Bulletin 80-19: Failures of Mercury-Wetted Matrix Relays in Reactor Protective Systems of Operating Nuclear Power Plants Designed by Combustion Engineering
SSINS No.: 6820
Accession No.:
8006190022
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
July 31, 1980
IE Bulletin No. 80-19
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 July 31, 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 or not mercury-wetted relays
are used in 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 nuclear power plants using mercury-wetted relays
in 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. 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 mercury-wetted matrix relays in 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
this bulletin.
Approved by GAO, B180225 (ROO72); clearance expires July 31, 1980. Approval
was given under a blanket clearance specifically for identified generic
problems.
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