ML20059E522
| ML20059E522 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 08/19/1993 |
| From: | Rueger G PACIFIC GAS & ELECTRIC CO. |
| To: | |
| References | |
| OLA-2-I-MFP-074, OLA-2-I-MFP-74, NUDOCS 9401110256 | |
| Download: ML20059E522 (7) | |
Text
e..
e si i wi.
4 n.,y#,
A hV Y N ** 5 b '~S h\\f0 hot.O Y GG 204653 Pacific Gas and Electric Company 77 Beale Street, Room 1451 Gregory. uegn P.O. Box 770000 SeniorVice President and San Francisco,CA 94177 General Maiia~ger' ~ ~ 0 415 S 73-4684 Nuc! car Power Generation Fax 415S73-2313
'93 DCT 28 P6 :09 March 10, 1993
$[ $
N PG&E Letter No. DCL-93-062 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk
~
Washington, D.C.
20555 Re:
Docket No. 50-323, OL-DPR-82 Diablo Canyon Unit 2 Licensee Event Report 2-92-003-01 Unit Shutdown Required Due to Inoperable High Pressure Turbine Stop Valve Gentlemen:
Pursuant to 10 CFR 50.73(a)(2)(1)(A), PG&E is submitting the enclosed revision to Licensee Event Report (LER) 2-92-003-00 concerning a unit shutdown due to an inoperable high pressure turbine stop valve. This revision is submitted to provide an update on the status of root cause investigations and minor editorial changes.
Revision bars in the margin in cate the revised LER sections. Another supplemental LER will be it ed to report the root cause and corrective actions.
This event did not affect the health and safety of the public.
Sincerely, t
y) <w Jk y Gregory M. Rueger i
cc:
Ann P. Hodgdon John B. Martin Mary H. Miller i
Sheri R. Peterson CPUC Diablo Distribution
"## * * " C " $8'"
INP0 M" s 6 475-o L A no y vngg 74 la *e m ::e IF KL 6
/RrQ 3 DC2-92-MM-N013 m.,
~
__ c yn o v Enclosure
~NW xe,w 1074S/85K/PSN/2246 d
An~n kilehYhkL:.n 8-l9.-3 C*
o=-
.,m, byGi!Er _
6 9401110256 930819 PDR ADOCK 05000275 9
PDR.
l l
UCENSEE EVENT REPORT (LER) 204653 1
i I
FACRJTY Kamas (1)
DocKf7 NUMB pt (2)
PAC E (Il l
j DIABLO CANYON UNIT 2 0l5l0 0l0l3l2l3 1
'l 6 UNIT SHUTDOWN REQUIRED DUE TO INOPERABLE HIGH PRESSURE TURBINE STOP VALVE 1
imam l
fVENT 8&TEtEl tst NUMast tel REPORT DATE (76 OTetR F ACILITIES INVOLVED (8) 6404 DAY YR YM SE REvnee0N
,AON DAY TR DOCKET Ntmest (S) 0 5
0 0
0 0l0l3 0l1 03 10 93 0
5 0
0 0
03 22 92 92 RA THI5 REPoaf IS SUBMITTED PUR$UANT TO THE REQUIREMENT 5 OF 10 CFRt (11) 1 LNL x
10 CFR 50.73te)(2)tficA) 0]9l2 OTHER (Specify in Abstract below and in text, NRC Form 366A) uct=str coerT Act rom Tees tar (121 Tttronour wunata i
DAVID P. SISK, SENIOR REGULATORY COMPLIANCE ENGINEER
^"
805 545-4420 cAust sisitM coMecNENT FAC.
R A E CAUSE SYSTEM COMPONENT FAC.
R A E X
SlB RITlVl Wl 1l 2l 0 Y
l lll l l l-1 III III I
Ill lll SuretadaxT AL RooRT ExrEcTsD ai4)
D"'
EXPECTED SUBMISSION lXl YES (if yes, conplete EXPECTED SUBMISSION DATE) l l NO 06 30 93 CSTKACT (15) l l
l On March 22, 1992, at 2143 PST with Unit 2 in Mode 1 (Power Operation) at approximately 92 percent power, an Unusual Event was declared and a manual shutdown was commenced in accordancs sith Technical Specification (TS) 3.3.4.1,
" Turbine Overspeed Protection," when it was determined that one high pressure turbine stop valve (FCV-144) was inoperable.
An emergency report was made to the NRC in accordance with 10 CFR 50.72(a)(1)(i).
FCV-144 was disassembled and it was determined that the nut that retains the valve disc to the valve swing arm had disengaged, allowing the valve disc to become separated from the valve swing arn. When the disc separated from the swing arm, it caused a partial blockage of Main Steam Lead 2.
The root cause and corrective actions of this event have not yet been determined.
4 A supplemental LER will be issued to report the root cause and corrective actions.
l l
l l 1074S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 204653 ease (si uay m y miun e m
= cur m ca m DIABLO CANYON UNIT 2 0l5l0l0l0l3l2l3 92
- 0l0l3 0l1 2 l"l6 1 EAT (17)
I.
Plant Conditions l
Unit 2 was in Mode 1 (Power Operation) at approximately 92 percent power.
11.
Descriotion of Event A.
sumary:
On March 22,1992, at 2143 PST, an Unusual Event (UE) was declared and a manual shutdown was commenced in accordance with Technical Specification (TS) 3.3.4.1, " Turbine Overspeed Protection," when it was determined that one high pressure (HP) turbine stop valve (FCV-144)(SB)(RTV) was inoperable due to a flow restriction.
B.
Background:
FCV-144 is a hydraulically-actuated swing check valve provided as part of the turbine overspeed protection system (JJ).
FCV-144 protects the HP turbine (SB)(TRB) from overspeed if the associated turbine governor valve (SB)(FCV) downstream of FCV-144 should fail to close when the overspeed trip or the normal trip mechanism operates. Overspeed protection is necessary to preclude turbine rotor failure and associated turbine generated missiles.
TS 3.3.4.1 requires one turbine overspeed protection sy: tem to be operable. With one of the HP turbine stop valves inoperable, the valve is required to be returned to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or the plant is required to be shut down within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
- C.
Event Descriptioni'"
l On March 22, 1992, at 1240 PST, control room (NA) operators observed a rapid load reduction in power of approximately 10 percent.
Investigations were initiated and it was determined that Main Steam Lead 2 had an unidentified flow restriction. An above normal pressure differential was measured across FCV-144, indicating that there was a restriction within the valve.
On March 22, 1992, at 2130 PST, as a result of the investigations, FCV-144 was declared inoperable. A UE was declared at 2143 PST and a manual shutdown to Mode 3 (Hot Shutdown) was commenced.
On March 22, 1992, at 2210 PST, an emergency report was made to the NRC in accordance with 10 CFR 50.72(a)(1)(i).
On March 23,1992, at 0127 PST, the main steam isolation valves l
(MSIV)(SB)(V) were closed prior to the unit trip in compliance with TS 3.3.4.1, and the UE was subsequently terminated.
1074S/85K
LICENSEE EVENT REPORT (LER) TEXT CONTINUATIEN 204653 Fac2LITY mnME (2) ooctri seuMota (2) ten enseca
- s) paec 0) ma sumamm p
DIABLO CANYON UNIT 2 0l5l0l0l0l3l2l3 92
- 0l0l3 0l1 3 l'l6 TExi (2P)
D.
Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
E.
Dates and Approximate Times for Major Occurrences:
1.
March 22, 1992, at 2143 PST:
Event / Discovery Date: A UE was declared and a manual shutdown was commenced in accordance with TS 3.3.4.1.
2.
March 22, 1992, at 2210 PST: An emergency report was made to the NRC in accordance with 10 CFR 50.72(a)(1)(i).
3.
March 23, 1992, at 0136 PST: Unit entered Mode 3.
TS 3.3.4.1 exited.
F.
Other Systems or Secondary functions Affected:
None.
G.
Method of D_iscovery:
Control room operators observed a rapid load reduction in power of approximately 10 percent.
Investigations were initiated and it was determined that FCV-144 had an unidentified flow restriction.
H.
Operators Actions:
Once FCV-144 was determined inoperable, a manual shutdown to Mode 3 was commenced. During the shutdown, the turbine was tripped and the main steam isolation valves were closed prior to separating the generator from the grid. This was done to prevent the possibility of l
overspeeding the turbine if the turbine governor valve downstream of FCV-144 failed to close.
I.
Safety System Responses:
None.
III.
Cause of the Event A.
Immediate Cause:
~
J The immediate cause of the event was an unidentified flow restriction in Main Steam Lead number 2.
FCV-144 was disassembled and it was determined that the nut that retains the valve disc to the valve swing 1074S/85K
-y
- r
.,y.
9-,
--.wy
,e f.
'i UCENSEE EVENT REPORT (LER) TEXT CONTINUATION 204653 DOCKET feMER (2)
Lin sa. meta
- s)
PAtt (3)
FAC2L277 at4PE (2) vtAn sumanent l
DIABLO CANYON UNIT 2 0l5l0l0l0l3l2l3 92
- 0l0l3l 0l1 4 l'l6 TEXT (17) arm had disengaged from the disc stem, allowing the valve disc to become separated from the valve swing arm. When the disc separated from the swing arm, it caused a partial blockage of steam flow through l
Main Steam Lead 2..
B.
Root Cause:
l The root cause of this event is being investigated.
Inspection of the valve parts following disassembly identified no failed components other than the valve nut and disc stem joint. The two postulated modes of failure of this joint are: (1) unscrewing of the nut off the I
stem; or (2) a failure of the nut / disc stem threaded joint.
l (1)
Unscrewing of the nut off the stem j
l The pins that secure the nut from rotation can possibly fall out i
due to inadequate peening or staking of the retaining nut.
Subsequently, unscrewing of the nut from the disc stem could l
cause separation of the disc from the swing arm. However, investigations do not indicate that FCV-144 failed in this mode.
i (2)
Failure of the nut / disc stem threaded joint i
The physical evidence indicates that the nut was stripped off the disc stem while the pins were still in place.
Laboratory testing performed on the failed components, in addition to calculations performed in a " weak link analysis," show that the following two factors likely contributed to the failure of the threaded connection: degraded threads and a higher than usual force acting on the valve disc.
The following scenarios were evaluated to determine the conditions of excessive force.
l (i)
A combination of excessive clearances and poor thread fit j
could lead to accelerated thread wear as a result of buffeting of the disc in the steam flow. This wear could significantly reduce the strength of the threaded joint.
The buffeting of the disc may have increased if the internal bonnet stop was too high, which may have held the valve disc too close to the steam flow, even though the valve was in its fully open position.
Inspection of the internal stop on FCV-144 determined that the valve disc was clearly up out of the steam flow by approximately 1-1/2 inches in the full-open position. However, weakening of the nut / stem connection due to flow-induced buffeting could be a factor in the nut / stem connection failure.
l 1074S/85X
1 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 204653 J
' ractuvv swc (u oocKri HvMata (r) tra osuMeta ts) nar o)
~3 n
DIABLO CANYON UNIT 2 0l5l0l0l0l3l2l3 92 0l0l3 0l1 5 l'l6 mi on l
(ii)
Improper dash pot adjustment in the hydraulic actuator could cause the hydraulic piston to reach the bottom of its stroke prior to the disc fully seating. Similarly, if there was an obstruction or binding in the hydraulic line of the actuator, the disc could be held up in mid-stream while closing. Under both of these conditions, the disc could be exposed to a full steam differential pressure, which could impart a very large tensile (pull) force on the nut stem threads. This large tensile force might result in stripping of the threaded connection, which may have been weakened either due to buffeting wear or poor manufacturing.
(iii) The valve disc could stick in midstream while closing due to binding of the valve shaft. This could impart an excessive tensile force on the disc nut / stem threads and begin to strip the nut off the stem. Although no physical evidence was specifically identified during the valve disassembly that confirms that the valve shaft binding occurred, the weak link analysis supports this theory.
This failure scenario is being investigated further.
The raot cause will be determined following additional inspections of the valve and actuator to validate one of the above postulated failure scenarios. These inspections are scheduled to be performed during the Unit 2 fifth refueling outage that started March 5,1993. A supplemental LER will be issued to report the root cause.
IV.
Analysis of the Event All four HP turbine governor vasves were operable during the period that FCV-144 was inoperable.
If a turbine overspeed condition had developed, the governor valves would have closed to stop the steam flow to the turbine to mitigate the overspeed condition.
The closing signal to the governor valves would have been provided by one of the following three independent overspeed protection mechanisms:
(1)
Overspeed Protection Controller (OPC) at 103 percent overspeed, (2)
Mechanical overspeed trip at 109 percent overspeed, or (3)
Electrical overspeed trip at 111.5 percent overspeed.
Thus, the inoperable HP turbine stop valve did not adversely affect the health and safety of the public.
1074S/85K
l l'
UCENSEE EVENT REPORT (LER) TEXT CONTINUATION 204653 FACILITV 8W E p)
DOCKET #ASSER Q)
LEA ER L6)
PAGE (3)
,i l
DIABLO CANYON UNIT 2 0l5l0l0l0l3l2l3 92 0l0l3 0l1 6 l"l6 l
nu un
-l V.
Corrective Actions
)
A.
Immediate Corrective Actions:
1 1.
FCV-144 was disassembled and repaired. The other three HP stop valves on Unit 2 were disassembled and inspected. The nuts on the other three stop valves were either re-peened or verified oto l
meet Westinghouse guidelines.
2.
The Unit I and Unit 2 stop valves have been otroke~ tested and are being acoustically monitored to demonstrate their operability.
B.
Corrective Actions to Prevent Recurrence:
Corrective actions to prevent recurrence will be determined once the root cause is finalized. They will be submitted in a supplemental LER.
i VI.
Additional Information i
A.
Failed Components:
FCV-144, High Pressure Turbine Stop Valve, Manufactured by l
Westinghouse Electric Corp. Model No. 723-J-119 l
B.
Previous LERs on Similar Problems:
None.
i 1
\\
1 l
i l
l l
1074S/85K
,