05000186/LER-2021-004, University of Missouri Research Reactor, Deviation from Technical Specification 3.2.g.10
| ML21327A305 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 11/23/2021 |
| From: | Robertson J Univ of Missouri - Columbia |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 2021-004-00 | |
| Download: ML21327A305 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| 1862021004R00 - NRC Website | |
text
Ei:) University of Missouri Research Reactor Center
1513 Research Park Drive Columbia, MO 65211
PHONE 573-882-4211 WEB murr.missouri.edu November 23, 2021
ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
REFERENCE: Docket No. 50-186 University of Missouri-Columbia Research Reactor Renewed Facility Operating License No. R-103
SUBJECT: Written communication as required by University of Missouri Research Reactor Technical Specification 6.6.c(3) regarding a deviation from Technical Specification 3.2.g.10
The enclosed document provides the University of Missouri-Columbia Research Reactor (MURR)
Licensee Event Report (LER) for an event that was discovered on November 15, 2021, and resulted in a deviation from MURR Technical Specification 3.2.g.10.
If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118.
\\~~
J. David Robertson, PhD Reactor Facility Director
JDR:jlm
Enclosure
xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Thomas Spencer, Vice Chancellor for Research and Economic Development Mr. Geoffrey Wertz, U.S. Nuclear Regulatory Commission Mr. Craig Bassett, U.S. Nuclear Regulatory Commission Enclosure U.S. Nuclear Regulatory Commission November 23, 2021
Licensee Event Report No. 21-04 -November 15, 2021 University of Missouri Research Reactor
Introduction
On November 15, 2021, while the reactor was shut down in preparation for scheduled routine maintenance, the Control Room operators shut down the pool coolant system and noticed that the ' Reflector Hi-Low Diff Pressure Scram' annunciator alann did not initiate as pool coolant flow rate decreased to zero. The Control Room operators contacted reactor management, and an investigation was initiated to determine if the safety scram function from the differential pressure across the reflector instrument channel (pressure transmitter PT 917 instrument channel) actuated correctly. Further testing revealed the low reflector differential pressure safety scram from the PT 917 instrument channel did not activate due to a failed alarm meter unit.
This is a deviation from Technical Specification (TS) 3.2.g.10.
TS 3.2.g states, "The reactor safety system and the number (N) of associated instrument channels necessary to provide the following scrams shall be operable whenever the reactor is in operation. Each of the safety system functions shall have 1 IN logic where N is the number of instrument channels required for the corresponding mode of operation. " TS 3.2.g.10 requires a differential pressure across the reflector scram at trip set points of 2.52 psi minimum and 8.00 psi maximum while the reactor is in Mode I operation. The failed alarm meter unit successfully provided the 8.00 psi maximum scram, but it failed to provide the 2.52 psi mm1mum scram.
The failed alarm meter unit was replaced with an exact spare, and the instrument channel was retested and documented to be back in compliance with TS 3.2.g.10. In accordance with TS 6.6.c(4), authorization was obtained from the Reactor Facility Director prior to returning the reactor to operation later that day.
Description of the PT 917 Instrument Channel
As described in Section 7.6.4.2, Pool Coolant System, of the MURR Safety Analysis Report (SAR), the PT 917 instrument channel automatically initiates a reactor scram due to a pool coolant system low flow condition. Additionally, PT 917 instrument channel scram serves as a backup to the two (2) pool coolant low flow instrument scrams.
"Differential pressure across the reactor pool reflector is monitored by PT 917. This instrument also initiates a reactor scram, serving as a back-up to the pool low flow scram. The transmitter senses pr essure at the hot leg of the pool coolant system near pool coolant isolation valve V509 and provides an input signal to an Alarm Meter Unit. If pressure increases above or decreases below a predetermined set point, a reactor scram and a "Reflector Hi-Low Diff Pressure Scram" annunciator alarm are initiated. The Alarm Meter Unit opens contacts in the process input string to E3B of the Reactor Safety System NCLUs, thereby interrupting power to the control blade electromagnets. The Alarm Meter Unit also provides reactor pool reflector differential pressure indication on the instrument panel."
Page 1 of 4 Enclosure U.S. Nuclear Regulatory Commi ssion November 23, 2021
Detailed Event Description
On November 15, 2021, while the reactor was shut down in preparation for scheduled routine maintenance, the Control Room operators shut down the pool coolant system and noticed that the ' Reflector Hi-Low Diff Pressure Scram ' annunciator alarm did not initiate as pool coolant flow rate decreased to zero. The Control Room operators contacted reactor management, and an investigation was initiated to determine if the safety scram function from the PT 917 instrument channel actuated correctly.
The scram set point check portion of compliance procedure CP-5, "PT-917 - 10 MW," was used to test whether the scram trip actuation amplifiers (T AAs) would reset with the pool coolant system shutdown and without preconditioning the instrument channel in any way. The scram T AAs did reset, indicating the PT 917 instrument channel scram was not operable. Testing revealed the high pressure PT 917 instrument channel scram, which satisfies the 8.00 psi maximum TS 3.2.g.10 requirement, worked; however, the low pressure PT 917 instrument channel scram, which satisfies the 2.52 psi minimum TS 3.2.g.10 requirement, failed.
Further testing revealed the safety scram signal from the PT 91 7 instrument channel did not occur due to a failed alarm meter unit. This is a deviation from TS 3.2.g.10.
TS 3.2.g states, "The reactor safety system and the number (NJ of associated instrument channels necessary to provide the following scrams shall be operable whenever the reactor is in operation. Each of the safety system functions shall have 1 IN logic where N is the number of instrument channels required for the corresponding mode of operation. " TS 3.2.g.10 requires a differential pressure across the reflector scram at trip set points of 2.52 psi minimum and 8.00 psi maximum while the reactor is in Mode I operation.
The failed alarm meter unit was replaced with an exact spare, and the PT 91 7 instrument channel was retested and documented to be back in compliance with TS 3.2.g.10 using CP-5. In accordance with TS 6.6.c( 4), authorization from the Reactor Facility Director was obtained prior to returning the reactor to operation later that day.
Safety Analysis
On July 19, 2021, the low differential pressure scram from the PT 917 instrument channel was documented to have occurred at 2.93 psi, well above the TS 3.2.g.10 minimum pressure of 2.52 psi. On November 15, 2021, the low differential pressure reflector scram from the PT 91 7 instrument channel was documented as inoperable. Therefore, some time between July 19, 2021, and November 15, 2021, the PT 917 low differential pressure reflector scram became inoperable. However, on the morning of November 8, 2021, one week prior to this event, the Interim Assistant Reactor Manager-Operations (ARM-0) was conducting training with a reactor operator trainee during a pool coolant system shutdown. The Interim ARM-0 was showing the trainee the proper annunciator indications for a shutdown lineup, which included verifying proper annunciator indications indicating that the PT 91 7 instrument channel was operable. Therefore, the PT 917 instrument channel low differential pressure scram was probably inoperable for a maximum of one (1) week of reactor operation.
Page 2 of 4 Enclosure U.S. Nuclear Regulatory Commission November 23, 2021
The basis of TS 3.2.g.10 states that the PT 917 scrams serves as a backup to the two (2) pool coolant low flow instrument scrams required by TS 3.2.g.9. Therefore, there are three (3) redundant pool coolant low flow scram instrument channels. Redundancy is incorporated into the MURR design and TS so that three (3) different pool flow safety channels would have to fail to allow the reactor to operate without pool coolant low flow protection. On November 15, 2021, only one (1) of two (2) low flow scrams from the PT 917 instrument channel failed. On June 14, 2021, pool coolant flow transmitters FT 912D and FT 912F low flow scrams operated correctly above the 850 gpm minimum flow required by TS 3.2.g.9.
The most conservative assumption is that the PT 917 instrument channel low differential pressure reflector scram was inoperable for the entire four (4) months between July 19, 2021, and November 15, 2021.
However, as stated above, it was probably only inoperable for a maximum of one (1) week. During that time, the high differential pressure scram from PT 917 instrument channel and the two (2) pool coolant low flow scrams from the FT 912D and FT 912F instrument channels provided low pool coolant flow protection to the reactor.
In summary, at no time was the ability to automatically scram the reactor due to a pool coolant low flow condition in doubt due to redundancies incorporated into the reactor design and TS.
Corrective Actions
The failed alarm meter unit was replaced with an exact spare, and the PT 917 instrument channel was retested and documented to be back in compliance with TS 3.2.g.10 using CP-5 later on November 15, 2021.
The failed PT 917 alarm meter unit is an LFE Corporation 195 Series Meter Relay Controller. The unit causes a scram trip by a mechanical meter needle passing through a light beam which momentarily breaks the light beam directed at a photo sensor. This model of meter relay controller has been used at MURR for more than 30 years without a failure to activate a scram until this event. When MURR has had failures from this unit in the past, spurious scrams have been activated due to light bulb failure or electrostatic charge causing the mechanical meter needle to pass through the light beam.
Evaluation of the failed meter relay controller indicated its light bulb did not appear properly seated in its holder. Though it is not known for sure that incorrect seating of the bulb caused this event, bench testing of the suspect alarm trip unit has shown that this meter relay controller can be made to fail in the non-safe, energized, no-trip condition if the light bulb is not correctly placed in its holder.
MURR has a plan to replace the PT 91 7 instrument channel alarm meter unit with a more modem alarm trip unit. However, until the PT 917 instrument channel is upgraded, the MURR electronic maintenance procedure EMP-41, "Meter Relay Unit Alarm Lamps," will be revised to ensure the technician changing the light bulb in a meter relay unit clearly understands how to ensure the light bulb is in the unit correctly and ensure the technician understands the importance of the bulb being properly seated in its holder. In addition, a second electronics technician will concurrently verify the bulb is being installed correctly.
Page 3 of 4 Enclosure U.S. Nuclear Regulatory Commission November 23, 2021
The Control Room operators' common practice to verify the correct plant shutdown indications on the annunciator panel is not currently in MURR procedures. However, this verification of annunciator indications reduced the time of this TS deviation. Therefore, verification of correct annunciator indications after the shutdown of the reactor, primary coolant system, and pool coolant system will be added to the reactor shut down, primary coolant system, and pool coolant system procedures.
Additionally, this event has been entered into the MURR Corrective Action Program as CAP No. 21-0120, and any additional information or corrective actions will be considered and documented in that CAP entry.
If there are any questions regarding this LER, please contact me at (573) 882-5118. I declare under penalty of perjury that the foregoing is true and correct.
ENDORSEMENT:
Sincerely, 6.~
Reactor Manager Reactor Facility Director
State of Missouri County of Boone
Subscribed and sworn before me this 23 rd day of November, 2021.
JACQUELINE L. MATYAS My Commission Expires March 26, 2023 Howard County Commission ##15634308
Page 4 of 4