05000328/LER-2011-001, Regarding Nuclear Instrumentation System Power Range Neutron Flux Trip Low Range Bistable Incorrectly Calibrated
| ML11236A247 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  (DPR-079) |
| Issue date: | 08/22/2011 |
| From: | John Carlin Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 11-001-00 | |
| Download: ML11236A247 (11) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3282011001R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Soddy Daisy, Tennessee 37384-2000 August 22, 2011 10 CFR 50.73 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Unit 2 Facility Operating License No. DPR-79 NRC Docket No. 50-328 Subject: License Event Report 32812011-001, "Nuclear Instrumentation System Power Range Neutron Flux Trip Low Range Bistable Incorrectly Calibrated" The enclosed licensee event report provides details concerning an event at the Sequoyah Nuclear Plant where a power range neutron flux low range trip instrumentation had been incorrectly calibrated during a refueling outage. The incorrect calibration and preparation for low power physics testing resulted in 2 of 4 instrument channels being inoperable during startup of the unit.
This report is being submitted in accordance with 10 CFR 50.73(a)(2)(i)(B), as any operation or condition prohibited by the Technical Specifications. The cause of the event is still under investigation. A supplement to this event report is expected to be submitted by September 29, 2011.
There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact G. M. Cook, Sequoyah Site Licensing Manager, at (423) 843-7170.
SieyiLC President Sequoyah Nuclear Plant Enclosure: Licensee Event Report - Nuclear Instrumentation System Power Range Neutron Flux Trip Low Range Bistable Incorrectly Calibrated cc: Regional Administrator-Region II
" ý NRC Senior Resident Inspector-Sequoyah Nuclear Plant k
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/20 (10-2010)
, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to, the dinformation collection.
- 3. PAGE Sequoyah Nuclear Plant Unit 2 05000328 1 OF 10
- 4. TITLE: Nuclear Instrumentation System Power Range Neutron Flux Trip Low Range Bistable Incorrectly Calibrated
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED S
FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SUMEN REV MONTH DAY YEAR NUMBER NO.
I FACILITY NAME DOCKET NUMBER 06 22 2011 2011 -
001 00 08 22 2011
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 2 l 20.2201(b)
D 20.2203(a)(3)(i)
[]
50.73(a)(2)(i)(C)
[]
50.73(a)(2)(vii)
El 20.2201(d)
[
20.2203(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
[
50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
[
20.2203(a)(4)
E] 50.73(a)(2)(ii)(B)
Ej 50.73(a)(2)(viii)(B)
El 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii)
. []
50.73(a)(2)(ix)(A)
- 10. POWER LEVEL El 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
El 50.73(a)(2)(iv)(A)
Ej.,50.73(a)(2)(x) 005 E] 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
E] 73.71(a)(4)
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
E] 50.73(a)(2)(v)(B) nl 73.71(a)(5)
El 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C) rI OTHER 20.2203(a)(2)(vi)
[
50.73(a)(2)(i)(B)
[
50.73(a)(2)(v)()
Specify in Abstract below (El or in LCO 3.0.4a - Specifies in part, when LCO conditions are not met, entering into a Mode or other specified condition shall only be made, if Actions permit continued operation in the Mode or condition for an unlimited period of time.
LCO 3.3.1 - Specifies in part, the minimum reactor trip system instrumentation channels to be Operable. The minimum number of Power Range Neutron Flux low range channels required to be operable is 3 of the 4 channels for Modes 1 and 2. When one of the four channels is inoperable, Action 2 allows for continued operation provided the inoperable channel, limited to 1 channel, be placed in tripped condition within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
LCO 3.10.3 - Specifies in part, the limitations of various LCOs may be suspended during Mode 2 for physics tests provided, Thermal Power does not exceed 5 percent of RTP, setpoints of Operable Intermediate and Power Range NIS Channels are set at less or equal to 25 percent of RTP, and the Reactor Coolant System (RCS) [EIIS Code AB]
lowest operating loop temperature is greater than or equal to 531 degrees Fahrenheit.
Actions for exceeding 5 percent Rated Thermal Power and RCS temperature limits are provided. No action is provided for failure to have necessary NIS Channels Operable.
Safety Limits and Limiting Safety System Settings (LSSS), 2.2.1, "Reactor trip System Instrumentation Setpoints," specifics, in part, the setpoints for the reactor trip system instrumentation. LSSS 2.2.1, defines the Applicability for these setpoints to be in accordance with LCO 3.3.1. If a condition of LSSS 2.2.1 is not met, the associated Action in LCO 3.3.1 is taken. This specification is mentioned here because of the direct connection to LCO 3.3.1 and the NIS Channel N44 being not in compliance with the setpoint requirement.
The TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(i)(B), any operation or condition prohibited by the TSs.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
NIS Channel N43 was inoperable because it was connected to the plant reactivity computer for low power physics testing. This channel was in a tripped condition in accordance with TS LCO 3.3.1, Action 2. In the tripped condition, one half of the 2 out of 4 logic was complete for a reactor trip.
NIS Channel N44 was inoperable due to the power range neutron flux low range bistable being incorrectly calibrated.
C.
Dates and Approximate Times of Major Occurrences
May 26, 2011 at With Unit 2 in Mode 6 following reactor shutdown for approximately 1130 scheduled maintenance, Maintenance personnel are DST authorized to perform calibration on the NIS Channel N44.
Maintenance personnel inadvertently calibrated the NIS N44 Power Range Neutron Flux Low Range Bistable setpoint.
June 2, 2011 Surveillance package for calibration of NIS Channel N44 acceptance criteria was reviewed and found satisfactory.
June 21, 2011 at With Unit 2 in Mode 3, NIS Channel N43 was removed 1230 DST from service and placed in tripped condition to comply with LCO 3.3.1, Action 2, for setup of the plant reactivity computer for low power physics testing.
June 22, 2011 at Unit 2 enters Mode 2.
0411 DST LCO 3.0.4a is applied for Mode 2 operation for known inoperable equipment: Penetration X-19 for post accident sampling line valves [EIIS Code IP] and NIS Channel N43.
However, compliance with LCO 3.3.1 was not met, because Operation personnel were unaware of the inoperable NIS Channel N44 due to the incorrect setpoint of the power range neutron low range bistable. LCO 3.0.3 applies at this time with both NIS Channels N43 and N44 inoperable since LCO 3.3.1, Action 2, does not provide actions for more than one inoperable channel. LCO 3.0.4a would not be met at this time, because LCO 3.0.3 does not permit continued operation in the Mode 2.
June 22, 2011 at LCO Actions are complied with for known inoperable 0411 DST equipment under LCO 3.6.3, "Containment Isolation Valves" and LCO 3.3.1 for entering into Mode 2.
June 22, 2011 at Low power physics testing commences. LCO 3.10.3 is 0502 DST entered. Because Operations personnel were unaware that NIS Channel N44 was inoperable and with NIS Channel N43 inoperable, compliance with LCO 3.10.3 was not met. LCO 3.0.3 would have been required to be entered, because LCO 3.10.3 does not provide an Action for failure to meet the LCO.
June 22, 2011 at Compliance with LCO 3.0.3 is not met for failure to be in 1111 DST Hot Standby, for inoperable NIS Channels N43 and N44.
June 22, 2011 at Low power physics testing is complete and LCO 3.10.3 is 1323 DST no longer invoked.
June 22, 2011 at LCO 3.3.1, Action 2, for NIS Channel N43 is exited with the 1453 DST channel restored to operable status. LCO 3.0.3 would not have applied because only one channel (i.e., NIS Channel N44) was inoperable.
June 22, 2011 at 1729 DST Unit 2 enters Mode 1 U
I -
June 23, 2011 at Operations personnel enter abnormal operating procedures 0222 DST for nuclear instrument malfunction due to NIS Channel N44 indicating light failure with RTP greater than 25 percent.
Upon discovery, LCO 3.3.1, Action 2, is entered to place NIS Channel N44 in tripped condition.
June 23, 2011 at LCO 3.3.1, Action 2, to place inoperable NIS Channel N44 0355 DST in tripped condition is complete.
June 23, 2011 at NIS Channel N44 is returned to operable status following 1830 DST maintenance and LCO 3.3.1, Action 2, is exited.
June 29, 2011 Investigation of the power range high neutron flux low range bistable card determined the card had not failed, but had been calibrated incorrectly.
D.
Other Systems or Secondary Functions Affected
No other systems or secondary function were affected.
E.
Method of Discovery
On June 23, 2011, at 0222 DST, during power ascension above 25 percent RTP, Operations personnel discovered that the NIS Channel N44 indicating light was "dark" and declared the channel inoperable.
F.
Operator Actions
Following the discovery, Operations personnel entered into an abnormal operating procedure for the inoperable nuclear instrumentation and complied with TSs actions for the associated inoperable equipment.
G.
Safety System Responses:
No safety system response was required.
Ill.
CAUSE OF THE EVENT
A.
Immediate Cause:
The immediate cause of the event was incorrect setting of the NIS Channel N44 for the power range neutron flux low range setpoint.
B.
Root Cause:
The root cause for this event is still under investigation. When the final investigation is completed, this report will be updated.
C.
Contributing Factor:
Contributing factor(s) for this event is still under investigation. When the final investigation is completed, this report will be updated.
IV.
ANALYSIS OF THE EVENT
The NIS power range neutron flux trip circuit trips the reactor when two of the four power range channels exceed the trip setpoint (i.e., two out of four logic). There are two independent bistables each with their own trip setting (a high and a low setting) per channel (four channels total). TS LSSS 2.2.1 indicates that the reactor trip nominal setpoints for the power range high neutron flux low range and the high range is 25 percent and 109 percent of RTP, respectively. The NIS Channel N44 low and high range settings had been calibrated to 60 percent of RTP. NIS Channels N41 and N42 were operable with low range setting within the TSs limits of 25 percent of RTP and the high range setting calibrated to 60 percent of RTP as intended to support low power physics testing. NIS Channel N43 was in tripped condition, providing one half the logic for a reactor trip, with this channel aligned to the reactivity computer.
The NIS power range neutron flux protection function provides reactor protection for events that result in an increase in reactor power as measured by an increase in neutron flux external to the reactor vessel. This function is, in part, credited for mitigation of the uncontrolled rod cluster control assembly (RCCA) bank [EIIS Code AA] withdrawal from a subcritical condition and withdrawal at power; excessive heat removal due to feedwater system [EIIS Code SJ] malfunctions; rupture of a control rod drive mechanism housing (i.e., RCCA Ejection); excessive load increase; accidental depressurization of the Main Steam System [EIIS Code SB]. Each of these events is discussed below. An assessment of the safety consequences and implication regarding low power physics testing is also considered.
An RCCA withdrawal accident is defined as an uncontrolled addition of reactivity to the reactor core caused by withdrawal of rod cluster control assemblies resulting in a power excursion. Such a transient could be caused by a malfunction of the reactor control or control rod drive systems. This could occur with the reactor either subcritical, at hot zero power or at power. The neutron flux response to a continuous reactivity insertion is characterized by a very fast rise attenuated by the reactivity feedback effect of the negative Doppler coefficient. The limiting transient for "at power" is analyzed to occur at full RTP with the power range neutron flux high range trip conservatively assumed to be 118 percent of RTP. Any rod withdrawal at power would have remained within the analyzed conditions since it would have been mitigated by the trip (i.e., 2 out of 4 logic to complete) provided by the power range neutron flux high range trip of NIS Channel N41, I
N42, or N44, as these were set to 60 percent of RTP. For the RCCA withdrawal from a subcritical and at zero power, the Updated Final Safety Analysis Report (UFSAR) assumes the power range neutron flux low range trip function is credited for initiating the reactor trip signal. The result of the analysis for subcritical and at zero power indicates nuclear power overshoots the RTP for a short time period with fuel thermal response lagging and remaining below RTP. As a result, departure from nucleate boiling (DNB) is maintained within limit. Given that the design of the power range high neutron flux instrumentation meets single-failure criteria, with one of 4 channels failed (i.e., NIS Channel N44,) and the NIS Channel N43 in tripped or operable condition, the 2 other operable channels, N41 and N42, were available to complete the logic for a reactor trip signal.
Excessive heat removal due to feedwater system malfunctions are means of increasing core power above full power. These malfunctions may occur as a result of reductions in feedwater temperature or additions of excessive feedwater. Either malfunction may be caused by equipment failure or an operator error. With the plant at no-load conditions, the addition of cold feedwater may cause a decrease in RCS temperature and thus a reactivity insertion due to the effects of the negative moderator coefficient of reactivity.
For the cases of an accidental full opening of one or more feedwater control valves with the reactor at zero power it has been determined in the UFSAR, the maximum reactivity insertion rate is less than the maximum reactivity insertion rate analyzed in the uncontrolled RCCA withdrawal from a subcritical condition discussed previously. It is noted in the UFSAR, that if this event occurs with the unit just critical at no load, the power range neutron flux low range trip may mitigate this event. As discussed, the design of the power range neutron flux instrumentation meets single-failure criteria, such that with one of 4 channels failed (i.e., NIS Channel N44) and the NIS Channel N43 in tripped or operable condition, the 2 other operable channels, N41 and N42, were available to complete the logic for a reactor trip signal.
In the case of continuous addition of excessive feedwater, the event is mitigated by the steam generator high-high level trip, which closes all feedwater regulator isolation valves, trips main feedwater pumps and trips the turbine.
The ejected RCCA event is initiated by an assumed mechanical failure of the control rod mechanism pressure housing which results in a rapid reactivity insertion. The responses are rapid with the power spike occurring over a small fraction of a second. The transient, as analyzed, is mitigated by the power range neutron flux low range trip (i.e.,
for hot zero power) and the power range neutron flux high range trip (i.e., for full power.)
As previously discussed, the power range neutron flux instrumentation meets single-failure criteria, such that with one of 4 channels failed (i.e., NIS Channel N44) and the NIS Channel N43 in tripped or operable condition, the 2 other operable channels, N41 and N42, were available to complete the logic for a reactor trip signal. As such, the analysis of the ejected rod event is not affected by the instrument channel condition as described in this LER.
An excessive load increase incident is defined as a rapid increase in the steam flow that causes a power mismatch between the reactor core power and the steam generator load demand. Protection against an excessive load increase accident is provided by the power range high neutron flux trip, overpower delta T trip and overtemperature delta T trip. The UFSAR evaluates these accidents without crediting a reactor trip and demonstrates that reactivity feedback limits power and prevents DNB from occurring.
The most severe core conditions resulting from an accidental depressurization of the main steam system are associated with an inadvertent opening of a single steam dump, relief or safety valve. The steam release as a consequence of this accident results in an initial increase in steam flow which decreases during the accident as the steam pressure falls. The energy removal from the RCS causes a reduction of coolant temperature and pressure. The analysis is performed to demonstrate that the following criterion is satisfied: Assuming a stuck rod cluster control assembly and a single failure in the Engineered Safety Features the DNB design basis will be met after reactor trip. Should the reactor be just critical or operating at power at the time of a steam release, the reactor is protected by the power range high neutron flux trip, overpower delta T trip and overtemperature delta T trip. As discussed previously, the design of the power range high neutron flux instrumentation meets single-failure criteria, such that with one of 4 channels failed (i.e., NIS Channel N44) and the NIS Channel N43 in tripped or operable condition, the 2 other operable channels, N41 and N42, were available to complete the logic for a reactor trip signal. As such, the event would be mitigated and the minimum DNB ratio remains above the limiting value, no consequential damage to the core or reactor system occurs.
During a portion of this event, low power physics testing was in progress to ensure that the operating characteristics of the core are consistent with the design predictions and that the core can be operated as designed. Although these Physics Tests are generally accomplished within the limits for all LCOs, conditions may occur when one or more LCOs must be suspended to make completion of Physics Tests possible or practical.
This is acceptable as long as the fuel design criteria are not violated. When one or more of the requirements specified in LCO 3.1.1.3, "Moderator Temperature Coefficient," LCO 3.1.1.4, "Minimum Temperature for Criticality," LCO 3.1.3.1, "Movable Control Assemblies," LCO 3.1.3.5, "Shutdown Rod Insertion Limit," and LCO 3.1.3.6, "Control Rod Insertion Limits," are suspended for Physics Tests, the fuel design criteria are preserved as long as the thermal power does not exceed 5 percent of RTP and RCS lowest operating loop temperature is greater than or equal to 531 degrees Fahrenheit.
During the nearly 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br /> of testing, the conditions for limitation suspension of the above LCOs were met, except for the unknown inoperable NIS Channel N44. During the physics testing, LCOs not met as allowed by LCO 3.10.3 included LCO 3.1.3.5 and LCO 3.1.3.6, when shutdown and control rods were inserted beyond the insertion limit specified in the Core Operating Limits Report. As in the other UFSAR analyzed event, this event would be mitigated by the reactor protection system [EIIS Code JC]. The I
condition of NIS Channel N44, during low power physics testing, would not have prevented a reactor trip,'as the design of the power range high neutron flux instrumentation meets single-failure criteria, such that with one of 4 channels failed (i.e.,
NIS Channel N44) and the NIS Channel N43 in tripped condition, the 2 other operable channels, N41 and N42, were available to complete the logic for a reactor trip signal.
V.
ASSESSMENT OF SAFETY CONSEQUENCES
Based on the above "Analysis of The Event," this event did not adversely affect the health and safety of plant personnel or the general public.
VI.
CORRECTIVE ACTIONS
A.
Immediate Corrective Actions
Operations personnel entered into TS LCO 3.3.1, Action 2, for the inoperable power range neutron flux instrumentation channel. The affected NIS channel card was identified and replaced and the channel was calibrated to meet TS limits.
B. Corrective Actions to Prevent Recurrence:
The root cause for this event is still under investigation. When the final investigation is completed, this report will be updated.
VII.
ADDITIONAL INFORMATION
A.
Failed Components:
The failed component, as a result of an incorrect calibration, was the Power Range High Neutron Flux bistable circuit card used for the low range trip setpoint. The card is manufactured by Westinghouse Electrical Company and is model number 3359C39G01.
B. Previous LERs on Similar Events:
A review of previous reportable events for the past 3 years did not identify any previous
similar events
C.
Additional Information
Human performance cause and circumstances for this event are still under investigation.
When the final investigation is completed, this report will be updated. Contributing factors discussed in this LER are being addressed within the Corrective Action Program under Problem Evaluation Report number 397142.
D. Safety System Functional Failure:
This event did not result in a safety system functional failure in accordance with 10 CFR 50.73(a)(2)(v).
E. Unplanned Scram with Complications:
This event did not result in a scram of Unit 2.
VIII.
COMMITMENTS
None.
I