United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 80-19: Revision 1, Failures of Mercury-Wetted Matrix Relays in Reactor Protective Systems of Operating Nuclear Power Plants Designed by Combustion Engineering

                                                            SSINS No.: 6820 
                                                            Accession No.: 
                                                            8006190052 

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C. 20555 
                                     
                               August 13, 1980

                                                       IE Bulletin No. 80-19
                                                       Revision 1 

FAILURES OF MERCURY-WETTED MATRIX RELAYS IN REACTOR PROTECTIVE SYSTEMS OF 
OPERATING NUCLEAR POWER PLANTS DESIGNED BY COMBUSTION ENGINEERING 
BACKGROUND: 

This bulletin addresses the failures of mercury-wetted relays used in the 
logic matrix of the reactor protective system (RPS) of nuclear power plants 
designed by Combustion Engineering (C-E). Except for Arkansas Nuclear One 
Unit 2 and Palisades, both of which use dry-contact matrix relays, the NRC 
understands that all other operating C-E plants use C.P. Clare Model 
HG2X-1011 mercury-wetted matrix relays in the RPS. 

Mercury-wetted matrix relays manufactured by the Adams and Westlake Company 
were initially used in the Palisades plant; however, because of repeated 
failures of these relays, they were subsequently replaced with relays having
dry-contacts. GTE, the manufacturer of these dry-contact relays, however, 
has since discontinued their production. Thus, although the dry-contact 
relays used at Palisades have performed without a failure since they were 
installed, they are not available for the other operating nuclear power 
plants designed by C-E. 

OPERATING EXPERIENCES AND EVALUATION: 

To date, operating nuclear power plants designed by C-E have reported 
thirty-one (31) failures of mercury-wetted relays used in the logic matrix 
of the RPS. 

Most of the reported failures were "failed-closed" type (i.e., the type that
could inhibit a reactor trip), and four of the reported events involved 
multiple failures (i.e., three relay failures were detected during two 
tests; two other failures were detected during two different tests). Because 
of the redundancy within the RPS, no reported event would have prevented a 
reactor trip; however, the build-up of coincident "failed-closed" failures 
of certain sets of relays could result in trip failures for off-normal 
events. 

The number of single and multiple relay failures reported gives rise to two 
concerns: (1) the total number of failures yields a much higher random 
failure rate than that used in other relay failure estimates*, and (2) the 
number of 

  *  Other relay failure estimates include (1) WASH-1400, "Reactor Safety 
     Study", NRC, October 1975; (2) IEEE Std 500-1977, "IEEE Guide to the 
     Collection and Presentation of Electrical, Electronic, and Sensing 
     Component Reliability Data for Nuclear Power Generating Stations", 
     IEEE, New York; and (3) NUREG/CR-0942, "Nuclear Plant Reliability Data 
     System, 1978 Annual Reports of Cumulative System and Component 
     Reliability", NRC. 
.

IE Bulletin No. 80-19, Revision 1                           August 13, 1980 
                                                            Page 2 of 2 

multiple failures detected suggests the presence of a common-mode failure 
mechanism. Such a common-mode failure mechanism could result in the build-up
of specific "failed-closed" failures which, in turn, could result in 
anticipated transients without scram (ATWS). Thus, the relatively high 
random failure rate and the suggested common-mode failure mechanism, 
indicate that plants using mercury-wetted matrix relays in the RPS are more 
susceptible to scram failures than predicted in other studies. 

ACTIONS TO BE TAKEN BY HOLDERS OF CONSTRUCTION PERMITS OR OPERATING LICENSES
FOR NUCLEAR POWER FACILITIES: 

1.   Review your facility to determine whether C.P. Clare Model HG2X-1011 
     mercury-wetted relays are used in the logic matrix of the RPS. If no 
     such relays are used, you should submit a negative declaration to this 
     effect and you need not respond to the remaining items in this 
     bulletin. Your negative declaration shall be submitted to the 
     appropriate NRC regional office within thirty (30) days of the date of 
     this bulletin and a copy forwarded to the Director, Division of Reactor 
     Operations Inspection, Office of Inspection and Enforcement, NRC, 
     Washington, D.C. 20555. 

2.   Licensees of operating facilities using the above relays in the logic 
     matrix of the RPS should increase the frequency of their surveillance 
     tests. Until further notice, or until the mercury-wetted relays have 
     been replaced with qualified relays of a different design, surveillance
     testing of the relays shall be initiated within ten (10) days of the 
     date of this bulletin and repeated at intervals not exceeding ten (10) 
     days thereafter. The additional surveillance testing applies when 
     operability of the RPS is required by the Technical Specification. Upon
     detecting a failed relay, the failed unit shall be replaced with a 
     qualified dry-contact relay or a new mercury-wetted relay. (The removed
     relay shall not be reused in the RPS.) 

3.   Nuclear power facilities which are using or whose design includes the 
     use of the above relays in the logic matrix of the RPS shall submit 
     either their plans and schedules for replacing the mercury-wetted 
     relays with qualified relays of a different design, or justification 
     for using the mercury-wetted relays. Responses to this item shall be 
     submitted to the offices listed in Item 1, above, within ninety (90) 
     days of the date of the original version of this bulletin, July 30, 
     1980. 

Approved by GAO, B180225 (ROO72); clearance expires July 31, 1980. 
(Application for renewal pending before GAO.) Approval was given under a 
blanket clearance specifically for identified generic problems. 
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