Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
ACCESSION #: 9405180436
LICENSEE EVENT REPORT (LER)
FACILITY NAME: CRYSTAL RIVER UNIT 3 (CR-3) PAGE: 1 OF 07
DOCKET NUMBER: 05000302
TITLE:
Due to a Lack of Engineering Review, Motor Operated Valves
With Brakes Could Fail to Perform Their Safety Function
Under Degraded Voltage Conditions
EVENT DATE: 07/06/93 LER #: 93-008-01 REPORT DATE: 05/12/94
OTHER FACILITIES INVOLVED: N/A DOCKET NO: 05000
OPERATING MODE: 1 POWER LEVEL: 100
THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR
SECTION:
50.73(a)(2)(ii)
LICENSEE CONTACT FOR THIS LER:
NAME: K. R. Wilson, Nuclear Licensing TELEPHONE: (904) 563-4549
Manager
COMPONENT FAILURE DESCRIPTION:
CAUSE: SYSTEM: COMPONENT: MANUFACTURER:
REPORTABLE NPRDS:
SUPPLEMENTAL REPORT EXPECTED: NO
ABSTRACT:
On July 6, 1993, Crystal River Unit 3 (CR-3) was in Mode 1 (POWER
OPERATION) at 100% Rated Therma
The
valve therefore may not be capable of performing its safety function.
This is a condition outside the design basis. The cause was a lack of
engineering review for the motor brake voltage requirements. On April
11, 1994, a periodic revision to the 1991 calculation identified an
additional 4 valves which may not function under degraded grid voltage
coincident with accident conditions. The 1994 revision was more
conservative than the original calculation and contained information
unavailable in 1991. Some of the corrective actions included locking
valves in place, brake removal, further analysis to assure performance
of valve safety functions, and a plant modification to eliminate a
valve safety function.
END OF ABSTRACT
TEXT PAGE 2 OF 7
EVENT DESCRIPTION:
On June 6, 1993, Crystal River Unit 3 (CR-3) was in Mode 1 (POWER
OPERATION) at 100% Rated Thermal Power producing 863 Megawatts. During
the process of reviewing the draft dedication and seismic qualification
plan for a spare motor and brake assembly for main feedwater crosstie
valve FWV-28 [SJ,ISV] a Limitorque motor operated valve (MOV), a
discrepancy was identified between the acceptance criteria for the
electrical brake minimum operating voltage versus the valve motor
minimum voltage acceptance criteria. A review was conducted to
determine the design requirements for the brakes and to identify other
safety related MOVs with similar brakes. Six other safety related
valves were found with this condition.
On June 28, 1993, a concern was then identified for the operability of
these valves. A calculation performed in 1991 showed that under
degraded voltage conditions, valve motors will have sufficient voltage
to accelerate their load. However, the motor brakes did not receive
degraded voltage operation review. The brake function is to protect
the valve from excessive mechanical thrust by stopping motor and valve
inertia after power cut off. The brake coil [CL] is connected in
parallel to the motor. The braking action is released with the
energizing of the brake coil. Upon power cut off, the coil is
deenergized and the brake clamps onto the valve motor shaft. The motor
brake has a more restrictive operating range of 460 Volts Alternating
Current (VAC) +/- 10 percent than analyzed for the valve operator.
This more restrictive motor brake operating voltage range results in
the possibility the MOVs may be exposed to voltages within design
operating ranges but lower than required to operate the brake coil.
With insufficient voltage to release the brake coil, the valves could
be unable to perform their safety function.
Limitorque Maintenance Update 92-2, Section 7 "Significance of Motor
Brakes on Actuator Motors" indicates that the brake does not
substantially minimize the inertial overshoot of the valve drive train
for this type of valve. The vendor confirmed this type of valve will
function properly without brakes.
On July 6, 1993, CR3 was in Mode 1 at 100 percent Rated Thermal Power
producing 859 Megawatts. At 1700 on that day, MUV-58 [BQ,ISV], the
normally closed High Pressure Injection (HPI) [BQ] suction valve from
the Borated Water Storage Tank (BWST) [BP,TK], was declared inoperable
and the Action Statement for Technical Specification (TS) Limiting
Condition for Operation (LCO) 3.5.2, ECCS Subsystems - Tavg > 280
degrees F, was entered. The TS was exited on July 7, 1993 at 0405 upon
removal of the stationary and rotating discs for MUV-58 which
permanently de-coupled the motor from the brake assembly. The
electrical power connections to the brake were also removed. This
modification was performed on MUV-73 [BQ,ISV], the normally open BWST
suction valve, as well.
TEXT PAGE 3 OF 7
On April 11, 1994, CR-3 was in MODE 5 (COLD SHUTDOWN), with reactor
coolant system temperature at approximately 115 degrees Fahrenheit.
The plant was engaged in a refueling outage. Engineering personnel
were performing periodic revision of calculations. Revision 1 to the
previously completed motor starting calculation, which would include
MOVs not considered in the previous revision, was in progress. The
original calculation (Revision 0 of CALC E-91-0018) which only
addressed block loaded motors, including MOVs, was being modified in a
conservative direction by applying a series of factors not available
during performance of the original calculation. Block loading is the
sequential connection of groups of devices to a bus. These factors
included the following:
A. Motor overload resistance;
B. A higher impedance of Engineered Safeguards (ES) bus [EA,BU] B
4160/480 transformer [EA,XFMR);
C. More accurate, but limiting, motor starting power factor;
D. More accurate vendor supplied actual cable impedance;
E. Motor torque correction factors based on Limitorque 10CFR21
report;
F. Locked rotor heating; and
G. A higher Second Level Undervoltage Relays (SLURs) [27]
setpoint, which offset some of the negative impact of the above
stated factors.
Since Revision 0 of the motor starting calculations did not model FWV-
28 (see Attachment - Simplified Main Feedwater Flow Diagram), an
estimate of voltage available at the motor terminals was made by
comparison with a similar valve addressed in Revision 0 of the
calculations. The estimate of 345 VAC was adequate to assure proper
operation of FWV-28, including brakes. Presently unverified motor
starting calculations now model FWV-28 utilizing the Revision 1
criteria. These calculations indicate that only 307 VAC would be
available at the motor terminals during accident conditions coincident
with degraded grid voltage. Although the 307 VAC at the motor
terminals still allows the valve operator to exceed minimum required
thrust values, the MOV brake release requires 345 VAC. The inability
of the MOV brake to release would prevent the motor from operating and
performing its intended safety function of closing on main feedwater
isolation.
Revision 1 of this motor starting calculation (presently unverified)
also indicates that three other MOVs would not have sufficient voltage
at their motor terminals to develop minimum required thrust during
accident conditions coincident with degraded grid voltage conditions.
Main feedwater suction isolation valves FWV-14 and FWV-15 [SJ,ISV] will
not receive adequate voltage under accident conditions coincident with
degraded grid voltage during its opening stroke. However, the closing
stroke is critical to safety by isolating main feedwater suction, and
the motor does have sufficient reduced voltage capability during its
closing stroke. Therefore FWV-14 and FWV-15 do not present any safety
concern.
TEXT PAGE 4 OF 7
FWV-30 does not have sufficient voltage capability at its motor
terminals at the beginning of the closure stroke, under accident
conditions coincident with degraded grid voltage, to equal or exceed
minimum required thrust requirements. However, sufficient voltage is
present to initiate MOV movement. As startup load surges reduce, ES
bus voltage increases and sufficient voltage for full thrust is present
at the end of the closing stroke. Therefore FWV-30 does not present
any safety concern.
These calculations established a condition outside the design basis and
are reported in accordance with 10 CFR 50.73(a)(2)(ii)(B).
CAUSE:
The cause of the 1993 condition was a lack of engineering review of the
motor brake voltage requirements. Although the motor brake symbols
appear on relevant drawings, the voltage requirements for the motor
brakes were not determined when degraded voltage calculations were
originally performed. In CR3's configuration, the brake is wired in
parallel with two phases of the three phase 460 VAC power feed to the
valve operator electric motor. As such, the brake was considered part
of the motor and did not receive a separate review.
The current (1994) condition was identified when new previously
unavailable analytical data was incorporated into Revision 1 of the
motor starting calculations.
EVENT EVALUATION:
In the 1993 report seven safety related valves were identified with
motor brakes installed. Four of the valves are maintained locked in
their safety function position during plant operation and are thus not
a concern. One valve with a motor brake manufactured by Sterns was
tested and is operable with voltages above 75 percent (345 VAC). The
remaining two valves (MUV-58 and MUV-73) have motor brakes manufactured
by Dings Dynamic Group.
The event of concern involves a postulated low voltage condition in the
230 kV switchyard at the low end of the SLUR setpoint coupled with an
Engineered Safeguards (ES) actuation and subsequent block loading.
With switchyard voltage at the low end of the SLUR setpoint and with
accident loads applied via block loading, MUV-73 and MUV-58 receive
377.1 VAC and 381.2 VAC, respectively, during BLOCK 1 loading.
The vendor for these motor brakes has indicated that the brake coil
could fail at a +/- 10 percent voltage differential. This places the
lower limit for motor brake voltage at 414 VAC. If voltage remains
above the SLUR setpoint, the Emergency
TEXT PAGE 5 OF 7
Diesel Generator (EDG) [EK,DG] will not connect to the bus and restore
adequate voltage. As long as voltages remain below 414 VAC,
insufficient voltage to release the brake coil could exist and the
valves might be unable to perform their safety function.
These valves provide HPI suction to the Makeup Pumps (MUP) [BQ,P] from
the BWST during accident conditions and are required to open on an HPI
signal. Normal plant configuration is for one of these valves to be
open and the other closed. Upon an HPI signal with voltages above the
SLUR setpoint, the MOVs with motor brakes will be block loaded to the
degraded bus. Assuming the closed valve fails to open, the operating
MUP will not be realigned to the BWST and will rapidly draw down the
Makeup Tank (MUT) [CB,TK]. Damage to that MUP will occur if suction
pressure is lost. At that point, and under an assumed simultaneous
single active failure of the other ES selected MUP to start, HPI would
be unavailable until operator action can be taken to align and start
the third MUP. The event could also be mitigated by operator action to
supply the ES buses with power from the EDGs.
The inability of the FWV-28 brake to release will prevent the motor
from operating and performing its intended safety function. The safety
function of FWV-28 is to provide feedwater system isolation in
conjunction with the feedwater block valves during events which result
in significant depressurization of one or both Once Through Steam
Generators (OTSG) [SG]. Upon reaching 600 psig pressure in an affected
OTSG the main feedwater isolation circuitry sends an isolation signal
to that train's feedwater pump, suction valve, block valves, and
crosstie valve FWV-28. Assuming that only one feedwater pump is in
operation, and that the running pump is on the opposite train as the
affected OTSG, the main feedwater isolation circuitry will send a
suction valve and block valve closure signal to the train associated
with the affected OTSG, a trip signal to the non-running pump and a
closure signal to FWV-28. A modification to the main feedwater
isolation logic circuitry will, when appropriate conditions are met,
provide an isolation signal to both feedwater trains, and a trip signal
to both main feedwater pumps. This action will eliminate the need to
assign a safety function to FWV-28.
The Revision 1 motor start calculations indicate that FWV-14 and FWV-15
will not receive adequate voltage under accident conditions coincident
with degraded grid voltage on their opening stroke. The inability of
FWV-14 and FWV-15 to open under accident conditions has no safety
significance since the safety function of these valves is to close and
provide main feedwater suction isolation. Engineering analysis via
these calculations has indicated that on their closing stroke the
motors have sufficient reduced voltage capability to operate
satisfactorily.
The unverified 1994 Revision 1 calculations indicate that FWV-30 does
not have sufficient voltage capability at its motor terminals under
accident conditions coincident with degraded grid voltage to overcome
minimum required thrust requirements at the instant actuation voltage
is applied to the motor terminals. This degraded voltage is the result
of other loads applied to the ES bus
TEXT PAGE 6 OF 7
simultaneously with the FWV-30 actuation. The 1994 Revision 1
calculations indicate that, at the instant the closure actuation
voltage is received at the FWV-30 motor terminals, sufficient voltage
for full thrust at valve closure and seating is not present, however
sufficient voltage is present to initiate MOV movement. As startup
load surges reduce, ES bus voltage increases and sufficient voltage for
full thrust is present at the end of the closing stroke. Therefore the
performance of FWV-30 does not present any safety concern.
CORRECTIVE ACTIONS:
MUV-58 and MUV-73 have had their motor brakes removed. Applicable
drawings have been revised to more accurately reflect the motor brake
option. The lack of a separate engineering review for these valve
operator subcomponents is considered an isolated event due to the
manner in which they were wired as part of the motor. Engineering
personnel have been advised of this occurrence to increase the
awareness for vendor supplied subcomponents when performing operability
reviews or preparing design packages. A modification to the main
feeedwater isolation logic will be installed during Refuel 9, which
will eliminate the need for assigning the safety function of main
feedwater crosstie valve FWV-28. Additionally, the calculations (E-91-
0018 Revision 1) will be verified prior to the end of July 1994.
PREVIOUS SIMILAR EVENTS:
There have been three previous Licensee Event Reports (LER) generated
due to potentially inadequate voltage to safety related components, two
of which included degraded grid voltage concerns (LER 92-007, LER 92-
010 and LER 92-020).
TEXT PAGE 7 OF 7
Figure "SIMPLIFIED MAIN FEEDWATER FLOW DIAGRAM" omitted.
ATTACHMENT TO 9405180436 PAGE 1 OF 1
Florida
Power
CORPORATION
Crystal River Unit 3
Docket No. 50-302
May 12, 1994
3F0594-11
U. S. Nuclear Regulatory Commission
Attention: Document Control Desk
Washington, D. C. 20555
Subject: Licensee Event Report (LER) 93-008-01
Dear Sir:
Attached is Licensee Event Report (LER) 93-008-01 which is submitted in
accordance with 10 CFR 50.73.
This supplement provides additional information which was previously
unavailable.
Sincerely,
G. L. Boldt
Vice President
Nuclear Production
GLB/JAF:rp
Attachment
xc: Regional Administrator, Region II
Project Manager, NRR
Senior Resident Inspector
CRYSTAL RIVER ENERGY COMPLEX: 15760 W Power Line St o
Crystal River, Florida 34428-6700 o (904) 795-6486
A Florida Progress Company
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