Three take homes.
1) they changed out the 2 stage for three stage...the problems only got worst.
- 2011: LER 2-2011-002, identified multiple SRV setpoint drift for 8 of the 11 SRVs. Corrective actions included replacement of the 2-stage SRVs with 3-stage SRVs during the Unit 2 Spring 2011 refueling outage which was considered at that time to be the long term fix for this corrosion bonding issue.
It sounds like they yanked out the three stage with this below eight failure. So why didn't Pilgrim take out their reliefs early like Hatch???
- 2012 with three stage: LER 1-2012-004, identified multiple SRV setpoint drift for 8 of the 11 SRVs
- 2013: LER 1-2014-003, identified multiple SRV setpoint drifts for 5 of the 11 two-stage SRVs installed on Unit 1.
- LER 1-2014-003, identified multiple SRV setpoint drifts for 5 of the 11 two-stage SRVs installed on Unit 1. The two-stage SRVs with platinum-coated pilot discs were
2) We have no idea of the magnitude of trhe leakage.
3) Hatch has problems with steam line vibration damaging these valves.
4) We know this state is totally inaccurate: "3-stage SRVs typically do not exhibit set point drift and the modified pilot reduces instances of vibration induced spurious openings and leak-by."
5) I don't think you ever can trust the public disclosure dance between a vendor and a licensee.
I believe for each of these outside tech spec lift inaccuracies, if the plant would have known it...they would have been required to shutdown.
JUL 10 2015
Surveillance Criteria
On May 11,
2015 at approximately 0923, Unit 2 was at 1 00 percent rated thermal power
(RTP) when the "as found" testing results of the 2-stage main steam
safety relief valves (SRVs) were received which indicated that two of eleven of
the Unit 2 SRVs had experienced a setpoint drift during the previous operating
cycle which resulted in their failure to meet the Technical Specification (TS)
opening setpoint of 1150 +1- 34.5 psig percent as required by TS Surveillance
Requirement (SR) 3.4.3.1.
The root
cause of the SRV setpoint drift is attributed to corrosion-induced bonding between
the pilot disc and seating surfaces. This conclusion is based on previous root
cause analyses and the repetitive nature of this condition at Hatch and within
the BWR industry. All 2-stage SRVs with platinum coated pilot seats were
removed from Unit 2 during the 2015 refueling outage and replaced with 3-stage
SRVs with amodified pilot. 3-stage SRVs typically do not exhibit set point
drift and the modified pilot reduces instances of vibration induced spurious
openings and leak-by.
A 3-stage
SRV with a similar modified pilot was installed on Unit 2 during the ·2013
refueling outage. Based upon "as-found" testing results, it was seen
that pressure lift setpoints were maintained during plant operation.
DESCRIPTION
OF EVENT
On May 11 2015, at approximately 0923, with
Unit 2 at 100 percent rated thermal power (RTP), "as-found" testing
of the 2-stage main steam safety relief valves (SRVs) (EllS Code RV) showed
that two of the ten main steam SRVs that were tested had experienced a drift in
pressure lift setpoint during the previous operating cycle such that the allowable
technical specification {TS) surveillance requirement (SA) 3.4.3.1 limit of
1150 +1-
34.5 (+/-
3%) psig had been exceeded. Below is a table illustrating the as found testing
results of Unit 2 SRVs that were removed from service during the Spring 2015
refueling outage and replaced with 3-stage SRVs
.
MPL Pilot Serial No. Lift Pressure Percent
Drift
2B21-F013B 1006 1155 0.40%
2B21-F013C 1231 1172 1.90%
2B21-F013D 303 1184 3.00%
2B21-F013E 315 1210 5.20%
2B21-F013F 1189 1179 2.50%
2B21-F013G 302 1174 2.10%
2B21-F013H 1230 1190 3.50%
2B21-F013K 1229 1164 1.20%
2B21-F013L 1228 1163 1.10%
2B21-F013M 1008 1179 2.50%
The 2-stage SRVs that were installed on Unit
2 during the previous cycle (Cycle 23) utilized platinum coated pilot discs.
The 3-stage SRVs currently installed on Unit 2 have a modified pilot that helps
reduce the possibility of inadvertent lift and leak by due to system vibration.
The one 3-stage SRV that was installed on Unit 2 during Cycle 23 was recently
successfully tested and found to be within the allowable TS SA pressure lift
setpoint limit of 1150 +1- 34.5 (+/- 3%) psig.
CAUSE OF
EVENT
The root cause of the SRV setpoint drift is
attributed to corrosion-induced bonding between the pilot disc and its seating surface.
This conclusion is based on previous root cause analyses and the repetitive
nature of this condition at Plant Hatch and in the industry. In General
Electric (GE) Service Information Letter (SIL) 196, Supplement 16, GE determined
that condensation of steam in the pilot chamber of Target Rock 2-stage SRVs can
cause oxygen and hydrogen dissolved in the steam to accumulate. As steam
condenses in the relatively stagnant pilot chamber, the dissolved gases are
released. In a volume such as the pilot chamber which is normally at
approximately a 1000 psig pressure and a temperature of 545 degrees F, the
total pressure consists primarily of water vapor partial pressure because 544.6
degrees F is the saturation temperature at 1000 psi g. This wet, hot,
high-oxygen atmosphere can be very corrosive and can increase the likelihood of
corrosion-induced bonding of the pilot disk to its seat. It was also noted that
proper insulation minimizes the accumulation rate of non-condensable gases and
the steady-state oxygen partial pressure. Despite improvements made in
maintaining the integrity of insulation for the previously installed 2-stage
SRVs and installing new platinum coated pilots, the corrosion-induced bonding
continued to occur as evidenced by the test results from this most recent
outage.
REPORTABILITY
ANALYSIS AND SAFETY ASSESSMENT
This event is reportable in accordance with
10 CFR 50.73(a)(2)(i)(B) because a condition occurred that is prohibited by TS
Surveillance Requirement (SR) 3.4.3.1. Specifically, an example of multiple
test failures is given in NUREG-1022, Revision 3, "Event Reporting
Guidelines 10 CFR 50.72 and 50.73" which describes the sequential testing
of safety valves. This example notes that "Sometimes multiple valves are
found to lift with set points outside of technical specification limits."
NUREG-1022 further states in the example
that "discrepancies found in TS surveillance tests should be assumed to occur
at the time of the test unless there is firm evidence, based on a review of relevant
information (e.g., the equipment history and the cause of failure), to indicate
that the discrepancy occurred earlier. However, the existence of similar
discrepancies in multiple valves is an indication that the discrepancies may
well have arisen over a period of time and the failure mode should be evaluated
to make this determination." Based on this guidance and the fact that the
development of the corrosion occurred over a period of time of plant operation,
the determination was made that this "as found" condition is
reportable under the reporting requirements of 10 CFR 50.73(a)(2)(i)(B). There
are eleven SRVs located on the four main steam lines within the drywell in
between the reactor pressure vessel (RPV) (EllS Code RPV) and the inboard main
steam isolation valves (MSIVs) (EllS Code ISV). These SRVs are required to be
operable during Modes 1, 2, and 3 to limit the peak pressure in the nuclear
system such that it will not exceed the applicable ASME Boiler and Pressure
Vessel Code Limits for the reactor coolant pressure boundary. The SRVs are
tested in accordance with TS surveillance requirement 3.4.3.1 in which the
valves are tested as directed by the In-Service Testing Program to verify lift
set points are within their specified limits to confirm they would perform
their required safety function of overpressure protection. The SRVs must
accommodate the most severe pressurization transient which, for the purposes of
demonstrating compliance with the ASME Code Limit of 1375 psig peak vessel
pressure, has been defined by an event involving the closure of all MSIVs with
a failure of the direct reactor protection system trip from the MSIV position
switches with the reactor ultimately shutting down as the result of a high
neutron flux trip (a scenario designated as MSIVF).
The results from this MSIVF event analysis
was performed by the Nuclear Fuels Department in order to bound the "as-found"
results of the U2 Cycle 21 2-stage SRVs pressure setpoint drift. The results
from this analysis showed a small increase in peak pressures relative to the
Hatch-2 Cycle 21 reload licensing analysis (ALA) results. The higher peak
pressures were due to the fact that eight of the eleven SRVs opened at pressures
higher than that which was assumed in the ALA. It should be noted that in this
analysis, the larger actual valve bore size was used in the calculations for
nine of the valves rather than the smaller bore size which was conservatively
assumed in the ALA. Therefore, higher steam flow capacities than those assumed
in the ALA were used in this analysis for those nine valves.
Based on the analysis, the calculated
minimum margin to the 1375 psig ASME Boiler and Pressure Vessel Code overpressure
limit for peak vessel pressure would have been 27.7 psig and the minimum margin
to the 1325 psig Tech Spec Safety Limit for the reactor steam dome pressure
would have been 2.9 psig during an MSIVF event during Cycle 21 operation.
Therefore, these test results show that in this case, where two of the eleven
SRVs would have opened at pressures higher than that which was assumed in the
RLA, the peak pressure at the bottom of the vessel would have remained below
the ASME Boiler and Pressure Vessel code limit and the peak RPV dome pressure
remained within the TS Safety limits.
Additionally, a highly reliable, though
non-credited, electrical actuation system serves as a redundant, independent method
to actuate the SRVs. During Cycle 23 this redundant electrical logic system was
fully functional. Based on the analyses performed, the overpressure protection system
would have continued to perform its required safety function if called upon in
its "as found" condition. Therefore, this event had no adverse impact
on nuclear safety and was of very low safety significance.
CORRECTIVE
ACTIONS
The 2-stage SRVs with platinum-coated pilot
discs were removed from Unit 2 during the 2015 refueling outage and replaced
with 3-stage SRVs that have a modified pilot. 3-stage SRVs typically do not
exhibit set point drift due to their design. The modified pilots will help
reduce spurious openings and leak-by due to system vibration.
ADDITIONAL INFORMATION
Other Systems Affected: None
Failed Components Information:
Master Parts List Number: 2B21-F013E, H
Manufacturer: Target Rock
Model Number: 7567F
Type: Relief Valve
Manufacturer Code: T020
EllS System Code: SB
Reportable to EPIX: Yes
Root Cause Code: B
EllS Component Code: RV
Commitment Information: This report does not
create any licensing commitments.
PREVIOUS SIMILAR EVENTS:
LER 1-2014-003, identified multiple SRV
setpoint drifts for 5 of the 11 two-stage SRVs installed on Unit 1. The two-stage
SRVs with platinum-coated pilot discs were removed from Unit 1 during the 2014
refueling outage and replaced with 3-stage SRVs that have a modified pilot. The
modified pilots will help reduce spurious openings and leak-by due to system
vibration.
LER 1-2012-004, identified multiple SRV
setpoint drift for 8 of the 11 SRVs. Corrective actions included replacement of
the 2-stage SRVs with 2-stage SRVs whose pilot discs had undergone a platinum
surface treatment which was considered at that time to be the long term fix for
this corrosion bonding issue.
LER 2-2011-002, identified multiple SRV
setpoint drift for 8 of the 11 SRVs. Corrective actions included replacement of
the 2-stage SRVs with 3-stage SRVs during the Unit 2 Spring 2011 refueling outage
which was considered at that time to be the long term fix for this corrosion
bonding issue. Subsequent to that outage the 3-stage SRVs exhibited signs of
unacceptable leakage which resulted in two separate outages that involved
changing out four SRVs during the first outage and the remaining seven SRVs
during the subsequent outage in May 2012. The 3-stage SRVs were replaced with
2-stage SRVs containing pilot discs that had undergone the platinum surface
treatment.
LER 1-2010-001, identified multiple SRV
setpoint drift for 5 of the 11 SRVs. Corrective actions included refurbishment
of the pilot valves and included the replacement of the pilot discs with discs
made from corrosion-induced bonding. These were the same actions that were
taken following similar failures reported in LEA 2-2009-001, since improved
results had been seen to some degree in the industry for at least one operating
cycle when these actions were implemented.
