ML20029A726

Provides Util Resolution for Achieving Diversity for Alternate Rod Injection Sys Per ATWS Rule (10CFR50.62).Util Will Replace Rosemount Trip Units W/Ge Trip Units During mid-cycle Outage Scheduled for Mar 1992

ML20029A726
Person / Time
Site:	Pilgrim
Issue date:	02/25/1991
From:	Gina Davis BOSTON EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BECO-91-021, BECO-91-21, NUDOCS 9103040145
Download: ML20029A726 (4)

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. 11 EOSTON EDISON P90nm Nuclear Power $tation Hocky Hill Road l Plymouth. Ma$58chusett6 02360 George W. Davis senio, vee noe.nt - Nuciat BECo 91 021 )

U.S. Nuclear Regulatory Commission Document Control Desk Hashington, DC 20555 ,

License DPR-35 Docket 50-293 ,

ATHS Rule, 10CFR50.02: Resolution to Alternate Rod Injection (ARI) Diversity Issue - l (i) Schedule for Rosemount Trip Units Replacement and (ii) Exemption R1 guest for Actiuttiga,, Belays Reolacement .

References:

(1) NRC Safety Evaluation Report, " Compliance with ATHS Rule 10CFR50.62",-dated June 6, 1989.

-(2) NRC Letter,; James H. Taylor to George J. Beck, Chairman, BHR Owners Group.. dated September 20, 1990. (Received by BECo on ,#

October 5, 1990).  ;

'(3) Long Term Program (LTP) Report BECo Letter to NRC #90-149, '

dated December 3, 1990.

This letter provides our-resolution for achieving diversity for Pilgrim Nuclear Power Station's Alternate Rod Injection System for compliance with the requirements of the Anticipated Transients:Hithout Scram (ATHS) Final Rule, ,

The ATHS Rule requires Boston Edison Company-(BECo) to implement an Alternate  :

Rod' Injection (ARI) system; a Standby Liquid Control System (SLCS) and to trip the-Reactor Recirculation Pt 4 (RPT) automatically under conditions intitcative  ;

of an ATHS. The Rule mandated schedule _to complete these modifications is

' Refueling Outage-(RF0) #8.=-The Rule also requires an explanation of the-schedule along with-a jus'ification if _the final implementation schedule:goes beyond RF0'#8. ,

BECo b d prev Misly implemented ARI and RPT. requirements in response-to NRC NUREG-0460,: Ihe SLCS was implemented-during RF0 #7, BECo provided information '

to'the NRC-regarding compliance with ATHS requirements through the BHR Owners-(Group Topical Report, NEDE-31096-P'and other submittals (see Reference 1 *

. Section 7.0). The NRC staff performed _a post-implementation review of: ARI and RPT portions of the'ATHS modifications'to determine compliance with the rule.

The NRC Safety Evaluation Report (SER) (Reference 1) concluded the Rosemount trip units and the actuation relays for the Pilgrim ARI system are not' diverse from the Reactor Protection System (RPS) and requested BECo provide a schedule for resolution of the diversity _ issue after NRC staff guidance is provided.

Concurrently, BECo sought to resolve the diversity issue'through the BHR C , Owner's-Group (BHROG). i 910304o145 910225 " 8g J (' ' 1-PDR ADOCK 05000293 P-PDR I0--..-.-.a

, BOSTON EDISON COMPANY U.S. Nuclear Pepulatory Commission j Page 2 l

The NRC has since resolved the diversity issue through issuance of a generic  ;

position to the BHROG in Reference 2. Also, Reference 2 states that the NRC will request licensees to propose a schedule for achieving compliance with the diversity requirement. This resolution to the diversity issue was made available to us by our NRC Project Manager on October 5, 1990.

BECo has re-reviewed the staff's position on the diversity requirement, the NRC l generic position presented to the BHROG in Reference 2, and the existing diversity afforded by the Pilgrim ARI system (as presented to the NRC staff in a meeting on December 9, 1989). Accordingly, in accordance with the SER I (Reference 1), our resolution to the diversity issue includes replacement of Rosemount Trip Units and an exemption request for Actuation Relays. These are discussed below.

(1) Raphitment of Rosemount Trip _Dalti: He will replace the Rosemount trip units with General Electric trip units during the mid-cycle outage presently scheduled for March 1992. This schedule is based on the lead-time needed to procure, install and test circuit boards and our existing schedule of commitments detailed in Reference (3). This schedule will be included in our next update to the LTP Report.

(ii) Lx1@pl191LRtquest for Actntion Relan: He request an exemption from the stPff position requiring the replacement of actuation relays. The basis for our exemption request is as follows:

a) The Agastat (manufactured by Amorace Company) relays installed in the ARI and RPS systems are simple devices compared to the Rosemount Trip Units and the failure modes of these relays are more completely understood.

These relays achieve diversity as defined in NUREG/CR-5460, "A Cause-Defense Approach to the Understanding and Analyses of Common Cause failures". This document defines diversity as either Equipment diversity, functional diversity or (Operating or Maintenance) Staff diversity. The PNPS ARI and RPS systems are functionally diverse because the relays have diverse energization states between the two systems. These relays are configured to perform their respective safety functions via opposite energization states. RPS relays are normally energized and must be de-energi7ed to insert control rods, whereas ARI relays are normally de-energized and must be energized to insert control rods. This type of functional diversity based upon opposite energization states for actuation of relays provides no evidence of a credible common mode failure which will prevent both relays from changing contact position to prevent insertion of control rods.

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EOSTON EDISON COMPANY U.S. Nuclear Regulatory Commission Page 3 b) He have reviewed the industry relay failure data available to date. This includes experience at PNPS, industry wide experience as documented in the NPRDS data base and manufacturer's experience. He have concluded that.no applicable common mode failure of these relays with opposite energization states has been evidenced by the failure data. The only potential common mode failures which have been identified in the design of these relays which would impact the diverse energization states is physical damage to the mechanical portion of the relays in their normal "non-initiating" positions. No positive individual accounts of this type of failure have been reported to NPRDS or the manufacturer. Also, no multiple or simultaneous (common mode) failures have been reported.

Of the 78 failures of this type of relay reported to NPRDS, 57 are not applicable to PNPS as they were due to the failure of relay coils to make proper contact or due to failuri. of the reiay coils caused by external conditions. None of these failure modes would have prevented both RPS and ARI from performing their safety functions. The remaining 21 reported failures were all single isolated incidences, not multiple simultaneous failures. This data shows that no poplicable common mode failures have been reported for this type of relay.

Common mode failures of other types of relays (e.g., HFA relays) in the industry have been due to prolonged energization states. The ATHS precursor event at Kahl (nuclear facility in Germany) was caused by common mode scram relay failures. This common mode failure was caused by a manufacturing defect which allowed the localized warmth due to prolonged energization to cause the release of a sticking agent in the relays. This sticking agent caused the mechanical portion of the relays to stick and not change position. This common mode mechanical coiaponent failure of relays would have been negated by the functional diversity of the relays in the PNPS ARI and RPS systems.

As discussed above, a common mode failure either due to manufacturer fault or power failure would actuate either an ARI or RPS trip function. Therefore, the existing relays by their design to actuate in opposite energ12ation states meet the underlying purpose of the ATHS Rule and replacement of these relays is not necessary. The cost of the significant design change needed to replace the relays is not justified based upon the low probability of an ATHS event compounded by the extremely low probability of an unknown common mode relay failure. Therefore, pursuant to the provisions of 10CFR50.12(a)(2)(li) and (iii), Boston Edison Company hereby requests specific exemption from the staff's stated position requiring replacement of the relays.

Should you have any questions regarding our exemption request, please contact our Licensing staff.

, CW4 .gg n G. H. Davis cc: See Page 4 l

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. BOSTON EDISDN COMPANY U.S. Nuclear R4gulatory Commission Page 4 cc: Mr. R. Eaton, Project Manager Division of Reactor Projects - !/II Office of Nuclear Reactor Regulation Mail Stop: 1401 U. S. Nuclear Regulatory Commission 1 White filet North 11555 Rockville Pike Rockville, HD 20852 U. S. Nuclear Regulatory Commission Region I 475 Allendale Road King of Prussia, PA 19406 Senior NRC Resident Inspector Pilgrim Nuclear Power Station HGL/ rec /5258 1

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