05000331/LER-1992-001, :on 920115,determined That Triple Low Level Trip Function May Not Be Functional at High Drywell Temps.On 910129,determined That HPCI & RCIC Trips May Not Function at High Drywell Temps.Level Instruments Calibr
| ML20092F420 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 02/14/1992 |
| From: | Axline J, Denise Wilson IES UTILITIES INC., (FORMERLY IOWA ELECTRIC LIGHT |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| DAEC-92-0068, DAEC-92-68, LER-92-001, LER-92-1, NUDOCS 9202190322 | |
| Download: ML20092F420 (8) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii) |
| 3311992001R00 - NRC Website | |
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- - Iowa Electric Light and Power Company 2
February 14, 1992 DAEC-92-0068 Mr. A. Bert Davis Regional Administrator Region III U. S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, IL 60137 Subject: Duane Arnold Energy Center Docket No: 50-331 Op. License DPR-49 Licensee Event Report #92-001 Gentlemen:
In accordance with 10 CFR 50.73 please find attached a copy cf the subject Licensee Event Report.
Very truly yours,
- Lwl 60 lOh David L. Wilson Plant Superintendent - Nuclear DLW/JA/eah cc
Director of Nuclear Reactor' Regulation Document Control Desk U.S. Nuclear Regulatory Commission l
Mail Station-P1-137 Washington,'D C. -20555 NRC Resident Inspector - DAEC l
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On 1/15/92, with the plant operating at 86*. power coasting down for a refuel t
outage, it was determined that -the triple low level trip functioii associated-with Yarway level switches may not be functional at high drywell temperatures.
On 1/29/92, during review of the effects of high drywell temperatures on other vessel level instrumentation, it was determined that the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) system automatic high level trips may not function at high drywell temperatures, Following verification of the problems on 1/15 and 1/29, the associated instrumentation was satisfactorily re-calibrated to ensure trips would occur, as required, under
- - high drywell temperature conditions.
The cause of the problem identified on 1/15 was incorrect information contained-in GE SIL 29S compounded by an incorrect statement in information provided by the instrument vendor and the verification calculation performed by General Electric.
The cause of the problem identified on 1/29 was failure to account for the effects of high drywell temperatures on the high level trip L
instrumentation when the instrument line reference legs were re-routed during the 1988 refuel outage.
Neither of the instrumentation problems had an effect on the safe operation of the plant.
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DESCRIPTION OF EVENT
TRIPLE LOW LEVEL INSTRUMENTATION 1
On January 15, 1992, with the plant operating at 86% power coasting down for a refuel outage, it was determined that the triple low level trip function associated with Yarway wide range (8-218 inches above top of I
active fuel) level switches, would not be functional with reference leg temperatures above approximately 420 degrees F.
This equates, worst case, to a drywell temperature of approximately 230 F., however, it should be.
noted that there is a significant time lag (approximately 20 - 30 minutes) between the time drywell temperatures reach 230 degrees and r.eference leg temperatures reach 420 degrees. The triple low level function associated with these switches provides initiation signals for Low Pressure Coolant Injection (LPCI), Core Spray (CS), Emergency Diesel Generators (EDGs),
t Group I Isolation (main steam lines), Group VII Isolation (Reactor l
Building Closed Cooling Water System (RBCCW) and well water containment cooling), and the Automatic Depressurization System (A05).
In addition to the above initiations, the River Water Supply (RWS) valves fail open and the radwaste dilution valves fail closed on the triple low level
- - signal.
The above determination was made during review of setpoint calculations as part of the Ouane Arnold Energy Center (DAEC) setpoint design basis reconstitution program.
At 2307 hours0.0267 days <br />0.641 hours <br />0.00381 weeks <br />8.778135e-4 months <br />, following General Electric verification of the DAEC setpoint calculations, 6 and 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Limiting Conditions for Operation (LCO) were entered as required by Primary Containment Isolation System (PCIS) and Emergency Core Cooling System (ECCS) instrumentation Technical Specifications, respectively. At 0005 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> on 1/16/92 the 6 hour6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> LC0 was exited when calibration of the PCIS triple low level instruments to the new setpoint was complete. At 0555 on 1/16/92, the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LC0 was exited following calibration of the remaining triple low level instruments.
HIGH LEVEL INSTRUMENTATION During review of the effects of high drywell temperatures on other vessel level instrumentation, it was identified that the instruments which feed into the feedwater level control system as well as instruments which provide the high level High Pressure Coolant Injection (HPCI), Reactor Core Isolation Cooling (RCIC), feed pump, and main turbine trips would be affected. The resultant effect would cause indicated level to be lower than actual for high drywell temperature conditions.
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the other with CC-4562) of instrument lines associated with the above j
M ' mmentation. Due to dif ferences in the variable leg lengths in the g
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Once vessel level exceeds the upper tap of
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Wu reference legs, indicated level will no longer increase.
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alculated variable leg temperatures at which the 211" HPCI and RCIC trip points would not be reached (because indicated level is lower than the trip point when vessel level exceeds the upper reference leg taps) is approximately 305 degrees F for instruments associated with the CC-4561 instrument lines and 205 degrees F for instruments associated with the CC-4562 instrument lines. Due to the configuration of the HPCI and RCIC logic (2 of 2 once), the HPCI and RCIC high level trips would become inoperable at a variable leg temperature of approximately 205 degrees F.
The feed pump and main turbine trips would become inoperable at approximately 305 degrees F due to their logic configuration (2 of 3 once with two instruments being connected to the CC-4561 instrument lines).
On 1/29/92, with the plant operating at 82% power, following verification of the above calculations, the level mit-hes associated with the HPCI and RCIC 211" high level trips were 9clered inoperable.
In compliance with Technical Specifications, a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LCO was entered at 2111 hours0.0244 days <br />0.586 hours <br />0.00349 weeks <br />8.032355e-4 months <br />.
At 0323 hours0.00374 days <br />0.0897 hours <br />5.340608e-4 weeks <br />1.229015e-4 months <br />, on 1/30/92, following re-calibration of the level instruments to ensure HPCI and RCIC trips would occur as required, the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> LC0 was exited.
II.
CAUSE OF EVENT
TRIPLE LOW LEVEL INSTRUMENTATION The cause of this event was incorrect information contained in General Electric (GE) Service Information Letter (SIL) 299 along with incorrect communication of information by the instrument vendor.
An additional factor whwh contributed to using the incorrect information provided by the SIL and the instrument vendor was the response GE provided when contracted by Iowa Electric to perform the verification of the instrument vendor's triple low level calculation. This verification restated that the SIL information was correct.
SIL 299 identified that large increases in drywell temperature, such as those which could occur during a pipe rupture in the drywell, would cause level instruments associated with Yarway sensing lines to be inaccurate.
The sit identified that "the indicated reactor vessel water level can be higher than the true level by 12.7% of the reference leg's total length (scale length + 6 inches top suppression + 6 inches t,ttom suppression)"
The example in the Sll for this calculation stated the. an instrument with
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of 222 inches, or +28 inches in the indicated reactor vessel water level".
What the SIL should have said was +18% of 222 inches, or +40 inches in the indicated reactor vessel water level.
This is equivalent to +28 inches (+12.7% of 222) measured dif ferential pressure at standard temperature (approximately 70 degrees F.) and atmospheric pressure at the Yarway differential pressure instrument.
The conversion factor between instrument differential pressure and indicated level is a factor of 1.4.
During initial review of SIL 299, DAEC personnel identified a discrepancy between measured drywell temperatures in the area of the two wide range Yarway instrument columns.
The instrument manufacturer was contacted to address this discrepancy.
The response provided by the manufacturer included calculations of the affects, identified in SIL 299, on the Yarway instrument columns.
These calculations were performed using units of differential pressure measured at the instrument, however, the last statement made in the calculation concluded that the ECCS setpoint change from 18.5 inches to 46.5 inches (setpoint increase of 28 inches) instrument scale was conservative.
Following receipt of the information from Yarway, General Electric was contracted to provide an independent verification of the Yarway triple low level calculation. GE performed an independent calculation with the results being similar to those in the Yarway calculation.
In conclusion, GE stated that the instrumentation correction recommended by SIL 299 is, therefore, considered verified.
HIGH LEVEL INSTRUMENTATION The cause for the HPCI and RCIC high level trips being inoperable at high drywell temperatures is the configuration of their associated instrument L
lines in the drywell. Both CC-4561 and CC-4562 reference legs have very short lengths (approximately 2') of vertical piping in the drywell, howeve,; the variable legs have approximately 13' and 32' of vertical piping in the drywell, respectively. The difference in vertical piping length in the drywell between the reference leg and variable leg, associated with an instrument, introduces inaccuracies in measured differential pressure with changes in drywell temperature.
These inaccuracios result in lower than actual level measurements at the instruments associated with CC-4561 and CC-4562 instrument lines for the current piping configuration.
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- - 8 n m,,,.....,,n.,,,.n..s.,,... m.., m o.,,,, w anon The reference legs which feed the narrow range vessel level instruments are shared by the fuel zone level instruments. To improve vessel level indication over a wide range of vessel level and high drywell temperatures (Ref Reg Guide 1.97 and Generic Letter 84-23), the-reference legs were lre-routed-to minimize the difference between the reference leg piping length and fuel zone variable leg piping length in the drywell, during the 1988 refuel outage. -This significantly reduced the fuel zone instrument error introduced during high drywell temperature conditions, as well as reversing-the direction of the error.such that actual. level would be higher than indicated. The effect on the narrow range vessel level _ instruments at high drywell temperatures was to reduce indicated
- - level-below actual level to the extent that at a variable leg temperature
Review of the design change package which re-routed the above reference legs identified that the effects of high drywell temperatures on the narrow range level instrumentation high-level trip setpoints were not calculated, and therefore, not addressed.
III.
ANALYSIS OF EVENT
Neither of the above setpoint discrepancies had an ef fect on the safe -
operation of the plant. _ Following verification of acceptable triple low level and high-level setpoints, the af fected instrumentation was re-calibrated satisfactorily, i
TRIPLE LOW' LEVEL INSTRUMENTATION To address the effects of the triple low level trip interrelation with drywell temperatures during other plant conditions, the accident scenario
-. discussed in SIL 299 was evaluated for the DAEC.
The level indication changes due to increasing drywell temperature occur rather slowly because the heat-up time of the Yarway reference-leg is calculated to be 20 to 30 minutes.. The drywell temperature related events of concern are steam line breaks in the drywell. Analysis of these events shows that the diverse parameter, high drywell pressure, will initiate the HPCI, LPCI, and CS parts of the ECCS as well as the EDGs, before double low or triple low reactor water level is reached. Although the low pressure'ECCS are initiated and ready to inject water on high drywell
- - pressure, they will not inject water into the vessel until vessel pressure is below the 450 psig interlock for the low pressure ECCS. Available early t
in the event is RCIC which is initiated on double low reactor water level l
and is not affected by the high drywell temperatures.
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For breaks that are larger than.1 square feet, the ADS (which is automatically initiated at triple low water level) is not required because the combination of the break area anJ the high pressure core cooling (RCIC as HPCI is~ assumed failed) will depressurize the reactor and allow the low pressure ECCS to inject into the vessel. For breaks that are smaller than,l' square feet, it is expected that RCIC will be able to maintain-reactor water level and therefore low pressure ECCS will not be required.
If the =1ow pressure ECCS is not required to maintain reactor water level then a depressurizattorc accomplished by ADS also would not be required.
Therefore, the operability of the triple low level instrument that provides the input-_to ADS logic would not be required.
SIL 299 supplement 1 states;that RCIC may be lost _due to high pressure at the turbine exhaust. This would result in low pressure ECCS and ADS being required.
The RCIC high exhaust pressure trip point at the DAEC occurs at a? pressure sufficiently high enough that the RCIC system will not-be affected by high drywell pressure..
HIGH LEVEL' INSTRUMENTATION To address _ the affects. of the high level trips interrelation with drywell
. temperatures during other plant conditions, a Loss of Offsite Power loss
' Of Cool ant A_ccident- (LOOP LOCA) event was _ reviewed. _ For large break -
LOCAs, HPCI and RCIC would not be required to function shortly after event initiation-as-vessel pressure would:be reduced to the point that low pressure-ECCS would begin to inject to ensure adequate core cooling.
For_ this _ reason, _ failure of-the high level trips to function at-high drywel_1 temperatures would have no adverse effect.
For small break LOOP LOCAs, vessel pressure would not reduce quickly enough-to allow low pressure ECCS to function early in the event.
Therefore, HPCI and/or RCIC-would be required to-maintain vessel level.
High drywell pressure would be reached soon into the event. initiating a SCRAM along with_HPCI. -At-the same time drywe_11 temperatures would continue to rise (Note: Emergency Operating Procedures (E0Ps). direct operators to -initiate a-plant shutdown if bulk average drywell temperatures cannot be maintained below 150 degrees F., therefore,
depending on the rate of drywell_ temperature /p.ressure increase, a shutdown
- - may be initiated before-the automatic scram occurs.) towards 280 degrees-which is the bulk' average drywell temperature at which E0Ps require drywell spray. The high level trip' instruments' ' variable leg temperatures
- - would lag behind drywell temperatures-by several minutes.
It should be noted that the two-narrow range level indicators associated with CC-4561 instrument lines would continue to function (with an offset), at a bulk
~ average drywell temperature of 280 degrees, for several minutes into the event.
It is expected _ that this information along with indicated C
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increasing' level on other control room ' instrumentation would provide the
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operators with adequate information to minimize vessel overfilling in the event that the high level trips did not occur automatically. In addition, the HPCI~ and RCIC turbines are constructed such that-introduction of some amount of_ water into the steam lines would not damage the turbines.
Therefore, following an automatic or manual trip of HPCI or RCIC, the units would still be_ functional and able to restart if level were to drop below their_ initiation setpoint.
IV. C0'RRECTIVE ACTIONS:
TRIPLE. LOW LEVEL INSTRUMENTATION Following verification of DAEC setpoint calcul'ations by GE, the affected Yarway level instruments were calibrated to the new setpoint of 64' inches.
Note:-The setpoint of 64 inches includes a correction factor for the SIL miscommunication along with an additional conservatism factor (approximately 10-inches) in accordance with new GE setpoint calculation
. methodology. Updating of procedures, training, and plant documentation to reflect the new triple low setpoint has been initiated.
HIGH LEVEL INSTRUMENTATION To correct for the loss of the HPCI and RCIC high level trips at high drywel1 temperatures, the setpoints on the-associated instruments were reduced to ensure high level ~ trips would occur as required. Due to the-large temperature affect on the instruments' associated with the CC-4562 instrument lines, the setpoints associated with these instruments had to
- - be reduced significantly (from 211" to 182").
The setpoint of 182" is below the normal vessel level of 192", therefore, half of the HPCI and RCIC trip logic is complete during-normal plant operation. As reduction in the setpoints-is not considered'to be an acceptable long. term corrective action, modifications will be made to appropriate logic and/or instrument lines to allow the high. level' trip points:to be acceptably l
above the normal operating range.
(Note: The setpoints for instruments g
associated with CC-4561_had to be' reduced by only 4_ inches.) Long term l-corrective actions that have been completed prior to submittal of the next l
_ update to the Integrated Living Schedule, as well as any pending corrective actions, will be discussed in that update. Guidance on the effects of drywell temperatures on level instrumentation was issued to the Operations Department.
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..<,,...ua..,,,, m... m,n,o In February of 1990, an Instrumentation and Controls Engineering (I&CE) grcup was formed within the Engincering Department.
This group is actively involved in the initial stages of the design change process and serves to support review of industry documentation related to instrumentation. Along with providing dedicated personnel, specifically for instrumentation and control work, the I&CE group is currently working on a setpoint design basis reconstitution program.
It was this program that identified the level instrumentation discrepancies.
Completion of this program will provide confidence that instrumentation setpoints are acceptable.
Program completion will be in accordance with the existing Integrated Living Schedule.
V. Additional Information
A, Previous Similar Events A review of DAEC LERs since 1984 did not identify any previous events where incorrect information in a SIL led to inadequate instrument setpoints.
In addition, no previous events were identified where failure to account for the affects of high drywell temperatures on instruments resulted in inadequate setpoints.
B. EIIS SYSTEM AND COMPONENT CODES Systems:
BO, BM, EK, JM, CC, SB, BS, BJ, BN, JE Components: LI, LIC, LIS This event is being reported pursuant to 10 CFR 50.73 (a)(2)(ii).
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