.

Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION

?? W W. p Johnsoh Vice President o

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