ML17157A048
| ML17157A048 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 02/23/1990 |
| From: | Anderson C, Barber G, Cheung L, Stair J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | |
| Shared Package | |
| ML17157A047 | List: |
| References | |
| 50-387-90-05, 50-387-90-5, 50-388-90-05, 50-388-90-5, NUDOCS 9003070295 | |
| Download: ML17157A048 (14) | |
See also: IR 05000387/1990005
Text
U.S.
NUCLEAR REGULATORY COMMISSION
REGION I
Report Nos.
50-387/90-05
50-388/90-05
Docket Nos.
50-387
50-388
License
Nos.
NPF-22
Licensee:
Penns
lvania Power
& Li ht
Com an
2 North Ninth Street
Allentown
Penns
lvania
18101
Facility Name:,Sus
uehanna
Steam Electric Station
Units
1 and
2
Inspection At:
Al 1 entown
Penn s
1 vani a
Inspection
Conducted:
Februar
3-9
1990
Inspectors:
Leonard
Cheung,
Senior
R
tor Engineer
Plant
Systems
Section,
ngineering
Branch, Divisions of Reactor Safety
da
e
G.
. Barber,
Senior
ide t Inspector,
dat
J.
R.
S ai,
R
s ent Ins
S
dat
Approved by:
C. J.
nderson,
Chief, Plant Systems
Section,
Engineering
Branch,
date
Ins ection Summa:
Ins ection
on Februar
3-9
1990
Ins ection
Re ort Nos.
50-387/90-05
and 50-388/90-05
Areas Ins ected:
A special
inspection
was conducted
to review licensee
actions
in response
to a loss of shutdown cooling event which occurred
on
February
3,
1990.
Results:
An Alert was declared
on February
3,
1990 due to
a loss of shutdown
cooling (SDC) for Unit
1 with the reactor coolant temperature
exceeding
200
degrees
F.
The loss of SDC was caused
by a failure of the "B" RPS bus
due to
a
ground fault i.n one of the circuit breakers.
The reactor coolant temperature
was stabilized at approximately
250 degrees
F and
SDC was returned to 'service
about
5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> later.
Onsite
and offsite emergency
response
organizations
were
activated in accordance
with the emergency
plans.
The licensee
actions
were
appropriate
and directed
toward plant safety.
Implementation of the emergency
plan onsite
was well coordinated
and effective.
900307(Qp~~ ygpp~(
ADOCK 0~0003~7
P11C
The root cause of the circuit breaker failure was promptly identified.
A design
modification of the Unit
1
power distribution panels
to deal with the root
cause
was completed before the conclusion of this inspection.
The licensee
made
an identical modification to the Unit 2
power distribution panels.
The Unit 2 modification was completed
on February
11,
1990.
The inspector
determined that the licensee's
corrective actions
were adequate.
No violations
were identified.
TABLE OF CONTENTS
~Pa
e
1.0
Introduction
2.0
Background
.
3.0
Sequence
of Events
.
0
~
~
~
1
4.0
Emergency
Plan Implementation
5.0
Reactor Protection
System
(RPS)
Power
Supply Electri
Design
.
5. 1
Power Distribution
5.2
Bus "8" Hardware Configurations
.
5.3
Previous
RPS Circuit Breaker Failures
5.4
Power
Supply Modifications
6.0
Findings
and Conclusions
cal
.
2
3
.
3
.
3
4
~
~
~
5
~
.
6
~
~
~
7.0
Exit Meeting
.
Appendix
1
Persons
Contacted
.
Figure
1, Circuit Breaker
CBSB Mounting Configuration
.
.
7
.
8
.
9
DETAILS
1 '
Introduction
During
a plant startup
on February
3,
1990 at 5:53 p.m.,
the licensee
declared
an Alert condition due to a loss of shutdown cooling
(SDC) with
the reactor coolant temperature
exceeding
200 degrees
F.
The loss of SDC
was caused
by the inability to restore
power to
a Reactor Protection
System
(RPS)
bus after it was deenergized
for surveillance testing.
Alternate
cooling was established
which stabilized the reactor coolant temperature
at approximately
250 degrees
F.
Shutdown cooling was reestablished
and
the core
was cooled to below 200 degrees
F by 12: 15 a.m.,
February
4,
1990,
The Alert was terminated at 12:25 a.m.,
February
4,
1990.
No radioactivity
was released offsite during the event.
'he
cause of the event
was
a ground fault on the "B" RPS bus.
An insulator
for one of the circuit breakers
on the
bus was found to be damaged,
which
provided
a shorting path to ground.
The licensee
evaluated
the
damaged
insulator
and determined that
a modification to the breaker
design
was
necessary.
Unit 2 scrammed
on February
6 due to an unrelated
problem.
A new type of distribution breaker
was installed in both of the Susquehanna
units prior to their startup.
~Back round
The Unit
1 reactor
had been previously
shutdown
on February
1,
1990 when
(EHC) system leak was discovered
on the Unit
1
number
4 control valve.
A power reduction
began at 8:55 a.m.,
February
1,
1990, to repair the leak.
The shutdown
was performed in a
controlled manner
by manually inserting individual control rods prior to
a manual reactor scram at
17 percent
power at 1:50 p.m.
Cold Shutdown
was
entered at 2:40 a.m.
on February 2,
1990.
It was necessary
to cooldown
below hot shutdown conditions,
since the
EHC leak could not be isolated
from the turbine bypass
system
and control of these
valves is needed for
decay heat
removal
in hot shutdown.
The licensee
was making preparations
to startup
from this short duration outage
when the loss of shutdown
cooling occurred.
3.0
Se uence of Events
During
a plant startup
on February
3,
1990 at 5:53 p.m., the licensee
declared
an alert condition due to a loss of shutdown cooling with reactor
temperatures
exceeding
200 degrees
F.
Prior to the loss of shutdown
cooling, the operators
were establishing
the conditions necessary
to test
the backup
power supplies for the "A" and "B" Reactor Protection
System
(RPS)
busses.
The licensee
expected certain
system isolations to occur as
a result of deenergizing
the "B" RPS bus,
such as,
(RWCU) and Residual
Heat
Removal
in the
SDC mode.
They prepared for the
isolations
and manually initiated the Standby
Gas Treatment
System
(SGTS).
One reactor recirculation
pump was started prior to the start of the
surveillance.
I
When the normal
power supply to the "B" RPS bus
was deenergized
for the
surveillance,
the backup
power supply circuit breakers
could not be closed
to reenergize
the bus.
The normal
and backup
supply breakers
were then
manually closed in sequence
but subsequently
tripped.
The deenergized
"B" RPS bus caused
a number of programmed isolations to occur,
including
the
system
and the reactor water cleanup
system
(RWCU).
The reactor coolant
began to heatup without the
SDC in operation.
The licensee
declared
an alert when the reactor coolant temperature
exceeded
200 degrees
F.
The loss of SDC procedure directed
the operators
to open
as
needed,
with the reactor coolant temperature
above
212 degrees
F.
Three
SRVs were opened.
The open
SRVs along with
the inflow from the control
rod drive system
in the feed
and bleed
mode,
were found to be sufficient to stabilize the reactor coolant temperature
at approximately
250 degrees
F.
Shutdown cooling was reestablished
at
11:02 p.m.
and the reactor coolant temperature
was below 200 degrees
F by
12:15 a.m.,
February 4,
1990.
The Alert was terminated at 12:25 a.m.
No
radioactivity was released offsite during the event
and
no personal
contamination
occurred.
4.0
Emer enc
Plan
Im lementation
At 5:25 p.m., the operations staff recognized that
an Alert condition would
be reached
following the loss of the "B" RPS
bus
and the inability to restore
shutdown cooling prior to reaching
200 degrees
F in the reactor coolant
system.
Callout for Technical
Support Center
(TSC) manning
commenced prior
to reaching
the Alert condition.
The Alert condition was correctly classified
and declared
at 5:53 p.m.
Accountability was initiated immediately following
the event declaration
and was completed within 29 minutes.
Notification
of the Alert was transmitted to the State
and County emergency
response
organizations
at 5:56 p.m.
and to the
NRC at 6:00 p.m.
At 6:30 p.m.
the
NRC Senior Resident
Inspector
was notified.
The
TSC was fully manned
and
activated at 6:45 p.m.
Assumption of command
and control by the
TSC occurred
at 7:30 p.m.
The Media Operations
Center
(MOC) was
manned at 7:25 p.m.
Operators
exhibited familiarity with the Abnormal Operating
Procedure
used
to provide
an alternate
method of Decay Heat
Removal during the loss of
shutdown cooling (SDC) and with the
Emergency
Plan
Implementing Procedures
used during the event.
The reactor coolant temperature
was stabilized at
approximately
250 degrees
F at about 8:00 p.m.
and maintained there until
SDC was restored
at 11:02 p.m.
Control of the plant was maintained at all
times throughout the event.
The Reactor Coolant temperature
was reduced
below 200 degrees
F at 12: 15 a.m.
on February
4,
and the Alert was terminated
at 12:25 a.m.
Overall
implementation of the emergency
plan onsite
was well coordinated
and effective.
However,
a few areas
where
improvements
could be
made
were noted with respect
to communications with the
news media during the
event
an'd should
be addressed
by the licensee
in order to preclude
any
future recurrences.
Specifically,
a delay in opening the
MOC exacerbated
the
news media desire for immediate information for ongoing events.
This
occurred
due to difficulty in contacting
the
on duty Public Information
Manager (PIM).
The first media contact
from the
MOC occurred at 7:45 p.m.
Although the licensee
has several
backup
PIMS, they were not immediately
available.
1
~P
imp
The licensee
held
a critique of their emergency
plan implementation
in
order to identify areas of weak performance
or other problems which had
been
encountered
during the emergency.
Their assessment
identified the
problems with communications,,with
the
news
media
and the
need to assure
that
a
PIM is available for press inquiries much earlier during an event.
A task force comprised of senior
management
and
headed
by the chief
executive officer has
been established
to explore various
ways of improving
communications with offsite agencies.
5.0
Reactor Protection
S stem
Power
Su
1
Electrical
Desi
n
5.1
Power Distribution
RPS bus "B" ( 120V AC single phase)
gets its power supply from 2 sources:
1) the normal
power supply from the
M-G set
"B" and 2) the alternate
power supply from MCC 18261 through
a step
down transformer.
A selector
switch located
on the main control board allows the operator
to select
the
power supply from either
one of these
power sources
through
2 contactors.
The
bus "B" power distribution is housed
in cabinet
1Y201B which contains
7 circuit breakers,
CB2B through
CB8B.
Circuit breaker
CB8B provides
control
power to the reactor trip system,
the nuclear
steam
supply shutoff
system
(NS4)
and
scram discharge
volume panel
"B".
When the incident
happened
on February
3,
1990, Unit
1 was in Condition
4 (cold shutdown).
The
RHR system
was in the shutdown cooling mode.
The
RHR inboard containment
isolation valve HV1515009
and outboard containment isolation valve HV151F008
were
open to provide the
shutdown cooling suction path.
When
bus "B"
lost power due to
a ground fault in circuit breaker
CBBB, several
of the
isolation valve interlocking relays
(K88 for the inboard valve and K95,
K92 and
K30 for the outboard valve) were deenergized,
causing
the inter-
locking contacts
to close.
The closure of these
contacts
allows the
valves'losing
coils to be energized,
thus closing the inboard
and outboard
isolation valves.
These
valves cannot
be reopened until power is restored
to
bus "B".
5.2
Bus "B" Hardware Confi uration
RPS bus "B" power distribution panel
1Y201B, about
3 feet wide and
4 feet
high, is classified
as non-safety related
equipment.
It contains
2
(K1B and K2B), one 2-pole,
200
amp circuit breaker
(CBlB, for
alternate
power supply),
two 100
amp circuit breakers
(CB2B and CB8B),
one
30
amp circuit breaker
(CB3B) and four 15
amp circuit breakers
(CB4B through
CB7B).
The two contactor s are operated
from the selector
switch in the
control
room to allow selection of the power source either from the "normal"
or the "alternate"
power supply.
The
seven single-pole,
type
TEB molded case circuit breakers
were mounted
on
a vertical metal
mounting plate.
Each of the circuit breakers
was supported
by two copper
extension bolts which were also
used for power connections,
one for power-in,
one for power-out.
For circuit breaker
CBBB, which had caused
the ground
fault problem,
the power-in bolt is about 3/8" in diameter
and 5" long
(see
Figure
1 for more detail),
One
end was secured
to the circuit breaker
and the other
end
was connected
to the power-in terminal lug.
The copper
extension bolt passed
through
a hole in the metal mounting plate.
An inner
cylindrical sleeve
about
30 mils thick made of impregnated
paper
separated
the copper extension bolt and the metal
mounting plate.
This sleeve
serves
as
an insulator
between
the power conductor (the bolt) and the metal
panel
which is at ground potential.
An outer sleeve
(the collar), slightly larger
and
much thicker than the inner sleeve,
also
made of impregnated
paper,
was
used for securing
the copper extension bolt.
The bolt was tightened
by lock-nuts,
compressing
the collar against
the metal
mounting plate.
The inner sleeve is brittle.
Damage'r
cracking of the sleeve
could
create
an electrical
path which in turn could cause
the insulation to
deteriorate,
leading to complete insulation failure,
In addition,
the
licensee
discovered that the inner sleeve of CB88 was shorter
than it
should
have
been,
leaving
a small portion of the copper extension bolt not
covered with insulation.
The licensee
determined that the ground fault
was
a result of the copper extension bolt touching the metal
mounting
plate.
The inner
removed
from CBBB indicated
excess
damage at one
end of the sleeve.
Part of the bolt was also
damaged
due to electric
arcing.
Excessive
leakage current from the bolt to the metal
panel
caused
the upstream circuit breakers
to trip, resulting in the loss of power to
bus "B".
5.3
Previous
RPS Circuit Breakers
Failures
The inspector
reviewed three
Licensee
Event Reports
(LERs) of previous
circuit breaker failures.
One applies
to Unit 1,
two apply to Unit 2.
LER 89-020 reported termination/mounting
problems with RPS circuit breaker
CB8B.
These
problems
caused
numerous
power interruptions
in the "B" RPS
distribution systems
of Unit l.
Each interruption resulted
in the actuation
of several
engineered
safety feature
(ESF)
systems
and components
(initiations of the Standby
Gas Treatment
System,
Control
Room Emergency
Outside Air Supply System,
isolations of HVAC Zones
I and III, Reactor
Water Cleanup
System
and Reactor Recirc
Pump cooling water isolation).
On
July 2,
1989,
the
ESF actuation
signals
occurred
three times,
caused
by
CB8B trips.
This circuit breaker
was subsequently
replaced.
On July 5,
1989,
the short circuit in the mounting stud of CBBB caused
the upstream
Electrical Protection
Assembly to trip, causing
loss of power to the
"B" bus.
The mounting stud was subsequently
reworked (changed
the orienta-
tion of the insulation sleeve),
LER 85-010 reported that
on February
23,
1985, circuit breaker
CB8B in
Unit 2
power distribution panel
2Y201B tripped prematurely three times,
each
caused
a reactor "half scram",
main
steam isolation valve logic
"half isolation", auto start of the 'B'tandby
Gas Treatment
System,
isolation of Reactor Water Cleanup
System,
closure of the containment
isolation valves for the Hydrogen-Oxygen Analyzers
and Containment
Radia-
tion Monitors and isolation of cooling water to the Reactor Recirculation
Pumps.
CBBB was subsequently
replaced.
LER 85-018 reported that
on May 29,
1985 circuit breaker
CB8B in 2Y201B
'ripped prematurely twice.
The circuit breaker
was subsequently
replaced.
These
LERs indicated that failures identical to the failure that occurred
on July 5,
1989 in
Unit 1 and similar failures
had occurred to the
trip logic circuit breakers.
For those
instances,
the licensee
corrected
the problems,
but they did not conduct
a rigorous root cause
analysis of
the failures.
This was
a result of the nonsafety classification of the
power
supply.
No loss of shutdown cooling occurred
as
a result of
these failures because
the reactor
was operating.
5.4
Power
Su
1
Modification
Following the February
3,
1990 loss of shutdown cooling incident,
the
licensee
decided to modify the design of the existing
RPS power distribution
panels
(1Y201A and
1Y201B).
On February
6,
1990,
the licensee
issued
Plant
Modification Package
DCP No. 90-2006.
The modification included the
following changes:
1.
Circuit breakers
CB3B through
CB8B, mounting plate
and hardware,
hot
bus
and standoff hardware
(CB2B is mounted separately
on
a non-metal
plate,
not affected)
were
removed
from panel
1Y201B.
2.
The above circuit breakers
were to be replaced with GE type
TEY
breakers,
mounted
on
a
GE 3-phase distribution panel
(P/N AEF31125B).
Only one
phase
was used.
3.
The
new panel
was to be mounted inside
1Y201B.
4.
The
same
changes
apply to panel
1Y201A, except the circuit breakers
to be replaced
were
CB2A through
CB7A.
(CB8A is mounted separately
on
a non-metal plate,
not affected).
5.
The modification was completely within panels
1Y201A and
1Y201B,
between existing circuit breaker
CBlA (for 1Y201A),
CB1B (for 1Y201B)
and existing load conductors.
The system logic was unchanged.
The above modification for Unit
1 was completed
on February 9,
1990.
In
order to shorten
the time during which power from the
RPS distribution
panel
was unavailable,
the licensee
installed
a temporary panel
which con-
tained
6 circuit breakers with current rating corresponding
to the
6
circuit breakers
to be replaced.
The
RPS power supply was transferred
to
the temporary
panel while the circuit breaker
replacements
were taking place.
The power supply transfer
was accomplished
through manipulation of the two
Electrical Protection Assemblies
upstream of the
RPS distribution panel.
A procedure
dated
February
2,
1990) entitled "Installation
and
Removal of Temporary
Power to
RPS Panels
Loads
1Y201A and
1Y2018 for
Performance
of Modification Work" had
been
developed
by the licensee
to
cover the power transfer activities, including circuit breaker testing.
The licensee
stated that the
power distribution panels for Unit 2 would
also
be modified.
The
same modification package
and the
same
procedure
for temporary
power transfer applied to Unit 2.
At the conclusion of this
inspection,
the modification work for Unit 2 had not been started.
However,
following the inspection
the inspector
was informed that the modifications
for both panels
(2Y201A, 2Y2018) were completed
on Sunday,
February
11,
1990.
The inspector
reviewed the modification package
and the procedure for
temporary
power transfer.
No deficiencies
were identified.
5.5
Ph sical Observation
of Unit
1
Power
Su
l
Modification
The inspector physically observed
the existing circuit breakers
and the
mounting hardware
before they were replaced,
and the
new circuit breakers
in Unit
1 after they were installed.
The inspector
observed
that the
mounting configuration of the
new circuit breakers
are substantially
different from the old ones.
The inspector determined that the
new
installation eliminates
the short circuit problem observed
in the old
circuit breakers.
The inspector also observed
the process
for the
RPS power supply to be
transferred
from the
bus "A" power distribution panel
to the temporary
panel,
including splicing of the affected cables.
No deficiencies
were identified.
6.0
Findin
s and Conclusions
The Senior Resident
Inspector
was contacted
at
home
and arrived onsite at
7:00 p.m.,
February
3,
1990.
The operators
completed actions
per procedure
ON-149-001,
Loss of RHR shutdown cooling mode
and continuously monitored
cooling effectiveness
to ensure
adequate
core cooling.
Other activities
were observed
throughout
the event.
It was noted that the licensee's
actions
were appropriate
and directed
toward plant safety.
The licensee
promptly recognized
that
an Alert condition would soon
be
reached
and
made the correct classification
and declaration.
In addition,
appropriate
emergency
response
organizations
were notified in a timely
manner.
The operators
were familiar with and properly followed the proce-
dures required for the event.
As a result, plant conditions were stabilized
in
a timely manner
pending recovery
from the event.
Overall implementation
of the emergency
plan onsite
was well coordinated
and effective,
and the
licensee's
handling of the event
was commendable.
10
Due to problemMs experienced
during the event,
the licensee
is taking
steps
to explore ways to improve communications with the
news media
and
local
emergency
response
organizations.
These
are
good initiatives
by the licensee
to ensure
timely and accurate
information is provided
to the public.
Following the loss of shutdown cooling,
the licensee
promptly determined
the cause
of the problem
and quickly initiated corrective actions to restore
the
shutdown cooling capability.
The root cause of the circuit breaker
failure was subsequently
identified.
A design modification of the Unit
1
power distribution panels
to deal with the root cause
was completed
before the conclusion of this inspection.
An identical modification was
made to the Unit 2
RPS power distribution panels.
The inspector
was informed
that the Unit 2 modification was completed
on February ll, 1990.
These
modifications should eliminate repeat
problems
in the future (identical or
similar circuit breaker failures occurred
in 1985 for Unit 2,
and in 1989
for Unit 1).
The inspector determined that the licensee's
corrective
actions for this event were adequate.
The licensee
completed
the review of their safety-related
equipment list
about
a week following conclusion of this inspection.
The licensee deter-
mined that
GE circuit breakers with similar mounting configurations
are
not used
in any safety-related
systems
in Susquehanna
Unit
1 or 2.
~Ei
M
At the conclusion of the inspection
on February
9,
1990,
the inspectors
met with the licensee
representatives
denoted
in Appendix
1.
The
inspectors
summarized
the
scope
and results of the inspection at that
time.
At no time during this inspection
was written material
given to the
licensee.
APPENDIX
1
Persons
Contacted
1.0
Penns
lvania
Power
& Li ht
Com an
J. Blakeslee,
Assistant
Supt. of Plants
R. Bogar, Electrical
Maintenance
Supervisor
AD Dominguey, Operation,
Senior Results
Engineer
D.
McGann,
Compliance
Engineer
- D. Roth, Senior Compliance
Engineer
- P.
Rusanowsky,
Compliance
Engineer
- E. Stanley,
Supt. of Plants
2.0
Commonwealth of Penns
lvania
DER/BRP
- S. Maingi, Nuclear Engineer
- D. Ney, Nuclear Engineer
- Denotes
those
personnel
present at the exit meeting
on February
9,
1990.
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