IR 05000298/2020050
| ML20113F037 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 04/22/2020 |
| From: | Jason Kozal NRC/RGN-IV/DRP/RPB-C |
| To: | Dent J Nebraska Public Power District (NPPD) |
| References | |
| IR 2020050 | |
| Download: ML20113F037 (35) | |
Text
April 22, 2020
SUBJECT:
COOPER NUCLEAR STATION - NRC SPECIAL INSPECTION REPORT 05000298/2020050
Dear Mr. Dent:
On December 17, 2019, the U.S. Nuclear Regulatory Commission (NRC) completed its initial assessment of the Division 2 service water discharge line clogging, which occurred on December 8, 2019, at the Cooper Nuclear Station. Based on our initial assessment, an inspection team was sent to your site on January 13, 2020. The clogging inhibited Division 2 service water system flow, rendering the system inoperable and unavailable. The NRCs initial evaluation satisfied the criteria in NRC Management Directive 8.3, NRC Incident Investigation Program, for conducting a special inspection. The basis for initiating this special inspection is further discussed in the Charter, which is included as an attachment to this report.
On March 27, 2020, the NRC completed its special inspection and discussed the results of this inspection with you and other members of your staff. The results of this inspection are documented in the enclosed report.
NRC inspectors documented one finding of very low safety significance (Green) in this report, which involved a violation of NRC requirements. The NRC is treating this violation as a non-cited violation (NCV) consistent with Section 2.3.2 of the Enforcement Policy.
If you contest this violation or its significance, you should provide a response within 30 days of the date of this inspection report, with the basis for your denial, to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, DC 20555-0001; with copies to the Regional Administrator, Region IV; the Director, Office of Enforcement; and the NRC resident inspector at the Cooper Nuclear Station.
J. Dent, Jr.
This letter, its enclosure, and your response (if any) will be made available for public inspection and copying at http://www.nrc.gov/reading-rm/adams.html and at the NRC Public Document Room in accordance with 10 CFR 2.390, Public Inspections, Exemptions, Requests for Withholding.
Sincerely, Jason W. Kozal, Chief Reactor Projects Branch C Division of Reactor Projects
Docket No. 05000298 License No. DPR-46
Enclosure:
Inspection Report 5000298/2020050 w/Attachment: Special Inspection Charter, dated December 18, 2019 (ADAMS A
Inspection Report
Docket Number(s):
05000298
License Number(s):
Report Number(s):
Enterprise Identifier: I-2020-050-0000
Licensee:
Nebraska Public Power District
Facility:
Cooper Nuclear Power Station
Location:
Brownville, NE
Inspection Dates:
January 13 to March 27, 2020
Inspectors:
N. Day, Comanche Peak Resident Inspector (Team Lead)
J. Drake., Senior Reactor Inspector
M. Stafford, Cooper Resident Inspector
D. Loveless, Senior Reactor Analyst
Approved By:
Jason W. Kozal, Chief
Reactor Projects Branch C
Division of Reactor Projects
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) continued monitoring the licensees performance by conducting a special inspection at Cooper Nuclear Station, in accordance with the Reactor Oversight Process. The Reactor Oversight Process is the NRCs program for overseeing the safe operation of commercial nuclear power reactors. Refer to https://www.nrc.gov/reactors/operating/oversight.html for more information.
List of Findings and Violations
Failure to Take All Design Parameters into Account during a Design Modification Cornerstone Significance Cross-cutting Aspect Inspection Procedure Mitigating Systems Green NCV 05000298/2020050-01 Open/Closed Not Applicable 93812Special Inspection A self-revealed, Green finding with an associated non-cited violation of 10 CFR Part 50,
Appendix B, Criterion III, Design Control, was identified for the licensees failure to ensure that all aspects of the environmental condition of silting were taken into account during the modification of the service water discharge outlet piping, CED 6035080, which was installed during October 2014.
As a result, in December 2019, the Division 2 service water discharge piping was clogged and covered by at least 15 feet of silt. This amount of silt degraded the system to the point that required flow was unable to be established through the Division 2 service water system and rendered the division inoperable and unavailable.
Additional Tracking Items
Type Issue Number Title Report Section Status LER 05000298/2019-003-00 Division 2 Service Water Discharge Blockage Resulting in Unplanned Service Water Inoperability 71153 Closed
INSPECTION SCOPE
Inspections were conducted using the appropriate portions of the inspection procedures (IPs) in effect at the beginning of the inspection unless otherwise noted. Currently approved IPs with their attached revision histories are located on the public website at http://www.nrc.gov/reading-rm/doc-collections/insp-manual/inspection-procedure/index.html.
Samples were declared complete when the IP requirements most appropriate to the inspection activity were met consistent with Inspection Manual Chapter (IMC) 2515, Light-Water Reactor Inspection Program - Operations Phase. The inspectors reviewed selected procedures and records, observed activities, and interviewed personnel to assess licensee performance and compliance with Commission rules and regulations, license conditions, site procedures, and standards.
OTHER ACTIVITIES
- TEMPORARY INSTRUCTIONS, INFREQUENT AND ABNORMAL
71153Follow-up of Events and Notices of Enforcement Discretion
The inspectors evaluated the following licensee event report (LER):
- (1) LER 05000298/2019-003-00, Division 2 Service Water Discharge Blockage Resulting in Unplanned Service Water Inoperability (ADAMS Accession No. ML20043D739).
The circumstances surrounding this event are documented in this Special Inspection Report. One non-cited violation associated with this event is documented in the Inspection Results section.
93812Special Inspection
In accordance with the attached Special Inspection Team Charter, the inspection team conducted inspection activities associated with clogging of the Division 2 service water discharge line in December 2019.
.1 Description of Event and Reactive Inspection Basis
On December 6, 2019, with the plant operating at 100 percent power and the Division 1 service water system in service, Cooper was unable to establish service water flow through the Division 2 reactor equipment cooling heat exchanger. Cooper entered a 30-day technical specification (TS) shutdown action statement for the Division 2 reactor equipment cooling system and began pursuing actions to determine the cause and verify appropriate system configuration (e.g., correctly aligned valves). At 5:00 a.m. on December 8, 2019, Cooper discovered that they were also unable to establish flow through the Division 2 emergency diesel generator, and subsequently declared the emergency diesel generator inoperable (7-day TS shutdown action statement) and Division 2 service water inoperable (30-day TS shutdown action statement). Through troubleshooting efforts, Cooper suspected that the cause of the inability to establish flow to these systems was silt buildup in the Division 2 service water discharge line. In order to provide service water cooling to any safety-related Division 2 systems, the flow must travel out the Division 2 discharge line.
Cooper established Division 1 service water flow through the Division 1 emergency diesel generator to rule out common cause failure on December 8, 2019, and Division 1 service water remained in service with continuous flow through its discharge line throughout the event. During their investigation, Cooper discovered blockage consisting of silt build-up at the discharge line of Division 2 service water. On December 9, 2019, manual sounding revealed approximately 13-18 feet of silt buildup covering the Division 2 discharge line and approximately 10-12 feet of silt buildup over the Division 1 discharge line. While flow through Division 1 continued unobstructed, discovery of several feet of silt covering both discharge lines could have resulted in loss of the ultimate heat sink under some scenarios.
Following discovery of the silt blockage, Cooper dredged the discharge canal to remove the silt covering both service water discharge lines, restoring Division 2 service water to an operable condition on December 12, 2019. As a compensatory measure to support continued operability of the service water system, Cooper established continuous flow through both trains of service water to prevent flow stagnation and potential silting in the discharge lines.
Cooper last successfully ran the Division 2 emergency diesel generator on November 11, 2019, and observed normal Division 2 service water cooling flow to the emergency diesel generator at that time. In addition, Cooper successfully backwashed the Division 2 reactor equipment cooling heat exchanger on November 23, 2019, and reported normal Division 2 service water system flow at that time.
Cooper experienced unusually high Missouri River water levels during the spring of 2019.
This likely contributed to increased rate of silt buildup in the discharge canal. In October 2019, Cooper took the Division 2 service water system out of service, resulting in stagnant flow conditions in the Division 2 service water discharge. This condition likely worsened the silt buildup that resulted in the Division 2 service water line becoming clogged at some time after the November 23, 2019, backwash of the reactor equipment cooling heat exchanger.
Cooper has a history of experiencing silting in the discharge canal. In November 2009, the licensee discovered several feet of silt accumulated above the service water discharge outlets in the discharge canal. At that time, the licensee had been trending a slow degradation of service water flow. Despite the discovery of several feet of silt at the discharge lines in 2009, the discharge line was never fully affected to the extent that occurred with this 2019 event.
Management Directive 8.3, NRC Incident Investigation Program, was used to evaluate the level of NRC response for this event. Based on the deterministic criteria and risk insights related to this event, the NRC determined that the appropriate level of response was to conduct a special inspection. The inspectors determined that the inspection did not need to be upgraded to an augmented inspection team response. This special inspection team was chartered to identify the circumstances surrounding this event and review Coopers actions to address the causes of the event.
discharge line clogging in December 2019.
The inspectors conducted a detailed review of the events related to the December 2019 service water discharge canal silting. The inspectors gathered information from station logs, the plant data system computer, licensee cause evaluations, condition reports, and interviews with plant operations personnel and engineering staff to develop the following timeline of the event.
Sequence of events:
DATE TIME CONDITIONS AND ACTIONS 10/24/2009
Divers entered the discharge canal and were unable to locate Division 1 service water discharge piping due to silt and sediment buildup; however, flow was capable of percolating through. Cooper discovered that the as-built configuration of the service water discharge piping did not match the approved design configuration.
10/27/2009
Cooper performed dredging in the discharge canal to remove the sediment.
11/9/2009
Cooper recognized the need for discharge canal maintenance to monitor and address silt levels in the discharge canal. A condition report was generated to create a maintenance strategy.
9/14/2010
The NRC issued a non-cited violation for the failure to maintain accurate design drawings of the service water system discharge piping in the Cooper Nuclear Station Integrated Inspection Report 2010-003 (ADAMS Accession No. ML102170525). The as-built service water discharge line configuration was vertically oriented piping with approximately 6 inches separating Division 1 from Division 2. The as-documented configuration was vertically oriented piping with approximately 24 feet separating Division 1 from Division 2.
3/26/2012
Cooper identified a buildup of approximately 9 feet of silt and sediment at the discharge outlet of the service water system in the discharge canal; however, there was no impact to service water functionality. Cooper performed dredging to remove the silt.
11/18/2012
Cooper dredged the discharge canal to support construction of a coffer dam for the upcoming service water piping modification.
2014
Cooper implemented a modification to service water discharge piping to modify the outlet configuration from approximately 6 inches apart to approximately 24 feet apart to remove the likelihood of a common cause of inoperability affecting both divisions due to the close proximity.
9/6/2016
Cooper cancelled the preventative maintenance strategy for periodic dredging. The basis for cancelling this was when service water testing was performed before and after dredging activities took place, no discernable change of service water performance was noted.
3/13/2019 12:15 p.m. Cooper appropriately entered the emergency procedure for flood conditions. River levels continued to rise.
3/15/2019
Cooper declared a notice of unusual event (NOUE) due to high river level of 899.05 feet above main sea level, as reported in Event Report 53934.
3/16/2019
Maximum Missouri River water level in 2019 peaked at 901.5 feet above main sea level. River levels begin to decrease.
3/24/2019
The NOUE was terminated due to river level of 896 feet above main sea level, with a lowering trend.
DATE TIME CONDITIONS AND ACTIONS 4/4/2019 4:05 a.m.
Cooper appropriately exited the emergency procedure for flood conditions.
6/16/2019 11:17 p.m.
Due to summer conditions (higher environmental heat loads and warmer river temperatures) Cooper utilized both Division 1 and Division 2 service water systems. Cooper remained in this configuration until October 2019.
9/20/2019 9:27 a.m.
Cooper appropriately entered the emergency procedure for flood conditions. River levels continued to rise.
10/6/2019
Due to exiting summer conditions (environmental heat loads decreasing and lowering river temperatures), Cooper secured Division 2 service water from operation. Cooper swapped divisions in this configuration as necessary to support maintenance and operations.
10/19/2019 3:00 p.m.
Cooper appropriately exited the emergency procedure for flood conditions.
11/12/2019
Cooper performed a monthly Division 2 emergency diesel generator surveillance; adequate Division 2 service water flow was achieved.
11/23/2019 11:06 a.m.
Cooper performed a backflush of the Division 2 reactor equipment cooling heat exchanger. During this backflush adequate Division 2 service water flow was achieved.
12/5/2019 2:40 a.m.
Division 1 service water pump A was taken out of service for planned maintenance, rendering Division 1 service water inoperable.
12/6/2019 2:15 a.m.
Planned maintenance of Division 1 service water pump A was completed. Cooper was unsuccessful in achieving flow through the Division 2 reactor equipment cooling heat exchanger during service water pump A post maintenance testing. Division 2 reactor equipment cooling was declared inoperable.
12/6/2019
Cooper planned and commenced troubleshooting.
12/7/2019 7:39 p.m.
Division 1 service water pump A was declared operable following successful post maintenance testing.
12/8/2019 4:18 a.m.
Troubleshooting discovered that the Division 2 emergency diesel generator did not have Division 2 service water flow.
12/8/2019 5:00 a.m.
Cooper declared the Division 2 emergency diesel generator and Division 2 service water systems inoperable.
12/9/2019
Cooper performed manual sounding in the service water discharge canal. Results of manual sounding identified at least 15 feet of silting in the discharge canal at the Division 2 service water discharge outlet.
12/11/2019 5:21 a.m.
Cooper implemented a temporary modification to provide an alternate service water flow path for the Division 2 emergency diesel generator. This temporary modification consisted of piping connections and fire hoses to restore a discharge flow path through service water cooled components. This restored system availability to the Division 2 emergency diesel generator.
DATE TIME CONDITIONS AND ACTIONS 12/12/2019 2:32 a.m.
Dredging activities began in the discharge canal. Dredging was performed in areas of interest for both Division 1 and Division 2 service water discharge piping.
12/12/2019 3:20 a.m.
During dredging activities, Division 2 service water flow returned to normal, restoring system availability to the Division 2 service water system, emergency diesel generator, and reactor equipment cooling systems.
12/13/2019 5:00 a.m.
The Division 2 service water system, emergency diesel generator, and residual heat removal system were declared operable following system evaluations and testing. At this point, Cooper added a compensatory measure to provide at least 1000 gallons per minute flow through each service water division to prevent silt buildup.
1/9/2020 1/29/2020 2/19/2020 3/11/2020
Cooper performed sounding in the service water discharge canal. Results of sounding conclude no significant buildup of silt in the discharge canal.
.3 Review the licensees causal evaluations and determine if they are being conducted at a
level of detail commensurate with the significance of the issues that were encountered for the degraded condition.
The inspectors reviewed the issues associated with this event, including those that Cooper had entered into the corrective action program and had identified as subjects for causal evaluations. The inspectors determined that Coopers causal evaluations were being conducted at the appropriate levels, commensurate with the significance of the associated issues. The team reviewed the A level, which is a significant condition adverse to quality level, root cause evaluation initiated by the licensee associated with the silting event and determined that it was appropriate for the development of required corrective actions.
.4 Review the licensees extent of condition evaluation to determine if the licensee has
adequately considered degradation of service water flow from other sources of debris and at the intake structure.
The inspectors reviewed the extent of condition evaluation performed as part of the larger root cause evaluation. The inspectors determined that the extent of condition evaluation was adequate. This extent of condition reviewed both divisions of service water as well as the nonsafety circulating water system. This evaluation reviewed other sources of blockage, including mollusks and plant debris. The evaluation also covered other potential locations for flow blockages, including at the intake of the system and within the systems themselves.
.5 Review completed and proposed corrective actions to determine if the licensee has/is taking
appropriate actions to address this condition.
The inspectors reviewed the completed and proposed corrective actions developed in the root cause evaluation. Specifically, dredging the discharge canal on December 12, 2019, returned system capability. On December 13, 2019, Cooper returned all affected systems to operable status following system testing and evaluation. Before December 2019, operation of the service water system during cooler environmental conditions included maintaining an in-service division with full flow and a standby division with no flow through the discharge piping. Post event system evaluations determined that the lack of flow through the standby division allowed the buildup of silt and debris. Cooper implemented a compensatory measure (i.e. corrective action) of ensuring at least 1000 gallons per minute flow through each service water discharge path, to prevent the silt from settling over the discharge outlets and impacting system capability. Additionally, Cooper began performing frequent monitoring (approximately every 3 weeks) of the discharge canal, using sounding techniques, to determine discharge canal sediment levels to inform any needed corrective actions.
The planned corrective actions are to develop a long-term periodic sounding strategy of the discharge canal, periodic dredging of the discharge canal, and perform a detailed study of the effects of silting considering particle size, system operation, and system configuration to further inform needed corrective actions. The inspectors determined that Coopers completed and proposed corrective actions are appropriate.
.6 Evaluate past silting/flooding events and determine if past licensee corrective actions to
preclude clogging of the service water line were adequate.
The inspectors review of a sample of historical records revealed that Cooper has internal operating experience associated with silting issues and overall degradation of service water system components since initial commercial operations. In November 1994, Cooper Condition Report CR-CNS-1994-0598 documented degradation on the Division 1 service water pump A. The condition report states, The apparent cause for the high current condition is silting of the pump suction due to prolonged periods of inoperation (idle pump).
Operations currently shifts operating SW (service water) pumps every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to limit this condition on the operable pumps. The inspectors noted that this practice was discontinued prior to the December 2019 clogging issue, as current operating procedures allowed prolonged periods of idle pump time.
Historically, Cooper had recurring issues with component degradation due to the debris and silt causing erosion and blockage of valves, piping, and heat exchangers.
Specifically, the inspectors noted:
02/21/2002 Reactor equipment cooling temperature control valve, SW-TCV-451A: flow greater than 1000 gpm when the valve is closed.
03/05/2003 Reactor equipment cooling temperature control valve, SW-TCV-451B valve body erosion on the upper half of the body downstream of the valve seat ring. This erosion has created deep channels in the valve disc/seat area.
03/29/2003 Division 1 service water pumps A and C system resistance flow acceptance criteria was not achieved during Procedure 6.SW.102.
03/18/2004 Reactor equipment cooling temperature control valve, SW-TCV-451B: valve will not go full close with the controller either in auto or manual. This valve is already written up for not properly controlling reactor equipment cooling temperature.
In October 2009, divers in the Cooper service water discharge canal identified a considerable amount of silt buildup and were unable to find the Division 1 service water discharge line. As part of the corrective actions, Cooper dredged the discharge canal and initiated actions to develop a monitoring and dredging strategy. The inspectors noted that Cooper dredged the discharge canal in October 2009, March 2012, and November 2012 under these programs, but never formulated a monitoring strategy. Cooper cancelled the periodic dredging strategy after completion of discharge piping modification, based solely on internal operating experience data from the pre-modification service water discharge piping configuration. Since Cooper did not have a monitoring strategy, e.g., periodic sounding, for the discharge canal, it is difficult to correlate what specific effects came solely from the 2019 flooding conditions. However, the inspectors do recognize the flooding conditions brought greater than normal amounts of sediment, further impacting the amount of debris in the discharge canal.
.7 Review the design bases documents (USAR, calculations, etc.) and determine if the
licensees operational practices with respect to potential service water degradation are consistent with these documents.
The inspectors reviewed design basis documents. The inspectors noted the service water system is designed to provide safety-related, essential cooling water for the removal of heat from safety-related equipment, such as the emergency diesel generators and reactor equipment cooling system heat exchangers, and to provide a supply of water for the residual heat removal service water heat exchangers using the residual heat removal service water (i.e. service water booster) pumps. The service water system is required for a safe reactor shutdown following a design basis accident or transient. The service water system also provides cooling water to turbine building non safety-related, nonessential loads and other unit components, as required, during normal operation.
In October 2009, the results of the Division 1 post loss of coolant accident flow testing indicated that the Division 1 service water system pumps A and C exceeded allowed operating range. During this Division 1 test, reactor equipment cooling heat exchanger flow had to be throttled (lowered) more than normal to achieve the required service water flow of 1400 gallons per minute to the diesel. Following this test, Cooper was unable to find the cause of the unexpected system resistance. Cooper modified design calculations to accept this condition and installed a chemical injection system (based on casual evaluation).
During the inspection, the inspectors reviewed the north coffer dam in the discharge canal.
The coffer dam was installed to enable excavation and installation of the discharge piping reconfiguration in October 2014. This was originally intended to be a temporary plant structure. During Change Notice 2 of Design Modification CED 6035080, Cooper evaluated the north coffer dam to remain a permanent structure in the discharge canal. Cooper was able to evaluate the north coffer dam for a design basis earthquake and develop a monitoring strategy for the structural components every 5 years utilizing a visual inspection methodology. The inspectors noted that the periodic monitoring strategy only included an above the surface water inspection and would not provide details on the below the water surface conditions of the coffer dam structural components. The inspectors noted that the current condition of the coffer dam is appropriate for service in that environment; however, the plan was deficient to determine any long-term degradation.
In 2014 the licensee modified the discharge piping design, shifting the discharge outlet of the service water system from the higher flow in the center of the discharge canal to a lower flow area near the west bank of the discharge canal. Cooper cancelled the periodic dredging strategy after completion of discharge piping modification, based solely on internal operating experience data from the pre-modification service water discharge piping configuration. The inspectors reviewed the modification of the service water discharge piping, described in CED 6035080, which was installed during October 2014. The inspectors concluded that this modification did not incorporate all necessary design parameters. The inspectors identified a violation of NRC requirements, which is documented in the inspection results section of this report.
.8 Evaluate the potential for common mode failure of the two trains of service water, given the
level of silting in the service water discharge canal.
The inspection team reviewed system performance data and December 2019 sounding results for both the Division 1 and 2 trains of the service water system. The December 9, 2019, sounding data concluded there was at least 15 feet of silt buildup over the Division 2 discharge outlet in the discharge canal. Inspectors reviewing this data concluded that the area adjacent to the Division 1 discharge piping had approximately 10 to 12 feet of silt buildup, and the area adjacent to the Division 2 discharge piping had approximately 13 to 18 feet of silt buildup. The inspectors noted that both divisions were susceptible to the environmental effects of silting. However, since Division 1 maintained the required flow conditions, any silt remained in a state of flow suspension. A brief loss of Division 1 service water flow (e.g., loss of offsite power and electrical restoration via emergency diesel generators) would not have allowed the silt to settle and clog the flow path.
In addition, on December 8, 2019, Cooper tested service water flow through the Division 1 emergency diesel generator piping, as required per technical specification actions. This test provided verification that the Division 1 service water system was not affected by discharge pipe clogging. Therefore, the inspectors concluded that there was not a common mode failure under design basis conditions.
.9 Evaluate the adequacy of licensee procedures to monitor and mitigate silt/debris buildup in
the discharge canal to maintain SW operability.
The service water system has two portions: a nonsafety-related portion and a safety-related portion, which cools the reactor equipment cooling heat exchanger, residual heat removal heat exchanger, and the emergency diesel generator. The safety and nonsafety portions have separate discharge lines. The focus of this inspection concerns the safety-related discharge lines, one for each division of service water.
During normal, cold weather plant operation, there is flow through one division of service water needed to cool one train of reactor equipment cooling. During warmer weather, there is flow through both divisions of service water. This flow helps to support system operability and assists in preventing silt buildup, as the flow maintains suspension of the silt in the vicinity of the discharge outlet. However, leading up to the December 2019 event, there was only continuous flow through the Division 1 service water system due to the colder weather and only one train being required for heat removal. This allowed silt to build up around the Division 2 discharge piping, thereby precipitating the event.
At the time of the event, Cooper did not have any procedures in place to monitor or mitigate silt buildup in the discharge canal. The inspectors believe that efforts in this area could have aided Cooper in making an informed decision on actions to take during periods of increased discharge canal silting. Additionally, the inspectors reviewed Coopers commitments and implementing procedures pertaining to Generic Letter 89-13. The inspectors did not identify any missed opportunities, specific to monitoring and/or dredging the discharge canal, discussed in the commitments of Generic Letter 89-13 or procedures.
The inspectors noted that historically, Cooper had a maintenance strategy/plan in place to periodically dredge the discharge canal. The plan was established in late 2009 to address silt buildup covering the service water discharge piping. From 2009 to 2016, Cooper dredged the discharge canal three times. In 2016, Cooper made the decision to cancel the dredging plan because they believed the composition of the silt was easily overcome by flow or the initiation of flow, and the fact that when service water testing was performed before and after dredging activities took place, no discernable change of service water performance was noted.
Following the 2019 service water discharge clogging event, Cooper implemented several actions to monitor and mitigate the buildup of silt. The licensee implemented a compensatory action to maintain at least 1000 gallons per minute of flow through both divisions of service water. This compensatory measure (i.e. corrective action) ensures that flow will be maintained in both divisions of safety-related piping to maintain suspension of silt. This modification involved changes to several plant procedures and is the basis for continued operability of the service water system. Additionally, Cooper has instituted a plan to sound the discharge canal every three weeks, observing the changes in silt depth.
Cooper plans to use this information to determine future actions as necessary.
.10 Evaluate the licensees implementation of internal and external operating experience
pertaining to degradation of service water systems from adverse environmental conditions and debris.
The team reviewed the available operating experience pertaining to degradation of service water systems caused by environmental conditions. The vast majority of these past experiences, both internal and external experience, cover degradation of the intake portion of service water systems. Cooper has a program in place to monitor silt buildup at the intake, and to take action as necessary.
The few pieces of operating experience available in relation to discharge canals did not apply to this situation. For internal operating experience, this involved large pieces of debris and silt buildup seen by Cooper, which did not impact system performance. For external operating experience, the discharge issues identified did not directly relate to an entire division of a system losing flow. The inspectors determined Coopers use of external operating experience was adequate.
The inspectors also considered Coopers corrective action documents to provide insights on internal operating experience. Of note, in 2009, while divers were in the discharge canal, a considerable amount of sediment was identified over the service water discharge line. To correct this condition, Cooper dredged the discharge canal in October 2009. However, after the modification to reconfigure the service water discharge lines for 24 feet of separation in 2014, Cooper did not perform dredging or sounding activities. Combining the lack of sounding and dredging with the new configuration, which was not evaluated for all aspects of the environmental condition of silting, the piping was particularly vulnerable to system degradation due to silting. The inspectors determined that a periodic maintenance strategy with sounding and dredging could have reduced the likelihood of the Division 2 service water system being impacted by at least 15 feet of silt.
.11 Determine whether the licensee appropriately evaluated the operability of systems affected
by this condition with regard to technical specification requirements.
The inspectors reviewed Coopers operability determinations and completion of required technical specification action statements associated with the sequence of events. The inspectors noted that considering time of discovery and methodical troubleshooting efforts, Cooper adequately evaluated all affected structures, systems, and components when needed. Although the Division 1 service water pump A was taken out of service for planned maintenance on December 5, 2019, this was reasonable, as there were no adverse conditions at the time to conclude there was degradation in the Division 2 service water systems (including the reactor equipment cooling and emergency diesel generator). Cooper had the first reasonable opportunity of finding the Division 2 service water system degradation during the return to service testing for service water pump A on December 6, 2019. As troubleshooting activities found additional insights that challenged the reasonable expectation of operability, Cooper appropriately entered the technical specification shutdown action statements and performed the required actions. Following discharge canal dredging, system evaluations, and tests, Cooper appropriately returned all affected systems to operable status on December 13, 2019. The inspectors reviewed these troubleshooting activities and return to service tests.
.12 Evaluate the licensees actions to comply with reporting requirements associated with this
event.
The inspectors noted that the Division 1 service water pump A was taken out of service for a planned maintenance outage. On December 6, 2019, when performing a return to service test, the issue with no flow through the Division 2 reactor equipment cooling system was identified. Through troubleshooting activities, on December 8, 2019, Cooper identified the lack of flow through the entire Division 2 service water system. Considering time of discovery for operability, the inspectors reviewed the sequence of events against 10 CFR 50.72 and 50.73 reporting requirements. The inspectors reviewed operability determinations, the Cooper procedure for reporting events to the NRC, and the Cooper procedure for determining system loss of safety function against NUREG-1022, Revision 3.
Reviewing the sequence of events, the inspectors considered the following reporting requirements:
- (1) Operation or Condition Prohibited by Technical Specifications (50.73(a)(2)(i)(B))
- (2) Event or Condition that Could Have Prevented Fulfillment of a Safety Function (50.72(b)(3)(v)) and 50.73(a)(2)(v))
- (3) Single Cause that Could Have Prevented Fulfillment of the Safety Functions of Trains or Channels in Different Systems (50.73(a)(2)(vii))
Cooper did not make an 8-hour event report for this event. Considering time of discovery, the inspectors conclude that Coopers decision to not make an 8-hour event report was appropriate.
On February 4, 2020, Cooper submitted LER 05000298/2019-003-00 (ADAMS Accession No. ML20043D739) to report the loss of Division 2 service water that was identified on December 8, 2020. The inspectors determined that Cooper appropriately complied with reporting requirements associated with this event.
The inspectors reviewed Cooper LER 05000298/2019-003-00 using Inspection Procedure 71153, as documented in Section 71153 of this report.
INSPECTION RESULTS
Failure to Take All Design Parameters into Account during a Design Modification Cornerstone Significance Cross-cutting Aspect Inspection Procedure Mitigating Systems Green NCV 05000298/2020050-01 Open/Closed Not Applicable 93812Special Inspection A self-revealed, Green finding with an associated non-cited violation of 10 CFR Part 50, Appendix B, Criterion III, Design Control, was identified for the licensees failure to ensure that all aspects of the environmental condition of silting were taken into account during the modification of the service water discharge outlet piping, CED 6035080, which was installed during October 2014. As a result, in December 2019, the Division 2 service water discharge piping was clogged and covered by at least 15 feet of silt. This amount of silt degraded the system to the point that required flow was unable to be established through the Division 2 service water system and rendered the division inoperable and unavailable.
Description:
On December 6, 2019, Cooper was unsuccessful in achieving service water system flow through the Division 2 reactor equipment cooling heat exchanger. On December 8, 2019, after troubleshooting, Cooper determined that all the safety-related systems that are supplied cooling water by the Division 2 service water system were impacted by a service water system blockage, rendering the systems inoperable and unavailable. These systems included the reactor equipment cooling system heat exchanger, emergency diesel generator, and residual heat removal system heat exchanger. On December 9, 2019, following troubleshooting and sounding of the discharge canal, Cooper identified at least 15 feet of silt on top of the Division 2 service water discharge outlet and initiated efforts to perform dredging in the discharge canal. On December 12, 2019, after dredging activities in the discharge canal, Cooper was able to clear the blockage and restore all flow capability back to the Division 2 service water system.
Dredging of the Division 1 discharge outlet was also performed. On December 13, 2019, the Division 2 service water train, emergency diesel generator, and residual heat removal system were declared operable following system evaluations and tests.
The inspectors noted that during the troubleshooting activities, Cooper pursued multiple paths to reduce overall plant risk and vulnerability due to the Division 2 service water blockage.
Specifically, on December 11, 2019, Cooper installed a temporary modification on the service water system, consisting of temporary piping and fire hoses, to reroute the downstream service water flow for the Division 2 emergency diesel generator. This modification restored the capability and availability of the Division 2 emergency diesel generator. In addition, Cooper was actively pursuing a similar temporary modification to restore capability and availability of the Division 2 reactor system cooling heat exchanger and residual heat removal heat exchanger.
This modification was never installed, but is now readily available for risk mitigation, if ever needed. The inspectors recognized these efforts as positive licensee performance to mitigate the consequences of downstream blockage and regain system capability but not operability.
Cooper established Work Order 5331801 for implementation of a 3-week sounding task, starting January 16, 2020, to better understand near term silting conditions. The inspectors reviewed these sounding results and noted that the discharge canal silting levels have remained consistent (approximately 4 feet) around the discharge piping. This means no appreciable increase in silting has occurred since the December 2019 dredging and change in the system operation (flow of 1000 gpm at all times).
The inspectors noted that Cooper had identified sediment buildup in the discharge canal in the past. In October 2009, while divers were performing an inspection of the Cooper discharge canal, they noted a considerable amount of sediment above the discharge piping for the Division 1 and Division 2 service water lines. Cooper dredged the silt in 2009.
In October 2009, Cooper believed that their discharge piping configuration had approximately 24 feet of separation between the two trains, per Drawing 2120, Revision 14. However, Cooper learned that the discharge piping existed as documented in Drawing 2120, Revision 13, which showed the lines configured approximately 6 inches apart from each other. The special inspection team inspectors noted that due to the discharge piping being in close proximity to each other, the effects of debris and silting were less of an impact at that time.
To monitor and mitigate the effects of debris buildup in the discharge canal, Cooper created Maintenance Plan 800000036721 following the October 2009 dredging. The inspectors noted that this maintenance plan did not include monitoring (e.g., periodic sounding) of the discharge canal. Dredging activities took place in October 2009, March 2012, and November 2012.
Cooper cancelled the maintenance plan in 2016 under the basis of engineering input to the work management system, Notification 11260417. The documented basis for cancelling this plan was that the composition of the silt was easily overcome by flow or the initiation of flow, as well as the experience that no discernable change of service water performance was noted after dredging.
The inspectors noted that this operating experience was from before the October 2014 modification of the discharge outlets, which is discussed below, and is not directly applicable to the existing configuration.
In October 2014, the station modified the discharge piping through Design Modification CED 6035080, to separate the discharge piping discharge outlets. The inspectors noted that this modification improved safety, in that it decreased the likelihood of a single point vulnerability (e.g., barge or large object debris) from affecting both the Division 1 and Division 2 discharge piping. This modification separated the discharge piping Division 1 from Division 2 by 24 feet, adjacent to the southwest bank on the discharge canal. The inspectors reviewed CED 6035080 and noted that the document discussed the environmental effects of silt and debris, but Cooper Calculation NEDC 11-139 demonstrated silt accumulation would not adversely impact the service water system. The basis of this assumption was again similar to Notification 11260417, in that, when silt was identified in the discharge canal adjacent to the service water discharge lines, no appreciable impacts were ever noted in the system performance. However, the inspectors noted that this assumption was not valid for the piping configurations being 24 feet apart. Furthermore, NEDC 11-139, Section 9, calculated the impact of silt buildup on system performance. The calculation determined that the accumulation of 16 feet above the discharge piping had a total increase of approximately 6 psig in system resistance, for a maximum system resistance of approximately 16 psig, which was well below a service water pump capacity of 75 psig. This calculation was incorrect and was a technical basis for implementation of the system modification in CED 6035080.
During the months of March 2019 through October 2019, the Missouri River levels were above flood stage, reaching a peak level of 901.5 feet on March 16, 2019. Cooper appropriately entered the emergency procedure for environmental flooding for the dates of March 13 through April 4, 2019, and again during September 20 through October 19, 2019. On March 15, 2019, Cooper declared a notice of unusual event due to high river level of 899.05 feet above main sea level, as documented in Event Report 53934. This was required per their Emergency Action Level HU 1.5. The inspectors noted that this information (long term site flooding) could have been an indicator for Cooper to evaluate the effects of flooding to see if any adverse impacts needed to be evaluated and, if necessary, corrected. It is appreciable that the flood conditions brought additional silt and debris, but it would have been difficult to determine any impact to discharge canal silting trends, as no sounding or dredging activities had taken place since the modification was installed in October 2014.
Prior to December 2019, during normal, cold weather plant operation, flow through one division of service water was needed to cool one train of reactor equipment cooling. During warmer weather, there is flow through both divisions of service water. This flow helps to support system operability and assists in preventing silt buildup, as the flow maintains suspension of the silt in the vicinity of the discharge outlet.
On October 6, 2019, due to lower environmental and river temperatures, and not needing both reactor equipment cooling heat exchangers in service, Cooper did not use both service water discharge lines in the discharge canal.
In December 2019, due to the service water discharge line separation of 24 feet, no monitoring or mitigation strategy for debris buildup, no system flow when during cold weather plant operation, and the increase of silt that occurs during flooding, the discharge canal had silt building up, which degraded the Division 2 service water capability.
Corrective Actions: Cooper performed dredging activities to restore system flow capability, modified system procedures and operations to ensure both Division 1 and Division 2 service water discharge lines have at least 1000 gallons per minute of flow to maintain silt in suspension, and performed a root cause evaluation. Coopers planned corrective actions are to develop a plan for periodic sounding of the discharge canal, periodic dredging of the discharge canal, and to perform a detailed study of the effects of silting, considering particle size, system operation, and system configuration to determine any additional corrective actions needed.
Corrective Action References: These issues were placed into the corrective action program as Condition Reports CR-CNS-2019-06302, CR-CNS-2019-06322, CR-CNS-2019-06323, and CR-CNS-2020-00739.
Performance Assessment:
Performance Deficiency: In 2014, Cooper failed to take into account all applicable design parameters when implementing a modification to the plant design. Specifically, when implementing plant modification CED 6035080, which was installed during October 2014, Cooper failed to take into account all aspects of the environmental condition of silting due to the inaccurate silting impact calculation contained in NEDC 11-139, Section 9.
Screening: The inspectors determined the performance deficiency was more than minor because it was associated with the Protection Against the External Factors attribute of the Mitigating Systems cornerstone, and adversely affected the objective to ensure the availability, reliability, and capability of systems that respond to initiating events to prevent undesirable consequences. The performance deficiency resulted in the Division 2 service water discharge piping becoming clogged and covered by at least 15 feet of silt. This condition degraded the system to the point that required flow was unable to be established, rendering the system inoperable.
Significance: The inspectors assessed the significance of the finding using Inspection Manual Chapter (IMC) 0609, Attachment 4, Initial Characterization of Findings, and Appendix A, The Significance Determination Process for Findings At-Power. The inspectors determined that the finding involved a degraded condition that represented a loss of the probabilistic risk assessment function of the Division 2 emergency diesel generator for greater than its technical specification allowed outage time. Therefore, a detailed risk evaluation was required in order to determine the safety significance.
The senior reactor analyst evaluated the risk of the subject performance deficiency in accordance with Appendix A, Section 6.0, Detailed Risk Evaluation. During the evaluation, the following assumptions were made:
Assumptions:
1) The subject performance deficiency resulted in the clogging of the Division 2 service water discharge piping.
2) The Cooper SPAR model, Version 8.58, as modified, is the best available tool for evaluating the risk of the subject performance deficiency, including the improved risk effect on the plant once the temporary modification of Division 2 emergency diesel generator was complete.
3) Condensate and feedwater would continue to be available following a failure of vital switchgear F.
4) The change in risk can be best divided into four timeframes:
a. The 12 days, 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> from the last time reactor equipment cooling heat exchanger B was backwashed on November 23, 2019, until operators failed to establish flow through the heat exchanger on December 6, 2019; b. The 2 day, 3-hour period from the failure to establish flow through the reactor equipment cooling heat exchanger until operators failed to establish flow through the Division 2 emergency diesel generator and the Division 2 service water was declared inoperable on December 8, 2019; c. The 3-day period from when Division 2 service water was declared inoperable until the Division 2 emergency diesel generator was made functional with a temporary modification on December 11, 2019; and d. The 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> from when the Division 2 emergency diesel generator was functional until flow was established through Division 2 service water on December 12, 2019.
5) The best estimate of the exposure time is 1/2 the total time (T/2) for timeframe a and the total time (T) for timeframes b, c and d.
6) In all cases, if operators were to have started the residual heat removal service water booster pumps, the increased system pressure would have cleared the blockage in the service water system discharge.
7) In all cases, the operators would likely have followed the emergency operating procedures and started the residual heat removal service water booster pumps early following a reactor scram.
8) The SPAR-H human reliability analysis is the best method to determine the likelihood that operators would fail to start the residual heat removal service water booster pumps.
9) At the time that residual heat removal service water booster pumps are needed, operators would be under high stress.
10) Because operators knew that the Division 2 service water discharge was clogged during most of the exposure period, the best value to use for the complexity of the diagnosis for starting residual heat removal service water booster pumps is Moderately Complex.
11) Because the operator actions are driven by emergency operating procedures, the procedure quality of the diagnosis for starting residual heat removal service water booster pumps is Diagnostic.
Conditional Core Damage Probability:
The analyst determined that the effect of failing the Division 2 service water discharge line was that the following components were essentially failed:
1) Division 2 reactor equipment cooling heat exchanger; 2) Division 2 residual heat removal system heat exchanger; and 3) Division 2 emergency diesel generator heat exchangers.
Using the site-specific SPAR model, Version 8.2, the analyst quantified the baseline core damage frequency for all internal initiators. During the initial baseline evaluation, the analyst noted that the SPAR initiating event for a failure of vital ac switchgear F assumed that the condensate and feedwater system were not available to provide water to the reactor vessel. The analyst modified Event Tree LOACB-F, Cooper Loss of Vital 4160 Vac, Bus F, to indicate the potential success of the main feedwater system as stated in Assumption 3.
Utilizing the modified SPAR model, and Assumptions 1, 2 and 3, the analyst quantified the conditional core damage frequency for each of the four timeframes discussed in Assumption 4. Components 1, 2, and 3 were failed as appropriate by setting each respective basic event to the House Event TRUE, indicating that the component had failed and that a potential for common cause failure of the opposite divisions components existed. The results of these runs of the SAPHIRE code are documented in the Table 2.
The dominant core damage sequences from these runs were as follows:
- Loss of dc switchgear A initiator, with:
o Failure of suppression pool cooling o Failure of containment spray o Failure to recover the power conversion system o Failure to vent the drywell o Failure to provide late injection
- Loss of offsite power initiator, with:
o Failure of suppression pool cooling o Failure of shutdown cooling o Failure of containment spray o Failure to recover the power conversion system o Failure to vent the drywell o Failure to provide late injection
- Loss of condenser heat sink initiator, with:
o Failure of suppression pool cooling o Failure of containment spray o Failure to recover the power conversion system o Failure to vent the drywell o Failure to provide late injection
- Loss of ac switchgear F initiator, with:
o Failure of the main feedwater system o Failure of suppression pool cooling o Failure of containment spray o Failure to recover the power conversion system o Failure to vent the drywell o Failure to provide late injection
Exposure Period:
In accordance with Assumption 4, the analyst quantified the following exposure periods (EXP) during which the plant was potentially impacted by the performance deficiency:
Timeframe A:
On November 23, 2019, operators performed a routine backwash of the Division 2 reactor equipment cooling system heat exchanger. Records of established flow rates and reviews of piping and instrumentation diagrams indicated that the system discharge was open at that time. On December 6, 2019, operators attempted to put Division 2 reactor equipment cooling heat exchanger in service and were unable to establish flow. At that time, the subsystem discharge was clogged. Also at the time, operators did not know that the entire division discharge was clogged; however, they knew that there was a system disruption affecting the reactor equipment cooling system.
The analyst utilized the Risk Assessment of Operational Event Handbook, Volume 1, Internal Events, to determine an appropriate exposure period for this timeframe.
The analyst noted that, based on best available information, the service water system discharge could have clogged at any time since the last flow was established on November 23, 2019. Therefore, in accordance with Volume 1, Section 2.4, Exposure Time = t/2 + Repair Time, the analyst selected t/2 for this timeframe.
The total time
- (t) of this timeframe was 12 days, 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br />. Half of this time (t/2) is 6 days, 7.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. This is the exposure time (6.32 days) for timeframe a as documented in Table 2.
Timeframe B:
On December 8, 2019, during troubleshooting of the Division 2 service water system, operators were unable to establish flow through Division 2 emergency diesel generator. Following an evaluation of the condition, the licensee declared Division 2 service water inoperable. Prior to this discovery, operators were unaware that the service water discharge was clogged.
The analyst utilized the Risk Assessment of Operational Event Handbook, Volume 1, Internal Events, to determine an appropriate exposure period for this timeframe.
The analyst noted that, based on best available information, the service water system discharge was known to be clogged throughout the entire timeframe from December 6 through December 8, 2019. Therefore, in accordance with Volume 1, Section 2.3, Exposure Time = t + Repair Time, the analyst selected the entire time t for this timeframe.
The total time
- (t) of this timeframe was 2 days, 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />. The total time
- (t) was selected. This is the exposure time (2.10 days) for timeframe b as documented in Table 2.
Timeframe C:
On December 11, 2019, the licensee installed a temporary modification, bypassing the Division 2 service water discharge and establishing flow to the Division 2 emergency diesel generator. During the 3-day window from when Division 2 service water was declared inoperable until Division 2 emergency diesel generator was made functional with a temporary modification, all Division 2 components cooled by service water were incapable of performing their risk-significant functions. During this timeframe, operators were aware that the service water discharge was clogged; however, the system was not removed from service.
The analyst utilized the Risk Assessment of Operational Event Handbook, Volume 1, Internal Events, to determine an appropriate exposure period for this timeframe.
The analyst noted that, based on best available information, the service water system discharge was known to be clogged throughout the entire timeframe.
Therefore, in accordance with Volume 1, Section 2.3, Exposure Time = t + Repair Time, the analyst selected the entire time t for this timeframe.
The total time
- (t) of this timeframe was approximately 3 days. The total time
- (t) was selected. This is the exposure time (3.01 days) for timeframe c as documented in Table 2.
Timeframe D:
On December 12, 2019, while the licensee was dredging the discharge canal, operators were able to establish flow through the Division 2 service water discharge.
This resulted in all Division 2 components being functional and capable of performing their risk-significant functions. During the 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> from when the Division 2 emergency diesel generator was functional until flow was established through Division 2 service water all Division 2 components except the Division 2 emergency diesel generator were incapable of performing their risk-significant functions. During this timeframe, operators were aware that the service water discharge was clogged; however, the system was not removed from service.
The analyst utilized the Risk Assessment of Operational Event Handbook, Volume 1, Internal Events, to determine an appropriate exposure period for this timeframe.
The analyst noted that, based on best available information, the service water system discharge was known to be clogged throughout the entire timeframe.
Therefore, in accordance with Volume 1, Section 2.3, Exposure Time = t + Repair Time, the analyst selected the entire time t for this timeframe.
The total time
- (t) of this timeframe was 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />. The total time
- (t) was selected.
This is the exposure time (0.916 days) for timeframe d as documented in Table 2.
Incremental Conditional Core Damage Probability (ICCDP):
For each of the timeframes, the analyst quantified the ICCDP by multiplying the CCDP by the annualized exposure period. The results are documented in the Table 2.
Recovery:
As described in Assumption 6, based on best available information, the analyst and inspectors assumed that, if operators were to have started the residual heat removal system service water booster pumps, the increased system pressure would have cleared the blockage in the service water system discharge. Therefore, the analyst evaluated the likelihood that operators would start the booster pumps when necessary. The analyst reviewed the emergency operating procedures that would be in effect for the dominant core damage sequences. In all cases, the analyst determined that the emergency operating procedures, combined with associated hard cards available at the control room operating panels, would direct operators to start the booster pumps.
The analyst utilized the human reliability analysis approach described in NUREG/CR-6883, SPAR-H Human Reliability Analysis Method, to estimate the probability that operators would fail to start the residual heat removal system service water booster pumps when required. Based on Assumptions 9, 10, and 11, the analyst modified the performance shaping factors as documented in Table 1. The estimated adjusted human reliability analysis probability was calculated, and the odds ratio applied to minimize the effects of the simplification in the model.
The total failure probability for diagnosis and actions was documented in Table 2.
Recovered ICCDP:
To estimate the change in core damage frequency as a result of the subject performance deficiency, the analyst applied the calculated failure probability to the ICCDPs. The associated ICCDP from each timeframe was multiplied by the probability that operators would fail to start the residual heat removal service water booster pumps. This calculation represented the assumption that once booster pumps were started, the clog in the Division 2 service water discharge line would clear and cooling would be restored to the divisional components.
The recovered ICCDPs were summed to determine the total CDF. This result, documented in Table 2, was 8.21E-08. Therefore, this finding was of very low safety significance (Green).
Large, Early Release Frequency:
In accordance with Manual Chapter 0609, Appendix H, Containment Integrity Significance Determination Process, Section 04.01, Types of Findings, the analyst determined that this was a Type A finding. Type A findings can influence the likelihood of accidents leading to core damage that are also identified as contributors to Large, Early Release Frequency. Appendix H, Section 04.02, LERF-Based Significance Determination Process, states that for Type A findings, if the total CDF for the finding is less than 1.0E-7 per reactor year, then the finding should be assigned a Green significance level.
External Events Conditional Core Damage Probability:
In accordance with Manual Chapter 0609, Appendix A, Section 0609A-05, Detailed Risk Evaluation, when the internal events detailed risk evaluation results are greater than or equal to 1.0E-7, the finding should be evaluated for external event risk contribution. In addition, the guidance allows that any internal event results that are less than 1.0E-7 can be evaluated for external event risk contribution at the discretion of the regional senior reactor analyst. The analyst determined that an evaluation of external risk would not be required because the result of the detailed risk evaluation was less than 1.0E-7, and no additional information indicated that the performance deficiency was particularly susceptible to external events.
Table 1 Failure to Start Residual Heat Removal Service Water Booster Pumps
Follow EOPs and Hard Card Performance Shaping Factor Diagnosis Action
PSF Level Multiplier PSF Level Multiplier
Time:
Nominal 1.00 Nominal 1.0 Stress:
High 2.0 High 2.0 Complexity:
Moderate 2.0 Nominal 1.0 Experience:
Nominal 1.0 Nominal 1.0 Procedures:
Diagnostic 0.5 Nominal 1.0 Ergonomics:
Nominal 1.0 Nominal 1.0 Fitness for Duty:
Nominal 1.0 Nominal 1.0 Work Processes:
Nominal 1.0 Nominal 1.0
Nominal 1.00E-02 1.00E-03
Adjusted 2.00E-02 2.00E-03
Odds Ratio:
1.98E-02 Odds Ratio:
2.00E-03 Failure to Recovery Probability:
2.18E-02
Table 2 SPAR Results
Timeframe Exposure Time (days)
CCDP ICCDP Recovery Recovered ICCDP
a 6.32 1.12E-04 1.95E-06 2.18E-02 4.24E-08 b
2.10 1.12E-04 6.48E-07 2.18E-02 1.41E-08 c
3.01 1.12E-04 1.12E-07 2.18E-02 2.02E-08 d
0.916 9.65E-05 2.42E-07 2.18E-02 5.28E-09
Total CDF 8.21E-08
Cross-Cutting Aspect: Not Present Performance. No cross-cutting aspect was assigned to this finding because the inspectors determined the finding did not reflect present licensee performance.
Enforcement:
Violation: Title 10 CFR 50, Appendix B, Criterion III, Design Control, in part requires that design control measures shall provide for verifying or checking the adequacy of design by the performance of design reviews, by the use of calculational methods, or by the performance of a suitable testing program. Contrary to the above, from October 2014 until December 2019, Cooper failed to ensure that design control measures provided for verifying the adequacy of design by the performance of design reviews, by the use of calculational methods, or by the performance of a suitable testing program. Specifically, Cooper failed to verify the adequacy of the service water system design when the system was modified for 24 feet of separation between the discharge outlets. Cooper failed to take all aspects of the environmental condition of silting into account during the design change process of CED 6035080 due to the inaccurate silting impact calculation contained in NEDC 11-139, Section 9.
Enforcement Action: This violation is being treated as a non-cited violation, consistent with Section 2.3.2 of the Enforcement Policy
EXIT MEETINGS AND DEBRIEFS
- On March 27, 2020, the inspectors presented the inspection results to Mr. J. Dent, Site Vice President, and other members of the licensee staff. The inspectors verified no proprietary information was retained by the inspection team.
DOCUMENTS REVIEWED
Inspection
Procedure
Type
Designation
Description or Title
Revision or
Date
93812
Calculations
NEDC 11-139
Station Service Water System Model Development and
Benchmark
0C1
NEDC 11-140
Review of ZNE Calculation 11-198 Rev. 1 "Cooper Nuclear
Station Service Water System Analysis
NEDC 96-029
Post LOCA Service Water Svstem Flow Variations With River
Level
NEDC 96-029
Post LOCA Service Water Svstem Flow Variations With River
Level
Corrective Action
Documents
CR-CNS-
1994-00530, 1994-00598, 2002-00953, 2003-01001, 2003-
01519, 2004-01340, 2004-02107, 2004-03050, 2005-08852,
2006-03301, 2006-03853, 2006-05299, 2006-05591, 2006-
08470, 2007-04958, 2007-05658, 2009-08065, 2009-08110,
2009-08676, 2009-08780, 2009-08848, 2009-08877, 2009-
09526, 2010-02345, 2010-02347, 2010-02529, 2010-02859,
2010-03689, 2010-05776, 2012-02067, 2012-02162, 2012-
05570, 2019-01530, 2019-03889, 2019-05121, 2019-05163,
2019-05967, 2019-06019, 2019-06302, 2019-06318, 2019-
06322, 2019-06323, 2019-06325, 2019-06326, 2019-06327,
2019-06339, 2019-06353, 2019-06384, 2019-06386, 2019-
06388, 2019-06399, 2019-06403, 2019-06470, 2019-06509,
2019-06518, 2019-06543, 2019-06566, 2019-06588, 2020-
00036
Corrective Action
Documents
Resulting from
Inspection
CR-CNS-
20-00739
Drawings
2006 Sheet 3
Circulating, Screen Wash & Service Water Systems
CNS DWG 2120
Yard Circulating and Service Water Piping Plan and Sections
SKE-6035080-C-
001
Headwall Installation Sketch
C1
Engineering
Changes
CED 6034480
CW/SW (Circulating Water/Service Water) Chemical Injection
Taps
01/26/2017
CED 6035080
Service Water Discharge Piping Replacement
2/24/2014
Inspection
Procedure
Type
Designation
Description or Title
Revision or
Date
DCN 12-0890
SW Discharge Piping Replacement
09/18/2012
CW/SW (Circulating Water/Service Water) Chemical Injection
System (Building/Equipment Installation)
10/29/2015
ECM DR-2014-
0008
Change Drawing 2120
2/17/2014
ECM DR-2014-
0496
Change Drawing 2120
2/17/2014
Engineering
Evaluations
EE 10-040
Configuration Documentation of the SW Discharge Piping
during Original Site Construction
Miscellaneous
Manual Sounding Results Dated: 12/9/19, 1/9/20, 1/29/20,
2/19/20, 3/11/20, and 4/1/20
6.1SW.101 2015
Performances
Service Water Surveillance Operation (Div 1)
04/30/2015
6.1SW.101 2016
Performances
Service Water Surveillance Operation (Div 1)
2/02/2016
6.1SW.101 2017
Performances
Service Water Surveillance Operation (Div 1)
01/05/2017
6.1SW.101 2018
Performances
Service Water Surveillance Operation (Div 1)
2/02/2018
6.2SW.101 2015
Performances
Service Water Surveillance Operation (Div 2)
2/13/2015
6.2SW.101 2016
Performances
Service Water Surveillance Operation (Div 2)
2/17/2016
6.2SW.101 2017
Performances
Service Water Surveillance Operation (Div 2)
01/25/2017
6.2SW.101 2018
Performances
Service Water Surveillance Operation (Div 2)
01/19/2018
CNSS907024
Response to Generic Letter 89-13
01/29/1990
Recent Operating Experience of Service Water Systems due to
External Conditions
07/31/2006
Maintenance Plan
800000015275, 800000034391, 800000036721
NLS9000459
Generic Letter 89-13 Recommended Inspection Program
10/15/1990
Notification
10700296, 10730847, 10827947, 10890761, 10939173,
11063685, 11260417, 11672289
NSD920007
Completion of Generic Letter 89-13 Actions
01/09/1992
Inspection
Procedure
Type
Designation
Description or Title
Revision or
Date
TCC-4908683
SW Discharge Pipe Repair Headwall & Tie-In Cofferdam
TCC-5328958
Diesel Generator Cooling Water Alternate Discharge Flow Path
and 1
TCC-5329190
RHR HX Alternate Discharge Flow Path
Procedures
0.5.CR
Condition Report Initiation Review, and Classification
2.2.20
Standby AC Power System (Diesel Generator)
107
2.2.20.1
Diesel Generator Operations
2.2.20.2
Operation Of Diesel Generators From Diesel Generator Rooms
2.2.65.1
REC (Reactor Equipment Cooling) Operations
2.2.65.1
REC (Reactor Equipment Cooling) Operations
2.2.70
RHR Service Water Booster Pump System
2.2.71
Service Water System
26
2.2.71
Service Water System
27
3.34
Heat Exchanger Program
3.48
Service Water Program Implementation
0, 1, and 2
5.2SW
Service Water Casualties
5.2SW
Service Water Casualties
5.3EMPWR
Emergency Power During Modes 1, 2, or 3
5.3SBO
Station Blackout
Emergency
Procedure 5.2SW
Service Water Casualties
NRC Generic Letter 89-13 Service Water Program
2C0
Emergency Operating Procedures Attachment 1
0-CNS-LI-108
Event Notification and Reporting
Work Orders
WO 27013, 4811153, 4819947, 4889729, 4895972, 4902025,
4905945, 4916945, 4938230, 4938231, 4973832, 5067128,
5167390, 5329082, 5329188, 5329190
December 18, 2019
MEMORANDUM TO:
Neil Day, Resident Inspector, Comanche Peak
Project Branch B
Division of Reactor Projects
FROM:
Anton Vegel, Director /RA/
Division of Reactor Projects
SUBJECT:
SPECIAL INSPECTION CHARTER TO EVALUATE CLOGGING OF
DIVISION 2 SERVICE WATER DISCHARGE LINE
AT COOPER NUCLEAR STATION
In response to silting in the service water discharge canal that blocked flow and rendered
several Division 2 components inoperable on December 6, 2019, at Cooper Nuclear Station, a
Special Inspection will be performed. You are hereby designated as the Special Inspection
team leader. The following members are assigned to your team:
James Drake, Senior Reactor Inspector, Division of Reactor Safety
Mike Stafford, Acting Senior Resident Inspector - Cooper, Division of Reactor Projects
A.
Basis
On December 6, 2019, with the plant operating at 100 percent power and the A service
water (SW) system in service, the licensee was unable to establish SW flow through the
B reactor equipment cooling (REC) heat exchanger. They entered a 30-day LCO for
B REC and began pursuing action to determine the cause and whether or not there were
any misaligned valves. At 5:00 a.m. on December 8, 2019, the licensee discovered that
they were also unable to establish flow through the No. 2 emergency diesel generator
(DG), and subsequently declared the DG inoperable (7-day LCO) and Division 2 SW
inoperable (30-day LCO). The licensee suspected that the cause of the inability to
establish flow to these systems was silt buildup in the Division 2 SW discharge line. In
order to provide SW cooling to any safety-related Division 2 systems, the flow must
travel out the Division 2 discharge line. The licensee established Division 1 SW flow
through the No. 1 DG to rule out common cause failure on December 8, and Division 1
SW remained in service with continuous flow through its discharge line throughout the
event. During their investigation, the licensee discovered blockage of silt build-up at the
discharge line of Division 2 S
- W. On the morning of Tuesday, December 10, manual
sounding revealed approximately 13-18 feet of silt buildup covering the Division 2
discharge line. Later that afternoon, Region IV learned that the station had also
discovered approximately 10-12 feet of silt buildup over the Division 1 discharge line.
While flow through Division 1 continued unobstructed, discovery of several feet of silt
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covering both discharge lines could have resulted in loss of the ultimate heat sink under
some scenarios.
Following discovery of the silt blockage, the licensee dredged the discharge canal to
remove the silt covering both SW discharge lines, restoring operability to both SW
divisions on December 12, 2019. As a compensatory measure to support operability of
the SW system, the licensee established continuous flow through both trains of SW to
prevent stagnation and silting in the discharge lines.
The licensee last successfully ran DG 2 on November 11, 2019, and observed normal
Division 2 SW cooling flow to the DG on that date. In addition, the licensee successfully
backwashed the B REC heat exchanger on November 23, 2019, and reported normal
Division 2 SW flow discharge at that time as well.
Cooper Nuclear Station experienced high Missouri River levels during the spring of
2019. This has likely contributed to increased silt buildup near the discharge canal over
time. In October of 2019, the licensee took the Division 2 SW system out of service,
resulting in stagnant conditions in the Division 2 SW discharge. This condition likely
exacerbated the silt buildup that resulted in the Division 2 SW line becoming fully
clogged at some time after the November 23 backwash of the REC heat exchanger.
Cooper Nuclear Station has a history of experiencing silting in the discharge canal and,
in November 2009, the licensee similarly discovered several feet of silt accumulated in
the discharge canal. At that time, the licensee had been trending a slow degradation of
service water flow. Despite the discovery of several feet of silt at the discharge lines in
2009, the discharge line was never fully plugged in the manner that occurred with this
2019 event.
Management Directive 8.3, NRC Incident Investigation Program, was used to evaluate
the level of NRC response for this event. In evaluating the deterministic criteria of
MD 8.3, it was determined that the degraded condition met four of the deterministic
criteria. The condition led to multiple failures in systems used to mitigate events,
unexpected system interactions, involved potential adverse generic implications, and
involved questions or concerns pertaining to licensee operational performance. In
evaluating the risk assessment criteria of MD 8.3, the estimated incremental conditional
core damage probability was determined to be between 1.2 x 10-6 - 8.9 x 10-6.
Based on the deterministic criteria and risk insights related to this event, Region IV
management determined that the appropriate level of NRC response was to conduct a
Special Inspection. This Special Inspection is chartered to identify the circumstances
surrounding this event and review the licensees actions to address the causes of the
event.
B.
Scope
The inspection is expected to perform data gathering and fact-finding to address the
following:
1.
Provide a recommendation to Region IV management as to whether the
inspection should be upgraded to an augmented inspection team response.
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2.
Develop a complete sequence of events related to the discovery of indications
that the Division 2 service water discharge line was clogged on December 6,
2019. The chronology should include the status of plant equipment and licensee
actions to mitigate the conditions. The timeline should consider any licensee
actions that occurred following the last known operability of the Division 2
equipment in November 2019, and should include any monitoring of the
discharge canal that has occurred (if any) in recent years and after the
March 2019 flooding event.
3.
Review the licensees causal evaluations and determine if they are being
conducted at a level of detail commensurate with the significance of the issues
that were encountered for the degraded condition.
4.
Review the licensees extent of condition evaluation to determine if the licensee
has adequately considered degradation of service water flow from other sources
of debris and at the intake structure.
5.
Review completed and proposed corrective actions to determine if the licensee
has/is taking appropriate actions to address this condition.
6.
Evaluate past silting/flooding events and determine if past licensee corrective
actions to preclude clogging of the service water line were adequate.
7.
Review the design bases documents (USAR, calculations, etc.) and determine if
the licensees operational practices with respect to potential service water
degradation are consistent with these documents.
8.
Evaluate the potential for common mode failure of the two trains of service water,
given the level of silting in the service water discharge canal.
9.
Evaluate the adequacy of licensee procedures to monitor and mitigate silt/debris
buildup in the discharge canal to maintain SW operability.
10.
Evaluate the licensees implementation of internal and external operating
experience pertaining to degradation of service water systems from adverse
environmental conditions and debris.
11.
Determine whether the licensee appropriately evaluated the operability of
systems affected by this condition with regard to technical specification
requirements.
2.
Evaluate the licensees actions to comply with reporting requirements associated
with this event.
13.
Collect data necessary to support completion of the significance determination
process, if applicable.
C.
Guidance
Inspection Procedure 93812, Special Inspection, provides additional guidance to
be used by the Special Inspection Team. Your duties will be as described in
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Inspection Procedure 93812. The inspection should emphasize fact-finding in its review
of the circumstances surrounding the event. It is not the responsibility of the team to
examine the regulatory process. Safety concerns identified that are not directly related
to the event should be reported to the Region IV office for appropriate action.
You will formally begin the Special Inspection with an entrance meeting to be conducted
no later than January 13, 2020, and perform preliminary information gathering from
December 18-20, 2019. The team will report to the Cooper Nuclear Station on
January 13, 2020, to complete the onsite portion of the inspection. You should provide a
daily briefing to Region IV management during the course of your inspections and prior
to your exit meeting. A report documenting the results of the inspection should be
issued within 45 days of the completion of the inspection.
This Charter may be modified should you develop significant new information that
warrants review.
Docket No. 50-298
License No. DPR-46
CONTACT:
Patricia Vossmar, DRP/C
817-200-1144
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SPECIAL INSPECTION CHARTER TO EVALUATE CLOGGING OF DIVISION 2
SERVICE WATER DISCHARGE LINE AT COOPER NUCLEAR STATION - December
18, 2019