ML20011A752

Submits Addl Info Re Independence of Four Channels of Reactor Protection & Engineered Safeguards Actuation Sys Measurement Logic,In Response to NRC 810416 Ltr

ML20011A752
Person / Time
Site:	Millstone
Issue date:	10/23/1981
From:	Counsil W NORTHEAST NUCLEAR ENERGY CO.
To:	Clark R Office of Nuclear Reactor Regulation
References
A01658, A1658, TAC-7089, NUDOCS 8110290518
Download: ML20011A752 (4)

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00T2 91981 )8 u.x,ta urnerr m o n H g sawuoet p (p October 23, 1981 Idi Docket No. 50-336 A31658 Director of Nuclear Reactor Regulation ATTN: Mr. R. A. Clark, Chief Operating Reactors Branch #3-U. S. Nuclear Regulatory Commission Washington, D. C. 20555

REFERENCES:

1. R. A. Clark letter to W. G. Counsil, dated April 16, 1981.

2. D. C. Switzer letter to G. Lear, dated September 21, 1977.

Gentlemen:

Millstone Nuclear Power Station, Unit No. 2 Reactor Protection and Engineered Safeguards Systems Actuation Logic In Reference 1, the NRC Staff informed Northeast Nuclear Energy Company (NNECO) of the continuing review of the Reactor Protec-tion System (RPS) inoperable channel condition at Millstone Unii, No. 2. The Staff requested that NNECO prc71de additional informa-tion in support of our current Technical dpecifications which permit unlimited bypass of one inoperable RPS channel. NNECO was also informed that the Staff's review has been expanded to include the Engineered Safeguards Actuation System (ESAS).

As noted in Reference 1, NNECO provided the Staff with informa-tion to support oper_ tion of the RPS in a two-out-of-three logic configuration, with an installed spare channel, in Reference 2.

It remains NNECO's position that the RPS as well as the ESAS is designed for a two-out-of-three logic configuration with an installed spare. As such, the current Technical Specifications which permit indefinite bypass of one channel of either the RPS or ESAS are appropriate.

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As requested by Reference 1,-NNECO.-hereby provides the following information with regard to the independence.of the four channels of both the-RPS and ESAS: measurement logic.

The' construction permit for Millstone Unit No. 2 was issued on December 11, 1970. Accordingly, the RPS and ESAS were: designed and constructed to the general requirements of the following IEEE Standards.

IEEE 279-1971 Criteria forfProtection Systems for Nuclear Power Generating Stations IEEE 323-1971 General Guide for. Qualifying Class I Electrical Equipment for Nuclaar Power-Generating Stations IEEE 338-1971 Periodic Testing of' Nuclear Power Generating Stetions Protection Systems IEEE 344-1971 Seismic. Qualification of Class I 31ectrical Equipment for Nuclear Power Generating Stations In addition, the requirements of IEEE Standards 308-1971, 336-1971, and 338-1971 were adhered to for both' systems.

l Physical and electrical separation of the circuits of the RPS is described in FSAR Sections 7.2.5 and 7.2.6.4. A detailed description of the physical-separation of the RPS channels was provided in Reference 2. This included "as-built" cable / raceway drawings marked up in color to illustrate the physical separation between channels of three~ typical RPS measurement parameters.

Spatial separation between cable trays carrying redundant cables is normally not less than four feet vertically and eighteen inches horizontally. Where these spacings between trays and redundant systems cannot be maintained, barriers are provided to preserve.

the physical and electrical.iategrity of the cables.

Each channel of the RPS is routed through a separate containment--

electrical penetration assembly. In the control room, each channel is located in a separate compartment. Mechanical and thermal barriers exist between these' compartments to preclude common event f ail ures.

Physical separation of the redundant channels of the ESAS is equivalent to that of the RPS and is described in FSAR Section e 8.7.

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NNECO has performed a' review of the failure modes which could a#~3ct the-ability of either the RPS or ESAS to perform as signed. No single. failure has been identified which would tevent either the RPS or ESAS'from completing its-design function. The following discussion is provided.

e A loss of signal for any reason will initiate a protective channel trip.

e Shorting of signal leads to each other at the bistable input. leads is equivalent to a loss of signal. Shorting-to a voltage source has no effect as the signal circuits are ungrounded. Single grounds of the signal circuit have no effect. Periodic surveillance of the systems assure that the circuits remain ungrounded.

o An open circuit in the signal leads is equivalent to a.

loss of signal.

e Four DC/AC inverters power the four vital instrument buses which provide independent 129 volt AC power for each measurement channel in the RPS and ESAS. Two inverters are supplied by the Division 1 safeguards battery and two inverters are supplied by-the Division 2 battery. . A loss' of either battery results in an undervoltage trip in the RPS by electromechanical means and is described in detail in Reference 2. An undervoltage condition in the ESAS will result in a trip of the.affected channel (s)~. The loss of one

- safeguards battery will not-inhibit the actuation of any safeguards function as redundant actuation logic is powered from the remaining safeguards hattery.

e A loss of DC power event occurred at Millstone Unit No. 2 on January 2, 1981 when the main feeder breaker connecting one safeguards battery and its charger outputs to a vital DC bus was inadvertently opened. -This event resulted in a reactor trip, as designed, since the reactor trip breakers trippad on undervoltage. This event demonstrated the adequacy of the design in this regard.

e NNECO considers an overvoltage condition on two RPS or ESAS measurement channels to.be an incredible event. To postu-late a significant ov(rvoltage event one would have to assume both chargers asscciated with one safeguards battery failed in such a way as to result in significant overvoltage to the two inverters. The design of the inverters provides for an output voltage regulation of 2 volts with an input voltage of up to 140 volt DC.

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k Based on.the review of the physical and electrical separation of o

the RPS,and ESAS channels, the possible_ failures which may.'ccur in either system,and the original design of the RPS'and ESAS, it remains'NNECO's position that current Technical-Specifications

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permitting the RPS and ESAS to operate.in a two-out-of-three-logic. configuration are appropriate. This position is'further-reinforced'by the fact that the Staff has. determined..that the1RPS design at numerous domestic Westinghouse ~ nuclear steam ~ supply systems utilizing a three channel RPS with two safety related battaries is acceptable. To require Millstone Unit No. 2 to Loperate in a mode beyond its design. basis would appear to. discourage licensees from innovations or improvements in design by. penalizing such innovation.

We trust you find this information responsive to the Reference 1 requests.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY-NUA W. G. Counsil Senior Vice President 4

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