Information Notice No. 87-34: SINGLE FAILURES IN AUXILIARY FEEDWATER SYSTEMS
SSINS No.: 6835
IN 87-34
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 24, 1987
Information Notice No. 87-34: SINGLE FAILURES IN AUXILIARY FEEDWATER
SYSTEMS
Addressees:
All holders of an operating license or a construction permit for
pressurized water reactor facilities
Purpose:
This information notice is being provided to alert addressees to potential
single failures of auxiliary feedwater pump start and protective pump trip
circuitry that could cause partial or complete loss of capability to
supply auxiliary feedwater (AFW) in conflict with the design basis. It
is expected that recipients,will review the information for applicability
to their facilities and consider actions, if appropriate, to avoid
similar problems. However, suggestions contained in this information
notice do not constitute NRC requirements; therefore, no specific action
or written response is required
Description of Circumstances:
On April 30, 1987, the licensee for the Indian Point Unit 2 nuclear power
plant identified a potential single failure in a portion of the pump start
circuitry that is common to both motor-driven auxiliary feedwater pumps
and that could prevent both pumps from starting automatically, in the
event of either low-low steam generator level or loss of main feedwater.
Such a single failure is in conflict with the design basis for the
system
This problem was discovered in the course of a review of the auxiliary
feedwater system being conducted by the licensee as followup to a number
of operational events that occurred in the fall of 1986.
The pump start circuitry at Indian Point Unit 2 was designed so that the
steam generator level and loss of feedwater start signals were routed
through contacts of the safety injection inhibit relays. The purpose of
these relays is to delay pump starts under safety injection conditions
until the safety injection sequencer calls for the pumps to start at the
appropriate time. If the contacts of either inhibit relay failed in the
open position, neither the low steam generator level nor the loss of
feedwater start signals would cause the
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IN 87-34
July 24, 1987
Page 2 of 3
motor-driven pumps to start automatically. Both inhibit relays are
normally deenergized and closed. Therefore, for the contact to fail in
the open position, a failure in which the relay remains energized or in
which the relay sticks open following actuation would have to occur.
Discussion:
When the potential single failure was identified, the licensee entered a
72-hour action statement, as required by the Technical Specification
Limiting Conditions for Operation. Within the 72-hour period, the
licensee completed a design modification to provide independent inhibit
relays for the two motor-driven auxiliary feedwater pumps, which satisfied
the single-failure criterion.
Subsequent to the design deficiency determination at Unit 2, a review was
conducted at Indian Point Unit 3. The licensee at that unit determined
that although the pump start circuitry at Unit 3 was in fact different
from that of Unit 2, it also failed to satisfy the single-failure
criterion. The motor-driven auxiliary feedwater pump start logic
circuitry at Indian Point Unit 3 incorporates a pump start (delay) relay
that responds to either a main feedwater pump trip or low-low steam
generator level signal. Processing of either signal requires the
operability of this relay, which is common to both motor-driven auxiliary
feedwater pump start logic circuits. Failure of this relay could
similarly prevent both pumps from starting automatically. Unit 3 is
currently shutdown for refueling and corrective actions are still being
planned.
Previous Similar Occurrence
On March 7, 1985, the licensee for the Salem Generating Station Units 1
and 2 reported (LER 50-272/85-01) to the NRC that the auxiliary feedwater
pump (AFP) trip circuitry did not meet the single-failure criterion. The
licensee discovered this in an ongoing review of design changes in which
the AFP circuitry was modified to provide protection of all three AFPs in
the event of a loss of water supply resulting from tornado damage to the
auxiliary feedwater storage tank. The licensee's review of this trip
circuitry revealed several potential single failures that could stem from
a single test switch circuit, a single suction pressure instrument, and a
single low suction pressure trip output relay. Failure of any one of
these protective features could have resulted in tripping all three AFPs.
The licensee immediately took interim corrective actions to eliminate the
potential single failure vulnerabilities. Subsequently, the licensee
modified the low suction pressure pump trip circuitry to provide the
protective feature without the potential for a single failure causing a
loss of all AFW.
These conditions serve to highlight the potentially generic aspects of
single- failure vulnerabilities in the automatic start and pump
protection logic arrangements for auxiliary feedwater pumps
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IN 87-34
July 24, 1987
Page 3 of 3
This information notice requires no specific action or written response.
If you have questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: R. Gallo, RI
(215) 337-5225
Samuel D. MacKay, NRR
(301) 492-8394
Attachment: List of Recently Issued Information Notices
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