ML17128A394: Difference between revisions
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==SUBJECT:== | ==SUBJECT:== | ||
Written communication as required by University of Missouri Research Reactor Technical Specification 6.6.c(3) regarding a deviation from Technical Specifications 3.2.a and 3.2.f.8. The attached documents provide the University of Missouri-Columbia Research Reactor (MURR) License Event Report (LER) for an event that occurred on April 20, 2017, that resulted in a deviation from MURR Technical Specifications 3.2.a and 3.2.f.8. If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118. | |||
Written communication as required by University of Missouri Research Reactor Technical Specification 6.6.c(3) regarding a deviation from Technical Specifications 3.2.a and 3.2.f.8. | |||
The attached documents provide the University of Missouri-Columbia Research Reactor (MURR) License Event Report (LER) for an event that occurred on April 20, 2017, that resulted in a deviation from MURR Technical Specifications 3.2.a and 3.2.f.8. | |||
If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118. | |||
Sincerely, Ralph A. Butler, P .E. Director RAB:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark Mcintosh, Vice Chancellor for Research, Graduate Development Mr. Geoffrey Wertz, U.S. Nuclear Regulatory Commission Mr. Johnny Eads, U.S. Nuclear Regulatory Commission Studies and Economic Attachment U.S. Nuclear Regulatory Commission May3, 2017 Licensee Event Report No. 17-04 -April 20, 2017 University of Missouri Research Reactor Introduction On April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received. | Sincerely, Ralph A. Butler, P .E. Director RAB:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark Mcintosh, Vice Chancellor for Research, Graduate Development Mr. Geoffrey Wertz, U.S. Nuclear Regulatory Commission Mr. Johnny Eads, U.S. Nuclear Regulatory Commission Studies and Economic Attachment U.S. Nuclear Regulatory Commission May3, 2017 Licensee Event Report No. 17-04 -April 20, 2017 University of Missouri Research Reactor Introduction On April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received. | ||
This alarm is initiated when any one (1) of the three (3) Nuclear Instrumentation (Nl) Power Range Monitor (PRM) channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. The regulating blade was deemed inoperable when it would not operate in either the manual or automatic control modes, so the reactor was immediately shut down by manual scram and all immediate and subsequent actions of reactor emergency procedures REP-2, "Manual Scram," and REP-7, "Rod Position Indication System Failure," | This alarm is initiated when any one (1) of the three (3) Nuclear Instrumentation (Nl) Power Range Monitor (PRM) channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. The regulating blade was deemed inoperable when it would not operate in either the manual or automatic control modes, so the reactor was immediately shut down by manual scram and all immediate and subsequent actions of reactor emergency procedures REP-2, "Manual Scram," and REP-7, "Rod Position Indication System Failure," were completed. | ||
were completed. | Failure of the regulating blade to be operable during reactor operation resulted in a deviation from Technical Specification (TS) 3.2.a, which states, "All control blades, including the regulating blade, shall be operable during reactor operation. " Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred. | ||
Failure of the regulating blade to be operable during reactor operation resulted in a deviation from Technical Specification (TS) 3.2.a, which states, "All control blades, including the regulating blade, shall be operable during reactor operation. | |||
" Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred. | |||
TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable. | TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable. | ||
Each of the rod run-in functions shall have 1 IN logic where N is the number of instrument channels required for the corresponding mode of operation." | Each of the rod run-in functions shall have 1 IN logic where N is the number of instrument channels required for the corresponding mode of operation." Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, ":S 10% withdrawn" or "bottomed," associated with the regulating blade must be operable when the reactor is in operation. | ||
Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, | |||
":S 10% withdrawn" or "bottomed," | |||
associated with the regulating blade must be operable when the reactor is in operation. | |||
The ":S 10% withdrawn" rod run-in function was not operable during this event. Description of the Regulating Blade and Drive Mechanism The reactivity of the reactor is controlled by five (5) neutron-absorbing control blades. Four (4) of the control blades, referred to as the shim blades, are used for coarse adjustments to the neutron density of the reactor core. The fifth control blade is the regulating blade. The low reactivity worth of this blade allows for very fine adjustments in the neutron density in order to maintain the reactor at the desired power level. The regulating blade is constructed of stainless steel with an overall length of approximately 30 inches, occupying about 18° of the circular arc around the outer reactor pressure vessel. The blade is driven at 40 inches per minute in both the inward and outward directions by its associated drive mechanism. | The ":S 10% withdrawn" rod run-in function was not operable during this event. Description of the Regulating Blade and Drive Mechanism The reactivity of the reactor is controlled by five (5) neutron-absorbing control blades. Four (4) of the control blades, referred to as the shim blades, are used for coarse adjustments to the neutron density of the reactor core. The fifth control blade is the regulating blade. The low reactivity worth of this blade allows for very fine adjustments in the neutron density in order to maintain the reactor at the desired power level. The regulating blade is constructed of stainless steel with an overall length of approximately 30 inches, occupying about 18° of the circular arc around the outer reactor pressure vessel. The blade is driven at 40 inches per minute in both the inward and outward directions by its associated drive mechanism. | ||
The regulating blade drive mechanism consists of a servomotor, a reduction | The regulating blade drive mechanism consists of a servomotor, a reduction gearbox, and a lead screw assembly. | ||
The lead screw assembly converts the rotating motion of the servomotor to the linear motion of the regulating blade. The drive mechanism, through a slave sprocket and chain arrangement, also drives a RPI encoder transducer and a rotary limit switch assembly. | The lead screw assembly converts the rotating motion of the servomotor to the linear motion of the regulating blade. The drive mechanism, through a slave sprocket and chain arrangement, also drives a RPI encoder transducer and a rotary limit switch assembly. | ||
The encoder transducer provides an analog signal to the RPI chassis, which converts the analog signal to a digital readout that is displayed on the control room instrument panel and control console. | The encoder transducer provides an analog signal to the RPI chassis, which converts the analog signal to a digital readout that is displayed on the control room instrument panel and control console. The rotary limit switch assembly actuates two (2) regulating blade position alarm functions (20% and 60% withdrawn) and a rod run-in(< 10% withdrawn). | ||
The rotary limit switch assembly actuates two (2) regulating blade position alarm functions (20% and 60% withdrawn) and a rod run-in(< | A second rod run-in is initiated by a limit switch, which is independent of the rotary limit switch assembly, when the regulating blade is fully inserted or "bottomed." Page 1of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 The regulating blade may be operated from the control console in either one of two modes: manual or automatic. | ||
10% withdrawn). | |||
A second rod run-in is initiated by a limit switch, which is independent of the rotary limit switch assembly, when the regulating blade is fully inserted or "bottomed." | |||
Page 1of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 The regulating blade may be operated from the control console in either one of two modes: manual or automatic. | |||
In the automatic control mode, the regulating blade controls reactor power by comparing the output signal from the NI Wide Range Monitor (WRM) with the setting of the power schedule potentiometer as determined by the Reactor Operator. | In the automatic control mode, the regulating blade controls reactor power by comparing the output signal from the NI Wide Range Monitor (WRM) with the setting of the power schedule potentiometer as determined by the Reactor Operator. | ||
If a mismatch does exist, a positive or negative output signal is generated and sent to the servomotor of the regulating blade drive mechanism, which repositions the regulating blade, stepwise, in a direction which minimizes the discrepancy between the power schedule setting and the actual power level. Over the course of the week, while in the automatic control mode, the regulating blade frequently shims to make minor adjustments to maintain power at the desired level. Detailed Event Description At 14:47 on April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received. | If a mismatch does exist, a positive or negative output signal is generated and sent to the servomotor of the regulating blade drive mechanism, which repositions the regulating blade, stepwise, in a direction which minimizes the discrepancy between the power schedule setting and the actual power level. Over the course of the week, while in the automatic control mode, the regulating blade frequently shims to make minor adjustments to maintain power at the desired level. Detailed Event Description At 14:47 on April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received. | ||
This alarm is initiated when any one (1) of the three (3) NI PRM channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. No additional annunciations were received. | This alarm is initiated when any one (1) of the three (3) NI PRM channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. No additional annunciations were received. | ||
The console operator immediately notified the Lead Senior Reactor Operator (LSRO) who was out of the reactor containment building on routine patrol. The LSRO directed another operator to the reactor bridge to inspect conditions near the control and regulating blade drive mechanisms. | The console operator immediately notified the Lead Senior Reactor Operator (LSRO) who was out of the reactor containment building on routine patrol. The LSRO directed another operator to the reactor bridge to inspect conditions near the control and regulating blade drive mechanisms. | ||
At 14:49, the LSRO arrived back at the reactor control room and tested the operation of the regulating blade in the manual control mode. When the regulating blade did not respond, the LSRO immediately initiated a manual reactor scram, and all immediate and subsequent actions of reactor emergency procedures REP-2, "Reactor Scram," and REP-7, "Rod Position Indication System Failure," | At 14:49, the LSRO arrived back at the reactor control room and tested the operation of the regulating blade in the manual control mode. When the regulating blade did not respond, the LSRO immediately initiated a manual reactor scram, and all immediate and subsequent actions of reactor emergency procedures REP-2, "Reactor Scram," and REP-7, "Rod Position Indication System Failure," were completed. | ||
were completed. | Failure of the regulating blade to be operable resulted in a deviation from TS 3.2.a., which states "All control blades, including the regulating blade, shall be operable during reactor operation." Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred. | ||
Failure of the regulating blade to be operable resulted in a deviation from TS 3.2.a., which states "All control blades, including the regulating blade, shall be operable during reactor operation." | |||
Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred. | |||
TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable. | TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable. | ||
Each of the rod run-in functions shall have JIN logic where N is the number of instrument channels required for the corresponding mode of operation." | Each of the rod run-in functions shall have JIN logic where N is the number of instrument channels required for the corresponding mode of operation." Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, ":S 10% withdrawn" or "bottomed," associated with the regulating blade must be operable when the reactor is in operation. | ||
Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, | |||
":S 10% withdrawn" or "bottomed," | |||
associated with the regulating blade must be operable when the reactor is in operation. | |||
The ":S 10% withdrawn" rod run-in function was not operable during this event. Initial investigation revealed that the drive chain for the Rod Position Indication (RPI) encoder on the regulating blade drive mechanism had disengaged from the RPI encoder slave sprocket and became bound in the lead screw adapter drive sprocket. | The ":S 10% withdrawn" rod run-in function was not operable during this event. Initial investigation revealed that the drive chain for the Rod Position Indication (RPI) encoder on the regulating blade drive mechanism had disengaged from the RPI encoder slave sprocket and became bound in the lead screw adapter drive sprocket. | ||
After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and troubleshooting. | After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and troubleshooting. | ||
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: 1. The bolts holding the regulating blade drive mechanism to the regulating blade upper housing were loose allowing the regulating blade drive mechanism to move enough that the RPI encoder chain could have impacted the outer tower housing during regulating blade operation, and 2. The idler sprockets for the RPI encoder drive chain were slightly out of vertical alignment from the main sprockets. | : 1. The bolts holding the regulating blade drive mechanism to the regulating blade upper housing were loose allowing the regulating blade drive mechanism to move enough that the RPI encoder chain could have impacted the outer tower housing during regulating blade operation, and 2. The idler sprockets for the RPI encoder drive chain were slightly out of vertical alignment from the main sprockets. | ||
Page 2of5 Attachment U.S. Nuclear Regulatory Commission May 3, 2017 Safety Analysis Preceding the failure, the reactor had been at continuous full power operation with the regulating blade properly maintaining power level in the automatic control mode for a period of 62 hours and 15 minutes since the last scheduled startup on April 18, 2017. The regulating blade had been fully operational during that period. | Page 2of5 Attachment U.S. Nuclear Regulatory Commission May 3, 2017 Safety Analysis Preceding the failure, the reactor had been at continuous full power operation with the regulating blade properly maintaining power level in the automatic control mode for a period of 62 hours and 15 minutes since the last scheduled startup on April 18, 2017. The regulating blade had been fully operational during that period. | ||
* A review of the NI WRM and PRM strip-chart recorders indicated an actual power level decrease starting about one (1) minute prior to receiving the "Channel 4, 5 or 6 Downscale" annunciator alarm; therefore, it appears that the regulating blade could have been inoperable for a period of approximately three (3) minutes before the reactor was shut down. The regulating blade and its associated rod run-in features are not part of the reactor safety system as defined by TS 1.24, which states, "The reactor safety system is that combination of sensing devices, electronic circuits and equipment, signal conditioning equipment, and electro-mechanical devices that serves to either effect a reactor scram, or activates the engineered safety features." | * A review of the NI WRM and PRM strip-chart recorders indicated an actual power level decrease starting about one (1) minute prior to receiving the "Channel 4, 5 or 6 Downscale" annunciator alarm; therefore, it appears that the regulating blade could have been inoperable for a period of approximately three (3) minutes before the reactor was shut down. The regulating blade and its associated rod run-in features are not part of the reactor safety system as defined by TS 1.24, which states, "The reactor safety system is that combination of sensing devices, electronic circuits and equipment, signal conditioning equipment, and electro-mechanical devices that serves to either effect a reactor scram, or activates the engineered safety features." When a reactor scram or rod rui:.-in occurs, the regulating blade is automatically shifted to manual control to prevent it from operating to maintain power. The basis for the rod run-in features associated with the regulating blade is to assure termination of a transient which, in automatic operation, is causing a rapid insertion of the regulating blade. The regulating blade "< 10% withdrawn" rod run-in is not required to prevent reaching a Limiting Safety System Setting (LSSS). The redundant regulating blade "bottomed" rod run-in was operable during the time the":::; 10% withdrawn" rod run-in was inoperable. | ||
When a reactor scram or rod rui:.-in occurs, the regulating blade is automatically shifted to manual control to prevent it from operating to maintain power. The basis for the rod run-in features associated with the regulating blade is to assure termination of a transient which, in automatic operation, is causing a rapid insertion of the regulating blade. The regulating blade "< 10% withdrawn" rod run-in is not required to prevent reaching a Limiting Safety System Setting (LSSS). The redundant regulating blade "bottomed" rod run-in was operable during the time the":::; | |||
10% withdrawn" rod run-in was inoperable. | |||
Corrective Action The reactor was shut down by manual scram when it was determined that the regulating blade was inoperable. | Corrective Action The reactor was shut down by manual scram when it was determined that the regulating blade was inoperable. | ||
After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and repairs. | After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and repairs. The RPI encoder drive chain was replaced. | ||
The RPI encoder drive chain was replaced. | |||
A complete alignment was performed on all the chain drives and sprocket assemblies for the regulating blade RPI encoder, rotary limit switch assembly, regulating blade drive assembly, and idler arms. Chain tension was adjusted on all drive chains after the alignments were completed. | A complete alignment was performed on all the chain drives and sprocket assemblies for the regulating blade RPI encoder, rotary limit switch assembly, regulating blade drive assembly, and idler arms. Chain tension was adjusted on all drive chains after the alignments were completed. | ||
The regulating blade drive mechanism was installed, and the bolts that secure it to the upper housing were tightened and verified tight by a second operator. | The regulating blade drive mechanism was installed, and the bolts that secure it to the upper housing were tightened and verified tight by a second operator. | ||
The applicable sections of Compliance Procedure-14 (CP-14), | The applicable sections of Compliance Procedure-14 (CP-14), "Regulating Rod 10% and Rod Bottom Rod Run-In Rod Not in Contact with Magnet Rod Run-In," and the "Regulating Blade Operation And Rod Run-In Function Test" portion of FM-57, "Long Form Startup Checksheet," were completed satisfactorily to verify proper operation of the regulating blade drive mechanism and its rotary limit switch assembly. | ||
"Regulating Rod 10% and Rod Bottom Rod Run-In Rod Not in Contact with Magnet Rod Run-In," | |||
and the "Regulating Blade Operation And Rod Run-In Function Test" portion of FM-57, "Long Form Startup Checksheet," | |||
were completed satisfactorily to verify proper operation of the regulating blade drive mechanism and its rotary limit switch assembly. | |||
Permission to restart the reactor was obtained from the Reactor Manager and Acting Reactor Facility Director in accordance with TS 6.6.c. To prevent future failures of this type, a checklist will be developed and used to re-install the regulating blade drive mechanism. | Permission to restart the reactor was obtained from the Reactor Manager and Acting Reactor Facility Director in accordance with TS 6.6.c. To prevent future failures of this type, a checklist will be developed and used to re-install the regulating blade drive mechanism. | ||
In addition, the checklist will require a second operator to verify the regulating blade drive mechanism hold-down bolts tight. Finally, Electronic Shop preventative maintenance Page 3of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 procedure EMP-12B, "Regulating Blade,'' | In addition, the checklist will require a second operator to verify the regulating blade drive mechanism hold-down bolts tight. Finally, Electronic Shop preventative maintenance Page 3of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 procedure EMP-12B, "Regulating Blade,'' will be revised to have the chain and sprockets alignment checked during every performance ofEMP-12B. | ||
will be revised to have the chain and sprockets alignment checked during every performance ofEMP-12B. | |||
Additionally, this event has been entered into the MURR Corrective Action Program as CAP entry No. 17-0059, and any additional improvements or corrective actions will be considered. | Additionally, this event has been entered into the MURR Corrective Action Program as CAP entry No. 17-0059, and any additional improvements or corrective actions will be considered. | ||
One possible corrective action that was previously identified from LER 15-01 as a long-term action would be to relocate the regulating blade rod run-in functions directly to the drive mechanism lead screw assembly similar to that of the current limit switches which provide the drive mechanism full-in and out stop functions. | One possible corrective action that was previously identified from LER 15-01 as a long-term action would be to relocate the regulating blade rod run-in functions directly to the drive mechanism lead screw assembly similar to that of the current limit switches which provide the drive mechanism full-in and out stop functions. | ||
| Line 83: | Line 59: | ||
Since MURR is having more frequent problems with the regulating blade drive mechanism, more resources are now being allocated to implement this potential modification. | Since MURR is having more frequent problems with the regulating blade drive mechanism, more resources are now being allocated to implement this potential modification. | ||
If there are any questions regarding this LER, please contact me at (573) 882-5118. | 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. | I declare under penalty of perjury that the foregoing is true and correct. Sincerely, f!::Me:ertY-Reactor Manager ENDORSEMENT: | ||
Sincerely, f!::Me:ertY-Reactor Manager ENDORSEMENT: | Reviewed and Approved, Director Page 4of5 JACQUELINE L MATYAS My Commission Expires March 26, 2019 Howard County Commlssion#15634308 Attachment U.S. Nuclear Regu lat ory Commission M ay 3 , 2017 Regulating Blade Drive Assembly Rotary Limit Switch Slave Sprocket Rotary Limit Switch Drive Chain Regulating Blade Gearbox RPI Encoder Sprockets RPI Encoder RPI Encoder Sprockets Page 5of5 RPI Encoder Drive Chain (Disengaged) | ||
Reviewed and Approved, Director Page 4of5 JACQUELINE L MATYAS My Commission Expires March 26, 2019 Howard County Commlssion#15634308 Attachment U.S. Nuclear | |||
Lead Screw Adapter Drive Sprocket Rotary Limit Switch Drive Chain RPI Encoder Drive Chain (Disengaged)}} | Lead Screw Adapter Drive Sprocket Rotary Limit Switch Drive Chain RPI Encoder Drive Chain (Disengaged)}} | ||
Revision as of 22:16, 7 July 2018
| ML17128A394 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 05/03/2017 |
| From: | Butler R A Univ of Missouri - Columbia |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| Download: ML17128A394 (6) | |
Text
EiJ University of Missouri May 3, 2017 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 Research Reactor Center 1513 Research Park Drive Columbia, MO 65211 PHONE 573-882-4211 WEB murr.missouri.edu
SUBJECT:
Written communication as required by University of Missouri Research Reactor Technical Specification 6.6.c(3) regarding a deviation from Technical Specifications 3.2.a and 3.2.f.8. The attached documents provide the University of Missouri-Columbia Research Reactor (MURR) License Event Report (LER) for an event that occurred on April 20, 2017, that resulted in a deviation from MURR Technical Specifications 3.2.a and 3.2.f.8. If you have any questions regarding this report, please contact Bruce A. Meffert, the facility Reactor Manager, at (573) 882-5118.
Sincerely, Ralph A. Butler, P .E. Director RAB:jlm Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Mark Mcintosh, Vice Chancellor for Research, Graduate Development Mr. Geoffrey Wertz, U.S. Nuclear Regulatory Commission Mr. Johnny Eads, U.S. Nuclear Regulatory Commission Studies and Economic Attachment U.S. Nuclear Regulatory Commission May3, 2017 Licensee Event Report No. 17-04 -April 20, 2017 University of Missouri Research Reactor Introduction On April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received.
This alarm is initiated when any one (1) of the three (3) Nuclear Instrumentation (Nl) Power Range Monitor (PRM) channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. The regulating blade was deemed inoperable when it would not operate in either the manual or automatic control modes, so the reactor was immediately shut down by manual scram and all immediate and subsequent actions of reactor emergency procedures REP-2, "Manual Scram," and REP-7, "Rod Position Indication System Failure," were completed.
Failure of the regulating blade to be operable during reactor operation resulted in a deviation from Technical Specification (TS) 3.2.a, which states, "All control blades, including the regulating blade, shall be operable during reactor operation. " Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred.
TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable.
Each of the rod run-in functions shall have 1 IN logic where N is the number of instrument channels required for the corresponding mode of operation." Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, ":S 10% withdrawn" or "bottomed," associated with the regulating blade must be operable when the reactor is in operation.
The ":S 10% withdrawn" rod run-in function was not operable during this event. Description of the Regulating Blade and Drive Mechanism The reactivity of the reactor is controlled by five (5) neutron-absorbing control blades. Four (4) of the control blades, referred to as the shim blades, are used for coarse adjustments to the neutron density of the reactor core. The fifth control blade is the regulating blade. The low reactivity worth of this blade allows for very fine adjustments in the neutron density in order to maintain the reactor at the desired power level. The regulating blade is constructed of stainless steel with an overall length of approximately 30 inches, occupying about 18° of the circular arc around the outer reactor pressure vessel. The blade is driven at 40 inches per minute in both the inward and outward directions by its associated drive mechanism.
The regulating blade drive mechanism consists of a servomotor, a reduction gearbox, and a lead screw assembly.
The lead screw assembly converts the rotating motion of the servomotor to the linear motion of the regulating blade. The drive mechanism, through a slave sprocket and chain arrangement, also drives a RPI encoder transducer and a rotary limit switch assembly.
The encoder transducer provides an analog signal to the RPI chassis, which converts the analog signal to a digital readout that is displayed on the control room instrument panel and control console. The rotary limit switch assembly actuates two (2) regulating blade position alarm functions (20% and 60% withdrawn) and a rod run-in(< 10% withdrawn).
A second rod run-in is initiated by a limit switch, which is independent of the rotary limit switch assembly, when the regulating blade is fully inserted or "bottomed." Page 1of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 The regulating blade may be operated from the control console in either one of two modes: manual or automatic.
In the automatic control mode, the regulating blade controls reactor power by comparing the output signal from the NI Wide Range Monitor (WRM) with the setting of the power schedule potentiometer as determined by the Reactor Operator.
If a mismatch does exist, a positive or negative output signal is generated and sent to the servomotor of the regulating blade drive mechanism, which repositions the regulating blade, stepwise, in a direction which minimizes the discrepancy between the power schedule setting and the actual power level. Over the course of the week, while in the automatic control mode, the regulating blade frequently shims to make minor adjustments to maintain power at the desired level. Detailed Event Description At 14:47 on April 20, 2017, with the reactor operating at 10 MW, a "Channel 4, 5 or 6 Downscale" annunciation was received.
This alarm is initiated when any one (1) of the three (3) NI PRM channels decrease below a power level set point of 95%. All PRM channels indicated a decreasing reactor power level. All primary and pool coolant system pressure, temperature, and flow indications were normal. No additional annunciations were received.
The console operator immediately notified the Lead Senior Reactor Operator (LSRO) who was out of the reactor containment building on routine patrol. The LSRO directed another operator to the reactor bridge to inspect conditions near the control and regulating blade drive mechanisms.
At 14:49, the LSRO arrived back at the reactor control room and tested the operation of the regulating blade in the manual control mode. When the regulating blade did not respond, the LSRO immediately initiated a manual reactor scram, and all immediate and subsequent actions of reactor emergency procedures REP-2, "Reactor Scram," and REP-7, "Rod Position Indication System Failure," were completed.
Failure of the regulating blade to be operable resulted in a deviation from TS 3.2.a., which states "All control blades, including the regulating blade, shall be operable during reactor operation." Additionally, with the regulating blade in an inoperable state, a deviation from TS 3.2.f.8 had also occurred.
TS 3.2.f.8 specifies that "The reactor shall not be operated unless the following rod run-in functions are operable.
Each of the rod run-in functions shall have JIN logic where N is the number of instrument channels required for the corresponding mode of operation." Rod Run-In Function No. 8 under this specification requires that the two (2) rod run-in functions, ":S 10% withdrawn" or "bottomed," associated with the regulating blade must be operable when the reactor is in operation.
The ":S 10% withdrawn" rod run-in function was not operable during this event. Initial investigation revealed that the drive chain for the Rod Position Indication (RPI) encoder on the regulating blade drive mechanism had disengaged from the RPI encoder slave sprocket and became bound in the lead screw adapter drive sprocket.
After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and troubleshooting.
Further investigation revealed two (2) potential causes for the separation of the drive chain from its sprockets:
- 1. The bolts holding the regulating blade drive mechanism to the regulating blade upper housing were loose allowing the regulating blade drive mechanism to move enough that the RPI encoder chain could have impacted the outer tower housing during regulating blade operation, and 2. The idler sprockets for the RPI encoder drive chain were slightly out of vertical alignment from the main sprockets.
Page 2of5 Attachment U.S. Nuclear Regulatory Commission May 3, 2017 Safety Analysis Preceding the failure, the reactor had been at continuous full power operation with the regulating blade properly maintaining power level in the automatic control mode for a period of 62 hours7.175926e-4 days <br />0.0172 hours <br />1.025132e-4 weeks <br />2.3591e-5 months <br /> and 15 minutes since the last scheduled startup on April 18, 2017. The regulating blade had been fully operational during that period.
- A review of the NI WRM and PRM strip-chart recorders indicated an actual power level decrease starting about one (1) minute prior to receiving the "Channel 4, 5 or 6 Downscale" annunciator alarm; therefore, it appears that the regulating blade could have been inoperable for a period of approximately three (3) minutes before the reactor was shut down. The regulating blade and its associated rod run-in features are not part of the reactor safety system as defined by TS 1.24, which states, "The reactor safety system is that combination of sensing devices, electronic circuits and equipment, signal conditioning equipment, and electro-mechanical devices that serves to either effect a reactor scram, or activates the engineered safety features." When a reactor scram or rod rui:.-in occurs, the regulating blade is automatically shifted to manual control to prevent it from operating to maintain power. The basis for the rod run-in features associated with the regulating blade is to assure termination of a transient which, in automatic operation, is causing a rapid insertion of the regulating blade. The regulating blade "< 10% withdrawn" rod run-in is not required to prevent reaching a Limiting Safety System Setting (LSSS). The redundant regulating blade "bottomed" rod run-in was operable during the time the":::; 10% withdrawn" rod run-in was inoperable.
Corrective Action The reactor was shut down by manual scram when it was determined that the regulating blade was inoperable.
After the reactor was shut down and secured, the regulating blade drive mechanism was removed for inspection and repairs. The RPI encoder drive chain was replaced.
A complete alignment was performed on all the chain drives and sprocket assemblies for the regulating blade RPI encoder, rotary limit switch assembly, regulating blade drive assembly, and idler arms. Chain tension was adjusted on all drive chains after the alignments were completed.
The regulating blade drive mechanism was installed, and the bolts that secure it to the upper housing were tightened and verified tight by a second operator.
The applicable sections of Compliance Procedure-14 (CP-14), "Regulating Rod 10% and Rod Bottom Rod Run-In Rod Not in Contact with Magnet Rod Run-In," and the "Regulating Blade Operation And Rod Run-In Function Test" portion of FM-57, "Long Form Startup Checksheet," were completed satisfactorily to verify proper operation of the regulating blade drive mechanism and its rotary limit switch assembly.
Permission to restart the reactor was obtained from the Reactor Manager and Acting Reactor Facility Director in accordance with TS 6.6.c. To prevent future failures of this type, a checklist will be developed and used to re-install the regulating blade drive mechanism.
In addition, the checklist will require a second operator to verify the regulating blade drive mechanism hold-down bolts tight. Finally, Electronic Shop preventative maintenance Page 3of5 Attachment U.S. Nuclear Regulatory Commission May3, 2017 procedure EMP-12B, "Regulating Blade, will be revised to have the chain and sprockets alignment checked during every performance ofEMP-12B.
Additionally, this event has been entered into the MURR Corrective Action Program as CAP entry No. 17-0059, and any additional improvements or corrective actions will be considered.
One possible corrective action that was previously identified from LER 15-01 as a long-term action would be to relocate the regulating blade rod run-in functions directly to the drive mechanism lead screw assembly similar to that of the current limit switches which provide the drive mechanism full-in and out stop functions.
This would eliminate the need for a drive chain and the difficulties associated with aligning and providing the correct tension for multiple drive chains, sprocket assemblies, and idler arms on the same component.
Since MURR is having more frequent problems with the regulating blade drive mechanism, more resources are now being allocated to implement this potential modification.
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. Sincerely, f!::Me:ertY-Reactor Manager ENDORSEMENT:
Reviewed and Approved, Director Page 4of5 JACQUELINE L MATYAS My Commission Expires March 26, 2019 Howard County Commlssion#15634308 Attachment U.S. Nuclear Regu lat ory Commission M ay 3 , 2017 Regulating Blade Drive Assembly Rotary Limit Switch Slave Sprocket Rotary Limit Switch Drive Chain Regulating Blade Gearbox RPI Encoder Sprockets RPI Encoder RPI Encoder Sprockets Page 5of5 RPI Encoder Drive Chain (Disengaged)
Lead Screw Adapter Drive Sprocket Rotary Limit Switch Drive Chain RPI Encoder Drive Chain (Disengaged)