ML20059H724
| ML20059H724 | |
| Person / Time | |
|---|---|
| Site: | Mcguire |
| Issue date: | 11/04/1993 |
| From: | Mcmeekin T DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 93-11, NUDOCS 9311100186 | |
| Download: ML20059H724 (14) | |
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7 g yc,,,,,5 McCture Nuclear Genmtion Department.
Vice President l
' 1:50 floem ferry Road (MG01A) '
(704)S75-48tXI:.
' Hunterst:tlie, NC280i#8985
.(TC4)B15-4SOR Tu,
.i.i DUKEPOWER;
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'Novemoer 4, 1993 i
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U.S.. Nuclear Regulatory Commission p
Document Control' Desk l
Washington, D.C.
' 20555
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Subject:
McGuire Nuclear Station Unit.2 Special Report No.' 93 Problem. Investigation Process No.: 2-M93-0879 q
^ Gentlemen:
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- .~ concerns a small primary coolant leak in Unit.2 Lower Containment. _
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' Attached you will find.Special Report No. 93-11.< This. incident'..
-No
.Technicalispecification Limit was violated.
This report is'being:sent.
for your,information.
This event..is considered to:beof no-significance.with respect to the health-and safety-:of;the public.:
j Very truly yours, q
v T.C. McMeekin l
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Mr. S.D.:Ebneter INPO Records. Center:J-
.1 Administrator, Region II LSuite 1500 M
- U.S.: Nuclear Regulatory-Commission
-1100 Circle?.75L: Parkway-1 101 Marietta:St.,.NW,. Suite 2900-Atlanta,;GA -30339.
Atlanta,'GA :30323 3
'Mr. Victor;Nerses
'Mr. George? Maxwell
'.U.S. Nuclear. Regulatory -Comraission NRC Resident Inspectorf H
Office of Nuclear Reactor Regulation-
'McGuire' Nuclear' Station--
H Washington, D.C.
20555 i
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Swindlehurst H.B. Tucker R.F. Cole D.B. Cook G.A. Copp
't C.A.-Paton M.E. Pacetti P.M. Abraham W.M. Griffin NSRB Support Staff (EC 12-A) l 1
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McGUIRE SAFETY REVIEW GROUP SPECIAL REPORT 1.
REPORT NUMBER: 93-011 2.
DATE OF REVIEW: September 9 - October 8, 1993 3.
SUBJECT DESCRIPTION: This review is submitted to the NRC as a special report of the circumstances relating to the incident described on.
Problem Investigation Process (PIP) 2-M93-0879.
The specific purpose of
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the review was to determine the cause(s) of the incident and corrective actions to prevent further problems of the type described by the PIP.
4.
EVALUATION AND COMMENT:
ABSTRACT:
On September 10, 1993, at 0015, a small primary coolant leak occurred when valve 2NV-2A, NC System Letdown Isolation to Regenerative Heat-Exchanger, was opened. Valve 2NV-2A had indicated that it had moved to the intermediate position on September 9, 1993, at 2252.
In actuality, the valve had closed sufficiently to stop flow, but not far enough to.
actuate the limit switch. At the time valve 2NV-2A moved to the intermediate position, valve 2NV-458A, B Letdown Orifice Outlet containment Isolation, remained open. This allowed the letdown piping to depressurize, resulting in void formation in the piping. Valve 2NV-458A was subsequently closed at 2304. When valve 2NV-2A was-re-opened the piping was repressurized, collapsing the void, which resulted in a water hammer. At this time, operations personnel received indications of a leak inside Containment. The actual leakage pathway was through a threaded nipple welded to valve 2NV-464, Letdown Line Containment Isolation Test Drain. Unit 2 was in Mode 3 (Hot Standby) at the time of the event.
Causes of Inadequate Procedure, Equipment Failure, and Unknown have been assigned to the event. Corrective actions include replacement of valve 2NV-464 and the threaded nipple, replacement-of the solenoid valve for valve 2NV-2A, and revision of several. Abnormal Operating procedures.
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DPC/MNS SPECIAL REPORT NO.93-011 PAGE 2 BACKGROUND 2 The charging and letdown functions of the chemical and Volume Control (NV) system are employed to maintain a programmed water level in the Reactor Coolant (NC) system pressurizer,_thus-maintaining proper NC system inventory during all phases of plant operation. This is achieved by means of a continuous feed and bleed process which is automatically controlled based upon pressurizer water level. The bleed rate can be-chosen to suit various plant operational requirements by selecting the proper combination of letdown orifices in the letdown flowpath.
Reactor coolant is discharged to the NV system from the NC system C cold leg..It then flows through the shell side of the regenerative heat'-
exchanger. The coolant then experiences a pressure reduction of approximately 1885 pounds per square inch (psig).in passing through a letdown orifice.
Downstream of the letdown heat exchanger a second pressure. reduction occurs.
This pressure reduction is performed by low-pressure letdown I
valve 2NV 124, the function of which is to maintain upstream pressure of about 300-350 psig. 'This pressure is high enough to prevent flashing y
downstream of the letdown orifices.
i Valves 2NV-1A and 2NV-2A, NC System Letdown Isolation to Regenerative Heat Exchanger, are Fall close (FC) Air Operated Valves (ADV).-These I
valves are controlled through solenoid valves 2NVSV-0001 for valve 2NV-IA and 2NVSV-0002 for 2NV-2A.
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valves 2NV-1A and 2NV-2A are cross-interlocked with the valves 2NV-457A, 2NV-458A and 2NV-35A, NC System Letdown Orifice Outlet Containment Isolation, which must all be closed in order to open either 2NV-1A or-2NV-2A.
Once open, valves 2NV-1A or.2NV-2A cannot be closed if any of-the Letdown Orifice Isolation valves are open, with the exception that-2NV-1A or.2NV-2A will automatically close on a low pressurizer level signal.
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t DPC/MNS SPECIAL REPORT NO.93-011 PAGE 3 Valves 2NV-457A, 2NV-458A and 2NV-35A are FC AOVs.
None of these valves
'l (2NV-457A, 2NV-458A, or 2NV-35A)-can be opened unless valves 2NV-1A and 2NV-2A are both open. These valves will close automatically on receipt of a pressurizer low level signal, if either valve 2NV-1A or 2NV-2A closes, or upon receip* of a Phase A Containment Isolation Signal
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The purpose of the interlocks between valves 2NV-1A, 2NV-2A, 2NV-457A, 2NV-458A, and 2NV-35A is to maintain pressure in the letdown piping, which will aid in preventing a water hammer.
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EVENT DESCRIPTION:
On September 8, 1993, Unit 2 was in Mode 3 (Hot Standby). Normal letdown was in service. At 0727, valve 2NV-2A closed, isolating
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1etdown flow.
i At the time the valve closed, OPS personnel entered procedure i
AP/2/A/5500/12, Loss of Letdown, Charging, or Seal Injection, and j
established excess letdown. Tha Shift Managdr was contacted and work order 93065641 was initiated to repair valve 2NV-2A.
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IAE personnel proceeded to check the Events Recorder for any associated t
alarms. The IAE personnel then checked the system for voltage at l
various points in the circuit and-for any mispositioned switches, links, or contacts. No problems were identified. The IAE personnel then
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requested that OPS personnel cycle the valve from the control switch.
The valve was cycled open and closed several times, with no problems detected. The IAE personnel consulted with Engineering personnel.
It was determined that the only component in the loop which could likely cause this type of movement was the solenoid valve.
By 1146 the IAE g
personnel had completed troubleshooting without having identified any specific problem. Control of the valve was returned to OPS personnel at 1220 on September 8, 1993.
f On September 9, 1993, at'2252, problems were again experienced with valve 2NV-2A.
OPS personnel obs9rved that letdown flow was decreasing _
and that valve 2NV-2A was indicating an intermediate (neither fully open f
nor fully closed) position. OPS personnel attempted to open valve 2NV-2A, however, the valve did not respond. OPS personnel then observed f
that letdown flow had decreased to O gallons per minute (gpm). They
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.DPC/MNS SPECIAL REPORT.NO.93-011 PAGB 4 also observed that valve 2NV-45BA was not closed. A decision was made to leave valve 2NV-458A open to aid IAE personnel in -troubleshooting the problem (i.e. preserve the as found conditions). Procedure AP/2/A/5500/12 was entered and excess letdown was established.
IAE personnel conducted an inspection of terminal cabinet 2ATC2 and found no problems with relays or contacts. They then opened links G-25, G-30, G-32, and F-46 in 2ATC2 to remove power from solenoid valve 2NVSV-0002, in an attempt to fail valve 2NV-2A completely closed. Valve 2NV-2A continued to indicate an intermediate position.
IAE personnel returned the links to their normal position.
Valve 2NV-458A was cycled to determine !.f the interlocks associated with valve 2NV-2A were operating properly. Valve 2NV-458 was closed at 2304, cycled open at 2328, then closed and left closed at 2328. Valve 2NV-458 had remained open for'approximately 12 minutes following the closure of valve 2NV-2A at 2252.
After discussion with OPS personnel, at 0012, on September 10, 1993, IAE personnel placed a jumper from terminal G-24 to G-30 in 2ATC2 to apply power directly to solenoid valve 2NVSV-0002. This action to open valve 2NV-2A was in concurrence with procedure AP/2/A/5500/12, which instructed OPS personnel to open valves 2NV-1A and 2NV-2A, during the process of establishing normal letdown flow.
At the time the jumper was placed in the circuit, valve 2NV-2A moved to the open position.
At 0015, OPS personnel received numerous indications of primary coolant leakage in Unit 2 Lower Containment. Those indications included numerous fire zone alarms, along with an increasing Containment pressure and a Pressurizer level decrease (as indicated on the trend recorder).
No annunciator alarms for Containment pressure or Pressurizer level were received. OPS personnel opened valve 2VQ-4, Containment Air Release To Unit Vent, to the fully open position to limit the containment pressure increase. All work on valve 2NV-2A was suspended at this time.
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SPECIAL REPORT NO.93-013 PAGE 5 I
OPS personnel entered procedure AP/2/A/5500/10, NC System Leakage Within
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Capacity Of Both NV Pumps. Valves 2NV-1A and 2NV-2A were closed at 0017.
Pressurizer level began to recover after the valves were closed.
OPS personnel continued with procedure AP/2/A/5500/10, and it was determined the leak had been isolated. During this time Containment temperature increased from approximately 104 degrees F to 106 degrees F.
l OPS and Radiation Protection (RP) personnel were dispatched to Unit 2 Lower Containment to determine where the primary leak had occurred.
Their investigation began at' valve 2NV-2A and proceeded to the Regenerative Heat Exchanger, i
t When the OPS and RP personnel reached the area of the Regenerative Heat Exchanger, they discovered steam coming from valve 2NV-464, Letdown Line Containment Isolation Test Drain.
c After discussion with the OPS personnel in the Control Room, the l
decision was made to tighten the valve in the closed direction and then tighten the pipe cap on the valve. OPS personnel were able to tighten
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(close) the valve an additional one-quarter turn, and the pipe cap was i
tightened an additional one-half turn.
When the valve was tightened the leak stopped.
Following isolation of the leak, inspection of the' threaded portion of the pipe and pipe cap revealed that the three quarter inch pipe nipple welded to the outlet side of valv,e 2NV-464 was cracked. This was a-i a
circumferential crack, in the threaded region of the nipple.
On September 10, 1993, at 0853,- valve 2NV-2A once again closed. This
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time the valve indicated a closed position, not intermediate.
OONCLUSION:
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This incident is assigned causes of Inadequate Procedure, Equipment Failure, and Unknown.
t P. cause of Inadequate Procedure is assigned because no procedural guidance existed to alert OPS personnel of potential' voiding in the i
letdown piping.
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DPC/MNS l
SPECIAL REPORT NO.93-011 l
PAGE 6 At the time valve 2NV-2A moved f rom the open position to an indicated '
intermediate position, OPS personnel realized that valve 2NV-458A had l
not closed.
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At that time, a decision was made to leave valve 2NV-45BA in the existing position to aid IAE personnel in troubleshooting the closure of valve 2NV-2A and the failure of 2NV-458A to close. However, OPS f
personnel did not realize that with letdown flow isolated and valve 2NV-l 458A open for greater than a few seconds, the potential for voiding in the letdown piping existed. Twelve minutes later, valve 2NV-458A was closed.
i When the letdown piping depressurized, voids were created in the letdown t
piping between valve 2NV-2A and the regenerative heat exchanger. This was due to the fact that water in this portion of the letdown piping is normally at NC system temperature and pressure (557 degrees F, 2235 j
poig).
The existing water temperature, along with the depressurization of the letdown piping, allowed the water to flash to steam.
Later in the incident, the decision was made to open valve 2NV-2A.
This decision was in concurrence with procedure AP/2/A/5500/12, during the process of establishing normal letdown. When valve 2NV-2A opened, the f
voided section of piping was repressurized, resulting in the collapse of l
I the steam voids. The collapse of the voids resulted in the water hammer. Subsequent to the water hammer, the primary coolant leak was f
identified.
I A cause of Equipment Failure is assigned to the closure of valve 2NV-2A.
Valve 2NV-2A actually closed spontaneously 3 times between September 8, t
1993, at 0727, and September 10, 1993, at 0843.
i IAE personnel conducted extensive troubleshooting. This included inspection of terminal cabinet 2ATC2 for voltage and weak connections at'
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various points in the circuit, along with inspection for any j
mispositioned links or relays, checking for any safety signals which I
could cause the valve to close, inspection of the control switch on the main control board, inspection of. limit switches, inspection of the valve for any air leaks, and inspection of the electrical penetration.
l through which the controls for valve 2NV-2A are powered.
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PAGE 7
.i It was determined by Component Engineering personnel that the most
'f plausible cause for a failure of-this type was an intermittent failure
.f of the solenold.
Solenoid 2NVSV-0002 and the air filter regulator were subsequently replaced.
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l The solenoid and filter regulator which were removed from service were disassembled and inspected. Approximately one teaspoon of non
_l radioactive water was found between the upper cover and the coil. A
.f small scuff, which was found on an 0-ring, is believed to have allowed the water to enter the cover during decontamination activities..
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A Hegger test was conducted on the solenoid coil. The indicated resistance was approximately one-half'of the manufacturer's expected f
value, but the coil exhibited no indication of being failed. -The l
solenoid internals were clean, with good physical appearance.
l The solenoid was reassembled and cycled. The solenoid cycled as expected. The solenoid was energized and heat was applied with a heat-gun.
A temperature of approximately 160 degrees F was maintained for i
over B hours. Test results were unchanged from the tests which were conducted at ambient temperature.
q The solenoid was sent to the Duke' Power Company Qualifications and Testing Facility (QTF) for further testing. The.QTF was asked to j
simulate the in plant conditions to determine the cause of the failure.-
A teaspoon of water was placed in the top cover of the solenoid. The
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solenoid was placed in an environmental chamber to maintain the ambient
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temperature at 110 degrees F.
The solenoid was thdn energized. An air source was also connected to the solenoid.
1 After'three days the solenoid failed to a unenergized positibn.
The solenoid was once again energized. After a short while the solenoid l
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failed again. These efforts duplicate the failure of the solenoid under j
operating conditions, possibly due'to moisture in the solenoid coll.
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Inspection of the filter regulator revealed no problems which might J
l contribute to the closure of valve 2NV-2A.
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DPC/MNS SPECIAL REPORT NO.93-011 1
PAGE 8 1
I A cause of Unknown is assigned to the failure of valve 2NV-2A to actuate the closed limit switch contacts en the evening of September 10, 1993.
As discussed above, valve 2NV-2A spontaneously closed on 3 separate occasions. The first and third time, on September 8 and September 10, f
1993, the closed limit switch operated properly. The second time that' valve 2NV-2A closed, on September 9, 1993, the closed limit switch contacts did not actuate.
Inspection of the limit switch during troubleshooting revealed no j
problems which should prevent proper operation of the switch. This is supported by the fact that the limit switch operated properly 2 of the 3 times valve 2NV-2A closed. To prevent further problems of this nature, IAE personnel adjusted the setting of the closed limit switch for valve f
2NV-2A to allow the contacts to actuate earlier during valve closure.
i The problems experienced with this limit switch can also explain why OPS i
personnel could not reopen valve 2NV-2A when the valve closed.
t The interlock between valve 2NV-2A and valve 2NV-458A is associated with the closed limit switch on valve 2NV-2A.
Since the closed contacts never actuated, the interlock did not cycle valve 2NV-458A closed.
The interlocks are also designed to prevent opening valve 2NV-2A unless valves 2NV-457A, 2NV-458A, and 2NV-35A are all closed. Since valve 2NV-l 458A remained open, the interlock to prevent opening 2NV-2A performed as expected.
I Later.in the incident, valve 2NV-458A was closed. At that time, the interlock to allow valve 2NV-2A to open would have been satisfied.
During the incident, valve 2NV-2A was reopened by jumpering power
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directly to the solenoid. However, it seems likely that the valve might
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also have been operated from the control board switch once the interlock f
I from valve 2NV-458A was satisfied.
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i A second cause of Unknown is assigned because it could not be conclusively determined how valve 2NV-464 became partially (approximately one quarter turn) opened. The information for the amount by which the valve and the pipe cap were turned was taken from a written l
statement from the operator who actually performed the work.
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DPC/MNS
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-SPECIAL REPORT NO.93-011 PAGE 9 During the Unit 2, EOC 8 outage, valve 2NV-464 was used as the test j
connection for a hydrostatic test.
The hydrostatic testing equipment was connected to the system using the threaded nipple welded to valve 2NV-464.
This test was conducted during the implementation of Nuclear j
Station Modift:ation 22413. The actual testing was performed under work order 93024063 using procedure MP/0/A/7650/55, controlling Procedure For Hydrostatic Testing Of Duke Class "A",
"B",And "C" Systems.
Documentation for the hydrostatic test indicates that valve 2NV-464 was closed and double verified following completion of the test.
No other documented activities which directly affected valve 2NV-464 took place l
between the completion of the test and the time the leak occurred.
l Several other activities took place in the area of valve 2NV-464 after the hydrostatic testing was completed. Those activities were the installation of piping insulation and the removal of lead shielding.
It is possible that valve 2NV-464 could have been bumped during these f
activities, causing the valve to be partially-opened. However, this l'
could not be confirmed.
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A metellurgical evaluation was performed on the failed three-quarter inch threaded pipe nipple.
Preliminary results of that evaluation indicate the nipple failed due to high cycle fatigue cracking. Two cracks were identified. These cracks originated from the root of two adjacent threads, at or near the end of the engagement area of the pipe
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cap.
This nipple has been used as a hydrostatic test connection several j
times in the past.
The nipple and valve 2NV-464 were subsequently l
l replaced.
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To prevent recurrence of this problem, several procedure changes were made.
A new " Subsequent Step" 1 was added to procedure AP/2/A/5500/12, Case I, Loss Of Letdown. This step instructs the operator to ensure letdown is isolated by ensuring valves 2NV-458A, 2 NV-457A', and 2NV-35A are closed.
Both procedures were also revised to instruct the operators, when establishing normal letdown, to "Obtain Staff evaluation for potential voiding of letdown line prior to proceeding to next step".
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DPC/ MMS SPECIAL REPORT NO.93-011 PAGE 10 Procedures AP/2/A/5500/07, Loss of Electrical Power, AP/2/A/5500/lO, NC System Leakage Within The Capacity Of Both NV Pumps, and AP/2/A/5500'35, ECCS Actuation During Plant Shutdown, were also revised to require this.
Staff evaluation prior to establishing normal letdown.
As a result of the water hammer of the letdown piping, Mechanical QC personnel conducted walkdown inspections of the affected piping. The portion of piping which was inspected was from valve 2NV-2A downstream to valves 2NV-458A, 2NV-457A, and 2NV-35A.
As a result of this inspection, 3 small snubbers, rated at approximately
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1000 pounds, were found to be damaged. The snubbers were replaced.. Two additional snubbers with similar ratings were replaced, even though they showed no indication of damage.
An Operability evaluation of the affected piping was completed by Engineering personnel. The evaluation found the system to be operable, after repairs.
There were no personnel injuries, radiation overexposures, or uncontrolled releases of radioactive material as a result of this incident.
Corrective Actions:
Immediate:
1)
OPS personnel isolated the primary coolant leak by closing valves 2NV-1A and 2NV-2A.
Subsequent: 1)
OPS and RP personnel entered Unit 2 Lower Containment and determined the leakage had occurred through a threaded nipple welded to valve 2NV-464.
2).
OPS personnel manually closed valve 2NV-458A.
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DPC/MNS i
SPF,CIAL REPORT NO.'93-011 PAGE 11 l
3)
IAE personnel and Component Engineering personnel
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investigated the closure of valve 2NV-2A and determined the most probable cause was an intermittent
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failure of solenoid valve 2NVSV-0002. The solenoid was subsequently replaced.
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Mechanical Maintenance personnel replaced' valve 2NV-464 and the threaded pipe nipple.
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Mechanical QC personnel performed a walkdown f
inspection of the affected letdown piping, components,
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and piping restraints. This inspection identified 3 small snubbers which were damaged; however, all five snubbers were replaced.-
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Engineering personnel performed an operability
'I evaluation of the affected letdown piping, components, j
and piping restraints. Following the completion of.
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repairs and successful leak testing at system operating temperature and pressure, the affected j
piping was declared operable.
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OPS Procedure Group personnel changed procedure.
AP/1 and 2/A/5500/12, adding a new subsequent step 1.
This step instructs OPS personnel to ensure j
letdown is isolated by ensuring valves 2NV-458A, 2NV-457A, and 2NV-35A are closed.
8)
OPS Procedure Group personnel changed procedures AP/1 and 2/A/5500/07, 10,12, and 35 to 32.struct the 1
operators, when establishing normal letdown, to t
"Obtain Staff evaluation for potential voiding of letdown line prior to proceeding to-next step".
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-SPECIAL REPORT NO. 93--011 PAGE 12 Planned:
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A Nuclear Station Modification (NSM) is currently l
under evaluation. This NSM would add pressure indication to the letdown. header in the vicinity of i
the regenerative heat exchanger..This indication will aid OPS personnel in evaluation the fluid conditions in the letdown header.
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