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| issue date = 01/12/2018 | | issue date = 01/12/2018 | ||
| title = Written Communication as Required by University of Missouri Research Reactor Technical Specification 6.6.c(3) Regarding a Deviation from Technical Specifications 3.4.b and 3.4.a(6) | | title = Written Communication as Required by University of Missouri Research Reactor Technical Specification 6.6.c(3) Regarding a Deviation from Technical Specifications 3.4.b and 3.4.a(6) | ||
| author name = Sanford M | | author name = Sanford M | ||
| author affiliation = Univ of Missouri | | author affiliation = Univ of Missouri | ||
| addressee name = | | addressee name = | ||
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=Text= | =Text= | ||
{{#Wiki_filter:(ii University of Missouri Research Reactor Center 1513 Research Park Drive Columbia, MO 65211 PHONE 573-882-4211 WEB murr.missouri.edu January 12, 2018 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station Pl-37 Washington, DC 20555-0001. | {{#Wiki_filter:(ii University of Missouri Research Reactor Center 1513 Research Park Drive Columbia, MO 65211 PHONE 573-882-4211 WEB murr.missouri.edu January 12, 2018 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station Pl-37 Washington, DC 20555-0001. | ||
==REFERENCE:== | ==REFERENCE:== | ||
Docket No. 50-186 University of Missouri-Columbia Research Reactor Renewed Facility Operating License No. R-103 | |||
==SUBJECT:== | ==SUBJECT:== | ||
Written communication as required by University of Missouri Research Reactor Technical Specification 6.6.c(3) regarding a deviation from Technical Specifications 3.4.b and 3.4.a(6) | |||
The attached document provides the University of Missouri-Columbia Research Reactor (MURR) | |||
Licensee Event Report (LER) for an event that occurred on December 30, 2017, that resulted in a deviation from MURR Technical Specificatio11:5 3.4.b and 3.4.a(6). | |||
If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118. | If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118. | ||
Sincerely, 1/{tvtr.-k~ | Sincerely, 1/{tvtr.-k~ | ||
Matthew R. Sanford Interim Reactor Facility Director MRS:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark McIntosh, Vice Chancellor for Research, Graduate Studies and Econo:rnic 1-.£ Z Z-. | Matthew R. Sanford Interim Reactor Facility Director MRS:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark McIntosh, Vice Chancellor for Research, Graduate Studies and Econo:rnic 1-.£ Z Z-. | ||
Development Mr. Geoffrey Wertz, U.S. NRC Mr. Johnny Eads, U.S. NRC jiJZO | Development Mr. Geoffrey Wertz, U.S. NRC Mr. Johnny Eads, U.S. NRC .jiJZO | ||
~--------------------------------------------- | |||
----Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Licensee Event Report No. 17 | ~--------------------------------------------- ---- | ||
The reactor was subsequently scrammed and secured by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical Specification (TS) 3.4.b; which states, "Reactor containment integrity shall be maintained at all times except when: (1) The reactor is secured,* | Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Licensee Event Report No. 17 December 30, 2017 University of Missouri Research Reactor Introduction On December 30, 2017, with the reactor operating at 10 MW in the automatic control mode, a Reactor Operator (RO) on routine patrol discovered that the differential pressure gauge in the reactor containment building was indicating zero (0) inches of water vacuum as compared to the laboratory building pressure. The reactor was subsequently scrammed and secured by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical Specification (TS) 3.4.b; which states, "Reactor containment integrity shall be maintained at all times except when: (1) The reactor is secured,*... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following condftions shall be satisfied: ... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas; ... " Investigation revealed the backup door in the reactor containment building exhaust air plenum had closed, causing a pressure increase in the reactor containment building. | ||
... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following condftions shall be satisfied: | Description of the Backup Doors As described in Section 6.2.3.4, Supply and Exhaust Ducts, of the MURR Safety Analysis Report (SAR), the supply and exhaust air ducts contain two (2) electric-motor-driven horizontal sliding doors, which are located in the ventilation plenums on the fifth level of the reactor containment building. Actuation of the reactor isolation or facility evacuation system will close these sliding doors sealing off the reactor containment building ventilation flow to the surrounding area. These sliding doors are the primary means of isolating the reactor containment ventilation flow during a reactor isolation or facility evacuation and are part of the containment system. | ||
... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas; ... " Investigation revealed the backup door in the reactor containment building exhaust air plenum had closed, causing a pressure increase in the reactor containment building. | As described in Section 6.2.4, Backup Doors, of the SAR, a second set of isolation doors, designated the backup doors, are located in the reactor containment building supply and exhaust plenums thereby providing redundancy for reactor containment building isolation. When the _ | ||
Description of the Backup Doors As described in Section 6.2.3.4, Supply and Exhaust Ducts, of the MURR Safety Analysis Report (SAR), the supply and exhaust air ducts contain two (2) electric-motor-driven horizontal sliding doors, which are located in the ventilation plenums on the fifth level of the reactor containment building. | backup doors are shut, the steel plenum chamber above the door becomes part of the containment system. | ||
Actuation of the reactor isolation or facility evacuation system will close these sliding doors sealing off the reactor containment building ventilation flow to the surrounding area. These sliding doors are the primary means of isolating the reactor containment ventilation flow during a reactor isolation or facility evacuation and are part of the containment system. As described in Section 6.2.4, Backup Doors, of the SAR, a second set of isolation doors, designated the backup doors, are located in the reactor containment building supply and exhaust plenums thereby providing redundancy for reactor containment building isolation. | The backup doors are constructed of ~-inch (0.64-cm) thick metal plate and reinforced to withstand 2.0-psig overpressure (13.8-kPa above atmosphere) (see Figure 1). Each door is held open against gravity by a pneumatic cylinder. Compressed air is supplied to the pneumatic cylinders from the facility main air compressors and the emergency air compressor. A %-inch (0.95-cm) rubber gasket installed in the door facing creates a seal for the backup doors when in the closed position. | ||
When the _ backup doors are shut, the steel plenum chamber above the door becomes part of the containment system. The backup doors are constructed of ~-inch (0.64-cm) thick metal plate and reinforced to withstand 2.0-psig overpressure (13.8-kPa above atmosphere) (see Figure 1). Each door is held open against gravity by a pneumatic cylinder. | Page 1 of 5 | ||
Compressed air is supplied to the pneumatic cylinders from the facility main air compressors and the emergency air compressor. | |||
A %-inch (0.95-cm) rubber gasket installed in the door facing creates a seal for the backup doors when in the closed position. | Attachment U.S. Nuclear Regulatory Commission January 12, 2018 The backup doors are normally kept open during reactor operation. A radiation level greater than the set point of either of the reactor bridge radiation monitors, or either of the exhaust plenum radiation monitors, will release air pressure from the cylinders and close both isolation doors automatically. Two solenoid-operated valves, installed in series, control the compressed air supply to each pneumatic cylinder. A closure signal will de-energize both solenoid-operated valves causing air to be vented from the pneumatic cylinder, allowing gravity to close the isolation door. Actuation of either of the solenoid-operated valves will close the backup door . | ||
Page 1 of 5 Attachment U.S. Nuclear Regulatory Commission January 12, 2018 The backup doors are normally kept open during reactor operation. | EXISTING t1<1* ST"L PL ON ALL DOOR SIDES AND TOP rRAME ON 5TH LEVEL | ||
A radiation level greater than the set point of either of the reactor bridge radiation monitors, or either of the exhaust plenum radiation monitors, will release air pressure from the cylinders and close both isolation doors automatically. | :ue* RUBBER SEAL ALL ARCJ\MD ' | ||
Two solenoid-operated valves, installed in series, control the compressed air supply to each pneumatic cylinder. | SUPPORT AT EDCiE OF' GRATING C2 aACn> | ||
A closure signal will de-energize both solenoid-operated valves causing air to be vented from the pneumatic cylinder, allowing gravity to close the isolation door. Actuation of either of the solenoid-operated valves will close the backup door . | _____ F'LDDR GRATING ___,// | ||
PLENUM TD IE REINFORCED TO VITHSTAND 2 PSI OVER PRESSURE DOUBLE ACTING PNEUMATIC CYL. | |||
This pressure reading is required to meet the requirements of surveillance TS 4.4.d, which states, "When required by Specification 3.4.b, containment integrity shall be verified to exist each shift." The RO on patrol immediately radioed the Lead Senior Reactor Operator (LSRO) about this abnormal indication. | ACTUATED BY TRIP Of' lSDLAT[ON SYSTEM | ||
Since MURR has only one differential pressure indication for reactor containment building pressure, the LSRO directed other personnel to investigate the various components of the reactor containment system to verify whether or not the differential pressure indication was valid. After a few minutes of investigation, the exhaust plenum backup door was found to be closed. Upon hearing the backup door was closed, the LSRO immediately scrammed and secured the reactor at 07: 17 by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical TS 3.4.b, which states, "Reactor containment integrity shall be mdintained at all times except when: (1) The reactor is secured, ... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following conditions shall be satisfied: | / | ||
... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas: ... " Closure of the exhaust plenum backup door blocked the path for air to exit the reactor containment building. | TYPICAL rOR BOTH ~ TD CLOS£ CJN LOSS OF' AIR. | ||
The supply fan continued to run, and the supply plenum was still open; therefore, air continued to be supplied to the building without air exhausting, which increased pressure in the reactor containment building, thus driving differential pressure to zero (0). After the reactor was secured, Reactor Operations management directed the LSRO to perform Compliance Procedure-20 (CP-20), which tests the mechanical and electrical portions of the air supply system to the backup door air cylinders. | DOORS ON REACTOR LOSS OF' ELEC. DR ISDLATltw TRIP STH LEVEL FIGURE 1 BACKUP DOOR Page 2 of 5 | ||
CP-20 satisfies the applicable requirements of surveillance TS 4.4.c. While preparing to conduct CP-20, it was observed that the air actuator of the exhaust plenum backup door would not open the door. In addition, operation of the supply plenum backup door was slower than normal. It was quickly determined that ice had formed in the compressed air supply lines to the backup doors' air cylinders (the temperature in the area surrounding the air supply line to the backup door cylinders was approximately 10 °F that morning). | |||
The compressed air supply piping was heated, water was drained from the piping, and electric heat trace with insulation was applied to the pipip.g to prevent freezing of any future moisture in the piping. Operation of the backup doors was tested successfully by completing CP-20 -both backup doors performed normally. | Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Detailed Event Description At approximately 06:57 on December 30, 2017, an RO on routine patrol (which is performed approximately every 4 hours) discovered that the differential pressure gauge in the reactor containment building was indicating zero (0) inches of water vacuum as compared to the laboratory building pressure. This pressure reading is required to meet the requirements of surveillance TS 4.4.d, which states, "When required by Specification 3.4.b, containment integrity shall be verified to exist each shift." The RO on patrol immediately radioed the Lead Senior Reactor Operator (LSRO) about this abnormal indication. Since MURR has only one differential pressure indication for reactor containment building pressure, the LSRO directed other personnel to investigate the various components of the reactor containment system to verify whether or not the differential pressure indication was valid. After a few minutes of investigation, the exhaust plenum backup door was found to be closed. Upon hearing the backup door was closed, the LSRO immediately scrammed and secured the reactor at 07: 17 by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical TS 3.4.b, which states, "Reactor containment integrity shall be mdintained at all times except when: | ||
As required by TS 6.6.c(4), permission to start up the reactor was received from the Interim Reactor Facility Director. | (1) The reactor is secured, ... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following conditions shall be satisfied: ... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas: ... " | ||
The reactor was started and obtained 10 MW operation at | Closure of the exhaust plenum backup door blocked the path for air to exit the reactor containment building. The supply fan continued to run, and the supply plenum was still open; therefore, air continued to be supplied to the building without air exhausting, which increased pressure in the reactor containment building, thus driving differential pressure to zero (0). | ||
Redundancy is incorporated into the system to ensure that no single component or circuit failure will render any portion of the containment system inoperative. | After the reactor was secured, Reactor Operations management directed the LSRO to perform Compliance Procedure-20 (CP-20), which tests the mechanical and electrical portions of the air supply system to the backup door air cylinders. CP-20 satisfies the applicable requirements of surveillance TS 4.4.c. While preparing to conduct CP-20, it was observed that the air actuator of the exhaust plenum backup door would not open the door. In addition, operation of the supply plenum backup door was slower than normal. It was quickly determined that ice had formed in the compressed air supply lines to the backup doors' air cylinders (the temperature in the area surrounding the air supply line to the backup door cylinders was approximately 10 °F that morning). The compressed air supply piping was heated, water was drained from the piping, and electric heat trace with insulation was applied to the pipip.g to prevent freezing of any future moisture in the piping. Operation of the backup doors was tested successfully by completing CP-20 - both backup doors performed normally. As required by TS 6.6.c(4), permission to start up the reactor was received from the Interim Reactor Facility Director. The reactor was started and obtained 10 MW operation at 17 :52 on December 30, 2017. | ||
Isolation of the reactor containment building can be automatically initiated by radiation detectors located at the reactor pool upper bridge and in the containment building exhaust plenum. Isolation can be manually activated by switches in the reactor control room or the facility lobby. Actuation of the facility lobby switch will also cause a facility evacuation. | Page 3 of 5 | ||
Preceding the discovery that the reactor containment building differential pressure decreased to zero (0) inches of water vacuum, the reactor had been operating at 10 MW in automatic control since December 26, 2017. During a routine patrol occurring between 02:30 and 03:40 on December 30, 2017, prior to the event, an RO logged the containment building differential pressure at 1.50 inches of water vacuum. The operators log the differential pressure approximately every four (4) hours. As stated above, surveillance TS 4.4.d requires reactor containment integrity to be verified to exist within a shift. Since MURR operates on 12-hour shifts, MURR routinely checks containment integrity three (3) times per shift. Therefore, the maximum time in which MURR may have deviated from TS 3.4.b was approximately four (4) hours. Negative pressure in the reactor containment building compared to the surrounding areas when the reactor is not secured is a TS requirement and good nuclear practice. | |||
However, no MURR safety analyses are based on the containment building pressure being negative. | Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Safety Analysis As described in Section 6.2.1, Introduction, of the SAR, the containment system is designed to completely isolate the reactor containment building, thereby preventing or mitigating any uncontrolled release of radioactive materials to the environment during an accident. | ||
Though the backup door was closed and unable to open due to ice blockage of its compressed air supply line, the reactor isolation and facility evacuation systems were fully operational and capable of isolating the reactor containment building from the surrounding areas. In addition, the air supply lines to all the other components of the reactor containment system are housed inside the building in controlled temperature environments. | Redundancy is incorporated into the system to ensure that no single component or circuit failure will render any portion of the containment system inoperative. Isolation of the reactor containment building can be automatically initiated by radiation detectors located at the reactor pool upper bridge and in the containment building exhaust plenum. Isolation can be manually activated by switches in the reactor control room or the facility lobby. Actuation of the facility lobby switch will also cause a facility evacuation. | ||
Therefore, reactor isolation and facility evacuation systems could perform their safety functions as described in the SAR. The backup doors failed to their 'fail safe' position, which is closed. Corrective Actions The cause of this event was lower than normal air temperatures surrounding the compressed air piping combined with moisture in the piping. This combination allowed ice to form and slowly block off the air supply to the exhaust plenum backup door air cylinder. | Preceding the discovery that the reactor containment building differential pressure decreased to zero (0) inches of water vacuum, the reactor had been operating at 10 MW in automatic control since December 26, 2017. During a routine patrol occurring between 02:30 and 03:40 on December 30, 2017, prior to the event, an RO logged the containment building differential pressure at 1.50 inches of water vacuum. The operators log the differential pressure approximately every four (4) hours. As stated above, surveillance TS 4.4.d requires reactor containment integrity to be verified to exist within a shift. Since MURR operates on 12-hour shifts, MURR routinely checks containment integrity three (3) times per shift. Therefore, the maximum time in which MURR may have deviated from TS 3.4.b was approximately four (4) hours. | ||
As the air pressure slowly bled out of the cylinder, the weight of the door overcame the upward force of the air cylinder, and the exhaust backup door closed. Prior to reactor startup on December 30, 2017, the water was drained from the compressed air lines, and heat trace with insulation was applied to the piping to prevent ice from forming in the lines as a short-term solution. | Negative pressure in the reactor containment building compared to the surrounding areas when the reactor is not secured is a TS requirement and good nuclear practice. However, no MURR safety analyses are based on the containment building pressure being negative. | ||
Until the compressed air quality is improved, Reactor Operations staff are also draining condensation for the air supply Page 4 of 5 Attachment U.S. Nuclear Regulatory Commission January 12, 2018 piping near the cylinders every week. MURR is currently investigating possible improvements to the main air system to reduce the amount of moisture in the compressed air piping. If Reactor Operations staff had continuous indication of reactor containment building differential pressure in the Control Room, they may have be able to prevent the deviation of TS 3.4.b by taking action to secure the reactor prior to the negative pressure decreasing below the TS 3.4.a(6) specification of 0.25 inches of water. MURR will be installing a continuous Control Room indication with a low differential pressure alarm in the future. In addition, a Reactor Emergency Procedure will be developed to provide Reactor Operations staff direction in case of a lowering reactor containment building differential pressure. | Though the backup door was closed and unable to open due to ice blockage of its compressed air supply line, the reactor isolation and facility evacuation systems were fully operational and capable of isolating the reactor containment building from the surrounding areas. In addition, the air supply lines to all the other components of the reactor containment system are housed inside the building in controlled temperature environments. Therefore, reactor isolation and facility evacuation systems could perform their safety functions as described in the SAR. The backup doors failed to their 'fail safe' position, which is closed. | ||
Additionally, this event has been entered into the MURR Corrective Action Program as CAP Number 17-0149. Any additional improvements 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. | Corrective Actions The cause of this event was lower than normal air temperatures surrounding the compressed air piping combined with moisture in the piping. This combination allowed ice to form and slowly block off the air supply to the exhaust plenum backup door air cylinder. As the air pressure slowly bled out of the cylinder, the weight of the door overcame the upward force of the air cylinder, and the exhaust backup door closed. Prior to reactor startup on December 30, 2017, the water was drained from the compressed air lines, and heat trace with insulation was applied to the piping to prevent ice from forming in the lines as a short-term solution. Until the compressed air quality is improved, Reactor Operations staff are also draining condensation for the air supply Page 4 of 5 | ||
I declare under penalty of perjury that the foregoing is true and correct. Sincerely, .&t1'1P- | |||
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 piping near the cylinders every week. MURR is currently investigating possible improvements to the main air system to reduce the amount of moisture in the compressed air piping. | |||
'{/p', ..... ~~!***-~Gf.,. | If Reactor Operations staff had continuous indication of reactor containment building differential pressure in the Control Room, they may have be able to prevent the deviation of TS 3.4.b by taking action to secure the reactor prior to the negative pressure decreasing below the TS 3.4.a(6) specification of 0.25 inches of water. MURR will be installing a continuous Control Room indication with a low differential pressure alarm in the future. In addition, a Reactor Emergency Procedure will be developed to provide Reactor Operations staff direction in case of a lowering reactor containment building differential pressure. | ||
=~: NOTARV*P~ | Additionally, this event has been entered into the MURR Corrective Action Program as CAP Number 17-0149. Any additional improvements 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, Reviewed and Approved, | |||
.&t1'1P- ;ft(~~ | |||
Bruce A. Meffer;{ Matthew R. Sanford Reactor Manager Interim Reactor Facility Director | |||
,,*UP.*,,, | |||
,.,?i..O,.'f '{/p', JACQUELINE L MATYAS | |||
..... ~~!***-~Gf.,. | |||
=~: | |||
NOTARV*P~ My Commission Expires March 26, 2019 | |||
~~*.. SEAL.~ Howard County | |||
*,'f,,orlX\,.~ | |||
....,, . Commission #15634308 Page 5 of 5}} | |||
Latest revision as of 19:28, 3 February 2020
| ML18017A233 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 01/12/2018 |
| From: | Sanford M Univ of Missouri |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 17-06 | |
| Download: ML18017A233 (6) | |
Text
(ii University of Missouri Research Reactor Center 1513 Research Park Drive Columbia, MO 65211 PHONE 573-882-4211 WEB murr.missouri.edu January 12, 2018 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station Pl-37 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 Specifications 3.4.b and 3.4.a(6)
The attached document provides the University of Missouri-Columbia Research Reactor (MURR)
Licensee Event Report (LER) for an event that occurred on December 30, 2017, that resulted in a deviation from MURR Technical Specificatio11:5 3.4.b and 3.4.a(6).
If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118.
Sincerely, 1/{tvtr.-k~
Matthew R. Sanford Interim Reactor Facility Director MRS:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark McIntosh, Vice Chancellor for Research, Graduate Studies and Econo:rnic 1-.£ Z Z-.
Development Mr. Geoffrey Wertz, U.S. NRC Mr. Johnny Eads, U.S. NRC .jiJZO
~--------------------------------------------- ----
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Licensee Event Report No. 17 December 30, 2017 University of Missouri Research Reactor Introduction On December 30, 2017, with the reactor operating at 10 MW in the automatic control mode, a Reactor Operator (RO) on routine patrol discovered that the differential pressure gauge in the reactor containment building was indicating zero (0) inches of water vacuum as compared to the laboratory building pressure. The reactor was subsequently scrammed and secured by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical Specification (TS) 3.4.b; which states, "Reactor containment integrity shall be maintained at all times except when: (1) The reactor is secured,*... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following condftions shall be satisfied: ... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas; ... " Investigation revealed the backup door in the reactor containment building exhaust air plenum had closed, causing a pressure increase in the reactor containment building.
Description of the Backup Doors As described in Section 6.2.3.4, Supply and Exhaust Ducts, of the MURR Safety Analysis Report (SAR), the supply and exhaust air ducts contain two (2) electric-motor-driven horizontal sliding doors, which are located in the ventilation plenums on the fifth level of the reactor containment building. Actuation of the reactor isolation or facility evacuation system will close these sliding doors sealing off the reactor containment building ventilation flow to the surrounding area. These sliding doors are the primary means of isolating the reactor containment ventilation flow during a reactor isolation or facility evacuation and are part of the containment system.
As described in Section 6.2.4, Backup Doors, of the SAR, a second set of isolation doors, designated the backup doors, are located in the reactor containment building supply and exhaust plenums thereby providing redundancy for reactor containment building isolation. When the _
backup doors are shut, the steel plenum chamber above the door becomes part of the containment system.
The backup doors are constructed of ~-inch (0.64-cm) thick metal plate and reinforced to withstand 2.0-psig overpressure (13.8-kPa above atmosphere) (see Figure 1). Each door is held open against gravity by a pneumatic cylinder. Compressed air is supplied to the pneumatic cylinders from the facility main air compressors and the emergency air compressor. A %-inch (0.95-cm) rubber gasket installed in the door facing creates a seal for the backup doors when in the closed position.
Page 1 of 5
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 The backup doors are normally kept open during reactor operation. A radiation level greater than the set point of either of the reactor bridge radiation monitors, or either of the exhaust plenum radiation monitors, will release air pressure from the cylinders and close both isolation doors automatically. Two solenoid-operated valves, installed in series, control the compressed air supply to each pneumatic cylinder. A closure signal will de-energize both solenoid-operated valves causing air to be vented from the pneumatic cylinder, allowing gravity to close the isolation door. Actuation of either of the solenoid-operated valves will close the backup door .
EXISTING t1<1* ST"L PL ON ALL DOOR SIDES AND TOP rRAME ON 5TH LEVEL
- ue* RUBBER SEAL ALL ARCJ\MD '
SUPPORT AT EDCiE OF' GRATING C2 aACn>
_____ F'LDDR GRATING ___,//
PLENUM TD IE REINFORCED TO VITHSTAND 2 PSI OVER PRESSURE DOUBLE ACTING PNEUMATIC CYL.
ACTUATED BY TRIP Of' lSDLAT[ON SYSTEM
/
TYPICAL rOR BOTH ~ TD CLOS£ CJN LOSS OF' AIR.
DOORS ON REACTOR LOSS OF' ELEC. DR ISDLATltw TRIP STH LEVEL FIGURE 1 BACKUP DOOR Page 2 of 5
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Detailed Event Description At approximately 06:57 on December 30, 2017, an RO on routine patrol (which is performed approximately every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />) discovered that the differential pressure gauge in the reactor containment building was indicating zero (0) inches of water vacuum as compared to the laboratory building pressure. This pressure reading is required to meet the requirements of surveillance TS 4.4.d, which states, "When required by Specification 3.4.b, containment integrity shall be verified to exist each shift." The RO on patrol immediately radioed the Lead Senior Reactor Operator (LSRO) about this abnormal indication. Since MURR has only one differential pressure indication for reactor containment building pressure, the LSRO directed other personnel to investigate the various components of the reactor containment system to verify whether or not the differential pressure indication was valid. After a few minutes of investigation, the exhaust plenum backup door was found to be closed. Upon hearing the backup door was closed, the LSRO immediately scrammed and secured the reactor at 07: 17 by placing the Master Control Switch (lSl) to the 'OFF' position, thus ending the time that MURR deviated from Technical TS 3.4.b, which states, "Reactor containment integrity shall be mdintained at all times except when:
(1) The reactor is secured, ... " Reactor containment integrity did not exist because one of its seven (7) requirements was not met. Specifically, TS 3.4.a states, "For reactor containment integrity to exist, the following conditions shall be satisfied: ... (6) The reactor containment building is at a negative pressure of at least 0.25 inches of water with respect to the surrounding areas: ... "
Closure of the exhaust plenum backup door blocked the path for air to exit the reactor containment building. The supply fan continued to run, and the supply plenum was still open; therefore, air continued to be supplied to the building without air exhausting, which increased pressure in the reactor containment building, thus driving differential pressure to zero (0).
After the reactor was secured, Reactor Operations management directed the LSRO to perform Compliance Procedure-20 (CP-20), which tests the mechanical and electrical portions of the air supply system to the backup door air cylinders. CP-20 satisfies the applicable requirements of surveillance TS 4.4.c. While preparing to conduct CP-20, it was observed that the air actuator of the exhaust plenum backup door would not open the door. In addition, operation of the supply plenum backup door was slower than normal. It was quickly determined that ice had formed in the compressed air supply lines to the backup doors' air cylinders (the temperature in the area surrounding the air supply line to the backup door cylinders was approximately 10 °F that morning). The compressed air supply piping was heated, water was drained from the piping, and electric heat trace with insulation was applied to the pipip.g to prevent freezing of any future moisture in the piping. Operation of the backup doors was tested successfully by completing CP-20 - both backup doors performed normally. As required by TS 6.6.c(4), permission to start up the reactor was received from the Interim Reactor Facility Director. The reactor was started and obtained 10 MW operation at 17 :52 on December 30, 2017.
Page 3 of 5
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 Safety Analysis As described in Section 6.2.1, Introduction, of the SAR, the containment system is designed to completely isolate the reactor containment building, thereby preventing or mitigating any uncontrolled release of radioactive materials to the environment during an accident.
Redundancy is incorporated into the system to ensure that no single component or circuit failure will render any portion of the containment system inoperative. Isolation of the reactor containment building can be automatically initiated by radiation detectors located at the reactor pool upper bridge and in the containment building exhaust plenum. Isolation can be manually activated by switches in the reactor control room or the facility lobby. Actuation of the facility lobby switch will also cause a facility evacuation.
Preceding the discovery that the reactor containment building differential pressure decreased to zero (0) inches of water vacuum, the reactor had been operating at 10 MW in automatic control since December 26, 2017. During a routine patrol occurring between 02:30 and 03:40 on December 30, 2017, prior to the event, an RO logged the containment building differential pressure at 1.50 inches of water vacuum. The operators log the differential pressure approximately every four (4) hours. As stated above, surveillance TS 4.4.d requires reactor containment integrity to be verified to exist within a shift. Since MURR operates on 12-hour shifts, MURR routinely checks containment integrity three (3) times per shift. Therefore, the maximum time in which MURR may have deviated from TS 3.4.b was approximately four (4) hours.
Negative pressure in the reactor containment building compared to the surrounding areas when the reactor is not secured is a TS requirement and good nuclear practice. However, no MURR safety analyses are based on the containment building pressure being negative.
Though the backup door was closed and unable to open due to ice blockage of its compressed air supply line, the reactor isolation and facility evacuation systems were fully operational and capable of isolating the reactor containment building from the surrounding areas. In addition, the air supply lines to all the other components of the reactor containment system are housed inside the building in controlled temperature environments. Therefore, reactor isolation and facility evacuation systems could perform their safety functions as described in the SAR. The backup doors failed to their 'fail safe' position, which is closed.
Corrective Actions The cause of this event was lower than normal air temperatures surrounding the compressed air piping combined with moisture in the piping. This combination allowed ice to form and slowly block off the air supply to the exhaust plenum backup door air cylinder. As the air pressure slowly bled out of the cylinder, the weight of the door overcame the upward force of the air cylinder, and the exhaust backup door closed. Prior to reactor startup on December 30, 2017, the water was drained from the compressed air lines, and heat trace with insulation was applied to the piping to prevent ice from forming in the lines as a short-term solution. Until the compressed air quality is improved, Reactor Operations staff are also draining condensation for the air supply Page 4 of 5
Attachment U.S. Nuclear Regulatory Commission January 12, 2018 piping near the cylinders every week. MURR is currently investigating possible improvements to the main air system to reduce the amount of moisture in the compressed air piping.
If Reactor Operations staff had continuous indication of reactor containment building differential pressure in the Control Room, they may have be able to prevent the deviation of TS 3.4.b by taking action to secure the reactor prior to the negative pressure decreasing below the TS 3.4.a(6) specification of 0.25 inches of water. MURR will be installing a continuous Control Room indication with a low differential pressure alarm in the future. In addition, a Reactor Emergency Procedure will be developed to provide Reactor Operations staff direction in case of a lowering reactor containment building differential pressure.
Additionally, this event has been entered into the MURR Corrective Action Program as CAP Number 17-0149. Any additional improvements 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, Reviewed and Approved,
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Bruce A. Meffer;{ Matthew R. Sanford Reactor Manager Interim Reactor Facility Director
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