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| Start date | Title | Description | Topic | |
|---|---|---|---|---|
| ENS 52967 | 13 September 2017 04:47:00 | Unusual Event Due to Letdown Relief Valve Discharge to the Pressurizer Relief Tank | At 0047 EDT, the licensee declared a notification of unusual event under emergency action level (EAL) S8 due to actuation of the letdown relief valve which resulted in reactor coolant system leakage to the pressurizer relief tank exceeding 10 gpm. The plant was in mode 3 at the time making preparations to proceed to cold shutdown to support their refueling outage. Operators were in the process of reducing reactor coolant system letdown from 120 gpm to 45 gpm when the letdown system relief valve lifted. The relief discharged to the pressurizer relief tank at approximately 12 gpm. Operators isolated the letdown line which reduced the leakage to approximately 1 gpm. Plant staff is in the process of determining where the 1 gpm leakage is from and evaluating the basis for terminating the unusual event. Notified DHS SWO, FEMA Operations Center, DHS NICC, FEMA National Watch Center (email) and Nuclear SSA (email).
At 0800 EDT, the licensee terminated the notification of unusual event. The basis for termination was that the letdown line was isolated which reduced leakage to less than the EAL threshold of 10 gpm. The cause of the letdown relief lifting is still under investigation. The licensee has notified the NRC Resident Inspector and will be notifying the state of Michigan and Berrien County. The licensee has issued a press release to the media concerning this event. Notified R3DO (Riemer), IRD MOC (Stapleton), NRR (Miller), DHS SWO, DHS NICC, FEMA Ops Center, FEMA National Watch Center (email) and Nuclear SSA (email).
The condition reported in Event Notification #52967 pursuant to 10 CFR 50.72(a)(1)(i) has been evaluated, and determined not to have met the threshold for classification as an Unusual Event under Emergency Action Level (EAL) S-8 and is being retracted. During the events of 9/13/17, Control Room staff calculated Reactor Coolant System (RCS) leakage to the Pressurizer Relief Tank (PRT) through the Letdown Relief Valve to be greater than the EAL S-8 threshold of 10 gallons per minute. Subsequent review of trends associated with PRT level increase during the event determined that the RCS leakage to the PRT was less than 10 gallons per minute and therefore did not represent an Unusual Event under EAL S-8. The NRC Resident Inspector was notified of this retraction. Notified R3DO (Riemer). | |
| ENS 52544 | 4 February 2017 01:24:00 | Unanalyzed Condition Due to Unaccounted Loads in Safe Shutdown Analysis | On February 3rd, 2017, during engineering modification reviews of electrical busses for a station switchyard transformer, electrical loads were identified on 4kV breakers that are not accounted for in the current safe shutdown analysis. Further reviews revealed the unanalyzed loads are associated with 26 cables that are routed through multiple fire zones in the Turbine Building and both Unit 1 and Unit 2 4kV rooms that have the potential to be affected from a fire event. Per NFPA 805 requirements, the cables need to be analyzed for overcurrent trip capability to demonstrate that the breakers will isolate a fault for a fire. Hourly Fire Watch tours have been established in the identified fire zones. The public health and safety is not impacted. This notification is being made in accordance with 10 CFR 50.72(b)(3)(ii)(B) as an unanalyzed condition. The NRC Resident Inspector was notified. This is a late notification for the 8 hour report.
The condition reported in EN# 52544 pursuant to 10 CFR 50.72(b)(3)(ii)(B) has been evaluated, and determined not to be an unanalyzed condition that significantly degraded plant safety and is being retracted. Fire Protection Engineering performed circuit analysis of the discovered loads and determined that the Fire Safety Analysis credited power supply for an established recovery action was lost due to a Turbine Building fire. This was determined to not be a significant impact to plant safety due to the availability of an alternate offsite 4kV power supply that was not impacted by this fire. Emergency Operating Procedures already contain guidance that directs operators to align the 4kV busses to this offsite power circuit and that can be accomplished within the time necessary to perform the recovery action. During periods when this offsite power circuit was removed from service over the past three years, risk was managed using the online risk management process and no additional actions would have been required for its use as the credited power source in the Fire Safety Analysis. The NRC Resident Inspector was notified of this retraction. Notified the R3DO (Skokowski). | Safe Shutdown Unanalyzed Condition Hourly Fire Watch |
| ENS 52456 | 22 December 2016 04:00:00 | Emergency Diesel Generators Declared Inoperable | With D.C. Cook Unit 1 in Mode 1 and 100 percent power and Unit 2 in Mode 4 during a refueling outage, the following emergency diesel generators (EDGs) were declared inoperable due to a discovered design and manufacturing issue involving some of the diesel fuel pumps/injectors in each of the following EDGs: Unit 1 CD (Train A) EDG, Unit 2 AB (Train B) EDG, and Unit 2 CD (Train A) EDG Due to both Unit 2 EDGs being inoperable, Unit 2 is required to be in Mode 5 by 1300 (EST) on 12/23/16. Unit 1 is required to restore its emergency diesel generator within 14 days (by 2300 (EST) on 1/04/17). In connection with both trains of Unit 2 EDGs being inoperable, this is being reported as an 8-hour report pursuant to 10CFR50.72(b)(3)(v)(D) as loss of safety function in connection with mitigating the consequences of an accident. The NRC Resident Inspector has been informed. Unit 1 maintenance will be prioritized over Unit 2 and Unit 2 will most likely proceed to Mode 5.
The condition reported in ENS 52456 pursuant to 10 CFR 50.72(b)(3)(v)(D) has been evaluated, and determined not to be a loss of safety function in connection with mitigating the consequences of an accident, and is being retracted. With D.C. Cook Unit 1 in Mode 1 and 100% power and Unit 2 in Mode 4 during a refueling outage, the following Emergency Diesel Generators (EDGs) were declared inoperable due to a discovered design and manufacturing issue involving some of the diesel fuel pumps/injectors in each of the following EDGs: Unit 1 CD (Train A) EDG, Unit 2 AB (Train B) EDG, and Unit 2 CD (Train A) EDG. Subsequent endurance testing was performed on selected Delivery Valve Holders (DVH), which were the affected components of the diesel fuel pumps/injectors, to evaluate the expected degradation of the DVHs during the EDG mission. This testing determined that the identified design and manufacturing issue would not have prevented the EDGs from performing their intended safety functions. Therefore, all EDGs were OPERABLE and did not result in a loss of safety function in connection with mitigating the consequences of an accident. The NRC Resident Inspector was notified of this retraction. Notified the R3DO (Kunowski). | |
| ENS 50149 | 25 May 2014 13:42:00 | Control Room Ventilation Boundary Door Inoperable | At 0942 on May 25, 2014, the Cook Nuclear Plant (CNP) declared both Control Room Emergency Ventilation trains inoperable in accordance with LCO 3.7.10 due to an inoperable Control Room Envelope when a control room boundary door was identified as not latching correctly during a security door check. The latch would not have been able to maintain the door closed during an event resulting in Control Room Pressurization. At this time, Security established a continuous door post and would have been able to maintain the door closed. At 1602 on May 25, 2014, repairs to the control room boundary door latch were completed restoring the Control Room Envelope to Operable. The licensee has notified the NRC Senior Resident Inspector. This notification should have been made within 8 hours of the event in accordance with 10 CFR 50.72 (b)(3)(v)(D) per guidance in section 3.2.7 of NUREG-1022 - Event or Condition that Could Have Prevented Fulfilment of a Safety Function, but was not recognized at that time.
The purpose of this report is to retract EN #50149 (May 25, 2014). On June 19, 2014, Cook Nuclear Plant concluded that the EN could be retracted based on the completion of a Maintenance Rule Evaluation (MRE) performed by Systems Engineering. By design, the Control Room Envelope (CRE) pressure boundary is required to be maintained at a positive pressure during all modes of operation and during any irradiated fuel movement. New information contained in the MRE concluded the CRE function was not lost as the control room boundary door remained closed without manual assistance during normal operations. The amount of make-up air during normal operation is similar to the design flow for accident mode (approx. 800 cfm). It was concluded that the sealing integrity was not lost, thus the CRE function was maintained. The CRE would have remained operable and LCO 3.7.10 would not have been entered for the identified condition. The licensee has notified the NRC Senior Resident Inspector. Notified the R3DO (Valos). | |
| ENS 45361 | 18 September 2009 14:30:00 | Inoperability of Both Edg'S Due to Valve Found Out of Position | At 02:00 on Friday, September 18, 2009, the Unit 2 AB Emergency Diesel Generator (EDG) Air Jet Assist Control Air Shutoff valve, which is required to be open, was discovered closed. At 10:30 on Friday, September 18, 2009, it was determined that with the valve closed, the EDG Air Jet Assist may not be able to support the EDG fast speed start to meet the EDG's Operability requirements. This condition renders the Unit 2 AB EDG inoperable. The valve was repositioned and verified open at 02:15 on Friday, September 18, 2009, restoring the EDG to operable status. Similar valves were verified to be in the correct position on all other EDGs in both Units. The Unit 2 AB Emergency Diesel Generator Air Jet Assist Control Air Shutoff valve was last confirmed open on July 21, 2009. The time the valve became closed is not known. Since July 21, 2009, there have been 3 periods of inoperability for Unit 2 CD EDG, the redundant Emergency Diesel Generator. During each of these periods, both of the Unit 2 EDGs are assumed to be inoperable. The periods are as follows: August 10, 2009 at 09:31 until August 10, 2009 at 23:00 August 11, 2009 at 07:30 until August 11, 2009 at 23:16 September 9, 2009 al 00:02 until September 9, 2009 at 04:40 This notification is being made in accordance with 10 CFR 50.72 (b)(3)(ii)(B) due to the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. The licensee has notified the NRC Senior Resident Inspector.
At 02:00 on Friday, September 18, 2009, the Unit 2 AB Emergency Diesel Generator (EDG) Air Jet Assist Control Air Shutoff valve, which is required to be open, was discovered closed. At 10:30 on Friday, September 18, 2009, it was determined that with the valve closed, the EDG Air Jet Assist may not be able to support the EDG fast speed start to meet the EDG's Operability requirements. This condition renders the Unit 2 AB EDG inoperable. The valve was repositioned and verified open at 02:15 on Friday, September 18, 2009, restoring the EDG to operable status. Since the last time the Turbocharger Air Jet Assist Control Air Shutoff valve was confirmed open, July 21, 2009, there were three periods where the redundant EDG was inoperable for planned maintenance and surveillance testing. During each of these periods, both of the Unit 2 EDGs are assumed to be inoperable. As such, Cook Nuclear Plant (CNP) submitted Event Notification 45361 in accordance with 10CFR50.72(b)(3)(ii)(B) due to the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. On September 25, 2009, CNP conducted two EDG fast speed starts, one with the Turbocharger Air Jet Assist Control Air Shutoff valve closed, and one with the valve open. Based on a review of the data collected and observed performance of the EDG, the determination has been made that having the Turbocharger Air Jet Assist Control Air valve closed does not adversely impact operability of an EDG. The circumstances discussed in the notification did not result in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety. The licensee has notified the NRC Resident Inspector. Notified R3DO (Lara). | Unanalyzed Condition |
| ENS 42536 | 29 April 2006 14:56:00 | Discovery of After-The- Fact Emergency Condition - Unusual Event Due to Rcs Lineup | D.C. Cook Unit 2 was in Mode 4 and heating up after Unit 2 Cycle 16 refueling outage when 2-IMO-340 (East RHR pump to Charging Pump Suction Header) was throttled open. RHR suction was aligned to Loop 2 Hot Leg with wide range RCS pressure at 337 psig and RCS average temperature 280 degrees Fahrenheit. Charging header safety valve 2-SV-56 lifted with a setting of 220 psig and relieved to the Pressurizer Relief Tank (PRT) between 10:56:00 and 10:58:30. 2-IMO-340 was then closed. Approximately, 120 gallons of reactor coolant were directed to the PRT during the approximate 2.5 minutes, resulting in a flow rate of approximately 48 gpm, greater than the 25 gallon per minute identified leak rate limit for an Unusual Event (10 CFR 50.72(a)(1)(i)). Time of discovery for leakage quantity and reportability was 13:21 on 4/29/06. Therefore, for approximately 2.5 minutes, D.C. Cook Unit 2 met the conditions for an Unusual Event. However, this fact was discovered after leakage was stopped. Unit 2 is stable in Mode 4 at 340 degrees Fahrenheit and 405 psig RCS pressure. An Unusual Event was NOT declared, but the conditions for an Unusual Event were met under Emergency Condition Criteria S-8 between 10:56:00 and 10:58:30 on 4/29/06. This ENS notification is being made within one hour of the discovery of the undeclared event. The licensee will notify the NRC Resident Inspector.
EN# 42536 reported that D.C. Cook Unit 2 satisfied the conditions for entry into an Unusual Event under Emergency Condition Criteria S-B, identified RCS leakage exceeding 25 GPM. This was identified after system alignment had been restored, ending the event, and was reported as an after the fact declaration that the conditions for an Unusual Event had been satisfied, but not declared. Subsequent review of this event has determined that the conditions for Emergency Condition Criteria S-B were not satisfied in that no identified RCS leakage occurred during this event. The diversion of 120 gallons of CVCS (Chemical Volume and Control System) inventory was the result of plant alignment by Operations personnel which was immediately recognized and terminated. Plant systems and components functioned as designed to terminate this event. The licensee will notify the NRC Resident Inspector. Notified R3DO (H. Peterson). | Time of Discovery |