Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 85-18
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 7, 1985
Information Notice No. 85-18: FAILURES OF UNDERVOLTAGE OUTPUT CIRCUIT
BOARDS IN THE WESTINGHOUSE-DESIGNED SOLID
STATE PROTECTION SYSTEM
Addressees:
All Westinghouse-designed pressurized water reactor power facilities holding
an operating license (OL) or a construction permit (CP).
Purpose:
This information notice is provided to alert recipients of a potentially
generic problem pertaining to short-circuit failures of the undervoltage
(UV) output circuit boards in the Westinghouse-designed solid state
protection system (SSPS). The failure of one UV output circuit would result
in the loss of automatic reactor trip redundancy; the unavailability of both
UV output circuits would result in the loss of the automatic trip function
of the reactor protection system and hence make the plant susceptible to an
anticipated transient without scram (ATWS) event.
It is expected that recipients will review the information contained in this
notice for applicability to their facilities and consider actions, if appro-
priate, to preclude similar failures from occurring at their facilities.
However, suggestions contained in this notice do not constitute NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances:
During a 4 1/2-year period, three UV output circuit failures have occurred
at North Anna Unit 2. As described later, Virginia Electric Power Company
(VEPCO) attributes these failures to inadequate surveillance, maintenance,
or modification procedures for the reactor trip breaker (RTB) switchgear
cubicle associated with the failed UV output circuit. The first failure
occurred in April of 1980, the second in May of 1983 and the third in
October of 1984. The first failure was detected and repaired before the
plant received an operating license. The second and third failures were
detected and repaired during refueling outages w and prior to returning to
power operation. Post-maintenance testing led to the timely, detection of
these last two failures.
In April of 1980, a short-circuit failure was detected in the B train UV
output circuit board at North Anna Unit 2. The Unit 2 maintenance history
for 1980 is no longer available, thus VEPCO cannot state with certainty that
this failure was maintenance related. However, because the UV coil for RTB-A
was replaced
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April 3, 1980 just before the B train UV output circuit failure, VEPCO
indicated that it is likely that some surveillance was performed on the B
train after the A train UV coil was replaced.
On May 13, 1983, a short-circuit failure was detected on the B train UV
output circuit board at North Anna Unit 2. In this instance, the failure was
preceded by maintenance activities on the bypass breaker. for RTB-A. Because
the UV trip coil for the bypass breaker for RTB-A is powered from the B
train UV output circuit, VEPCO has attributed this failure to a maintenance
action that inadvertently short-circuited the B train UV output circuit.
On October 29, 1984, another short-circuit failure was detected on a UV
output circuit board at North Anna Unit 2. In this instance, the failure was
preceded by the installation of the shunt trip relay, the relay that
actuates the automatic shunt trip feature of the RTBs; therefore, VEPCO
attributes this failure to inadequate plant procedures for implementing the
automatic shunt trip feature of the RTBs.
On January 12, 1985, while the plant was operating at approximately 96
percent power, RTB-A failed to open automatically on a valid reactor trip
signal at Sequoyah Unit 2; however, RTB-B opened automatically, causing the
control rods to insert. The failure was recognized immediately by the oper-
ator who then followed up the automatic scram demand with a manual scram
which opened RTB-B. This failure to trip was the direct result of a pre-
existing short-circuit failure on the A train UV output circuit. Sequoyah's
staff initially believed that maintenance had not been performed on external
components associated with the A train UV output circuit prior to this
event. The failure, therefore, was originally considered to have been a
random failure rather than a maintenance related failure. However, a
subsequent investigation revealed that surveillance activities had been
performed on the UV trip attachment of RTB-A prior to the event. As a
result, Sequoyah management now believes that a multimeter was being used to
check the voltage across the UV coil of RTB-A, and that the meter was set on
a current setting rather than on a voltage setting, thereby shorting out the
A train UV output circuit. In this regard, the Sequoyah failure appears to
be similar to those at North Anna.
Discussion:
The UV output circuit (see Attachment 1, UV Output Circuit) is designed so
that in the absence of a reactor trip signal from the universal logic boards
or switched inputs, the Darlington pair transistors Q3 and Q4 in the UV out-
put circuit will conduct current and energize the UV trip coil of the assoc-
iated RTB. A reactor trip input will result in the turning off of the Dar-
lington pair transistors Q3 and Q4, thereby interrupting current flow to the
UV trip coil of the associated RTB and causing the RTB to open. Similarly,
if the automatic shunt trip feature has been implemented, the turning off of
transistors Q3 and Q4 would also deenergize the associated shunt trip relay,
thereby tripping the same RTB by a diverse mechanism.
Failures of the type experienced at North Anna and Sequoyah have resulted in
the shorting of transistors Q3 or Q4. Such short-circuit failures are not
only
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IN 85-18
March 7, 1985
Page 3 of 3
not fail-safe (i.e., they do not cause a reactor trip) but they also would
prevent the functioning of any subsequent automatic reactor trip signal
because the failure would continuously maintain power to the UV trip coil of
the associated RTB. Since the Darlington pair transistors Q3 and Q4 are also
being used to control the automatic shunt trip feature of the RTBs, failures
of the type experienced at North Anna and Sequoyah would also incapacitate
the automatic shunt trip feature of the associated RTB. Thus, the coincident
failures of both the A and B train UV output circuits would disable both
automatic trip functions (UV trip and shunt trip) of both RTBs, thereby
exposing the plant to a potential ATWS event.
Because the UV output circuits are not continuously monitored for failures
and because each UV output circuit is functionally tested on a 60-day cycle,
one of the two redundant UV output circuits could be inoperable for as long
as 60 days before the failure was detected. Thus, although the probability
of an ATWS event occurring due to failed UV output circuit boards is very
low, the most likely scenario for such an event appears to be one where a
reactor trip demand occurs while one SSPS train is being tested and the
other train has a shortcircuited UV output circuit board. For example, if
the Sequoyah Unit 2 reactor trip of January 12, 1985 had occurred while the
B train SSPS was being tested, an automatic reactor trip would not have
occurred.
The described failures appear to be associated with maintenance activities
external to the SSPS, including the implementing of the automatic shunt trip
features of the RTBs. To preclude potential failures of the UV output
circuit boards while implementing the shunt trip feature, Westinghouse
informed North Anna that it is imperative that the SSPS UV output circuit
card be removed before performing such work. Prudent actions also would
include testing the UV output circuit by using the SSPS semiautomatic testor
after any maintenance work is performed on the associated RTB cubicle.
No specific action or written response is required by this information
notice; however, if you have any questions regarding this notice, please
contact the Regional Administrator of the appropriate NRC regional office or
the technical contacts listed below.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: I. Villalva, IE N. Merriweather, RII
(301) 492-9007 (404) 221-5577
Attachments:
1. UV Output Circuit
2. List of Recently Issued IE Information Notices