05000282/LER-2013-001
| Prairie Island Nuclear Generating Plant Unit 1 | |
| Event date: | 8-9-2013 |
|---|---|
| Report date: | 10-8-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2822013001R00 - NRC Website | |
GDC-19 requires protection of the control room operator against the threat of radiological hazards under normal and accident conditions. Following the Three Mile Island (TMI) accident in 1979, actions mandated by the Nuclear Regulatory Commission (NRC) required licensees to evaluate their control rooms to assure adequate protection of operators.
PINGP personnel began Control Room Envelope (CRE) testing in anticipation of issuance of additional TS requirements. The initial test was conducted on January 24, 1998. The test results did not meet the established acceptance criteria, and PINGP submitted LER 1-98-02 for excessive CRE leakage. Station personnel took action to address the in-leakage sources, and retested in July 1998.
The July 1998 test results met the acceptance criteria.
On December 06, 2004, station personnel completed Tracer Gas Testing of the Control Room per WO 0406901. The performers added the two gallons of water into the chiller room floor drains as described in the work order, step 6.1. The test personnel did not recognize that pouring the water down the drains could be preconditioning by improving the results while not representing the as-found condition. The test was completed satisfactorily.
On December 10, 2010, station personnel executed WO 0412432 (SP 1449) to conduct Tracer Gas Testing. The program engineer added the two gallons of water to the floor drains to ensure the loop seals1 were full, as prescribed in SP 1449, step 6.1, but did not recognize the activity as preconditioning. The test results met the surveillance acceptance criteria.
On November 21, 2011, station personnel wrote CAP 01297439 to perform a Root Cause Evaluation (RCE) on a potential white finding related to 12 Battery Charger installation issues. The root cause determined that corrective actions to address the extent of cause for the preconditioning aspect of the event would include a comprehensive examination of surveillance procedures to identify any potential preconditioning within the procedures. The results of the review were that 22 procedures needed a more detailed review. New CAPs were written to facilitate the reviews, and assigned as CEs to the appropriate subject matter experts. SP 1449 was among the 22 procedures identified as having potential preconditioning.
On January 16, 2012, the CRE program engineer completed his review of SP 1449 for preconditioning (from RCE 01297439). He determined that the step was not preconditioning because the loop seals were being kept full by the condensate from the 121/122 Safeguard Chiller Air Handler Units, and initiated PCR 01320860 to remove the step to fill the loop seals from SP 1449. SP 1449, Rev 1, was issued on February 12, 2012. The CE was approved by the supervisor, with the following closure comments:
"The condition was evaluated and concluded that the steps that may have been considered pre-conditioning are no longer applicable. PCR 01320860 was generated to remove the steps.
This CAP will be closed to the PCR. No additional actions are required.
There was no documented consideration given as to whether or not the step to add water to the floor drains invalidated the surveillance test conducted in 2010.
On July 8, 2013, a Nuclear Oversight assessor wrote CAP 01389231 "Potential Past Preconditioning, SP 1449 Air In-Leakage Test" because the 2010 Control Room Envelope Tracer Gas Test had "potential improper preconditioning." Station personnel performed an Operability Review (OPR), concluding that the CRE was "Operable But Nonconforming" because the 1998 test, where the loop seals were not filled, met the acceptance criteria.
On July 26, 2013, NOS personnel wrote CAP 01391663 "OPR Based Upon Non-Conservative Assumption" to identify that the OPR from CAP 01389231 "Potential past preconditioning, SP 1449 Air in leakage test" was based upon non-conservative assumptions from the 1998 test and that CRE operability had to be revisited. The non-conservative assumptions were related to having blanks in place instead of the ventilation dampers during the test, potentially causing the results to show lower in-leakage rates from eliminating the dampers as potential paths of in-leakage. The engineer who performed the evaluation for CAP 01391663 concluded that the OPR from CAP 01389231 did not have to be revised based upon the following:
These Dampers are bubble tight dampers. All six of the dampers were tested to 6 psid satisfactorily with no leakage. Additionally, two of the dampers were tested opposite the direction of flow at 1.4" H2O and also tested satisfactorily with no leakage. The other four dampers are associated with fan suctions. The fan suction tends to draw the damper closed.
On August 6, 2013, the Control Room Habitability (CRH) program Engineer wrote CAP 01392548 "Unacceptable Preconditioning Used To Meet TS.5.5.16".
The CAP writer adds his conclusion:
Prerequisite 6.1 of SP 1449 meets the criteria as stated in the 1ST industry document of unacceptable preconditioning and given that the site practice prior to this summer of not maintaining loop seals full of water, the performance in 2010 of SP 1449 did not reflect "a true 'as found' in-leakage measurement" and we cannot use that test to credit meeting TS 5.5.16.
As a result of the evaluation from August 6, 2013, the Control Room Supervisor determined that the CRE was Inoperable, and wrote CAP 01393090 "Control Room Envelope Inoperable". Control Room personnel entered LCO 3.7.10 Condition B1, and implemented mitigating actions. According to CAP 01393090, the mitigating actions are met by having established process by which the Control Room Chiller Room drains are filled every day. The SRO also documented that Condition B2 actions (verification that the mitigating actions are effective) have been implemented.
The CRE was considered inoperable due to unacceptable preconditioning during previous performances of SP 1449, Tracer Gas Test of Control Room. The unacceptable preconditioning was related to filling loop seals in the floor drains located in the 121 and 122 Control Room (CR) Chiller Rooms.
On September 13, 2013, troubleshooting efforts under Work Order 00486988 determined that a CRE boundary Control Damper2 CD-34177 (122 Control Room Outside Air Roof Isolation Damper) was the major source of in-leakage.
On September 15, 2013, a TMOD (EC 22746) "Install Blank Flanges for CD-34177, Control Room Outside Air Roof Isolation Damper" was installed to seal off CD-34177 and re-run SP 1449. The results of SP 1449 were completed SAT with the TMOD installed, thereby validating that the cause of SP 1449 failing to meet in-leakage acceptance criteria on September 10, 2013, was from leakage past CD-34177.
On September 17, 2013, engineering reported that SP1449 had been completed satisfactorily.
On October 3, 2013, engineering completed past operability for the unacceptable preconditioning associated with CAP 01392548. The previous performance of SP 1449 per WO 412432 was completed satisfactorily on December 10, 2010. The unacceptable preconditioning performed under SP 1449 on December 10, 2010 tests was validated as having a minimal affect with the recent performance of SP 1449 and ST 1449 per WO 485074 on September 17 and 18, 2013. This testing validates the CRE is operable with or without the floor drain loop seals being filled.
The conclusion from the past operability regarding the control damper provided no firm evidence that a discrepancy existed prior to the failure of SP 1449 on September 10, 2013, however the monthly visual inspections performed during SP 1112 were considered inadequate for identifying the dampers failure mechanism. The previous performance of SP 1449 per WO 412432 was completed SAT on December 10, 2010, providing firm evidence that CD-34177 was performing its intended function at that time. Therefore the time of discovery, given the inadequate visual inspection criteria in SP 1112, for the discrepancy with CD-34177 is December 10, 2010.
Based on historical and current testing information along with the equipment history of CD-34177, the CRE was inoperable from December 10, 2010. The Technical Specification Required Action Statement for 3.7.10 Condition B was exited on September 18, 2013.
EVENT ANALYSIS
The control room envelope consists of the control room and the two chiller rooms. The control room is a common structure that contains the controls for both Unit 1 and Unit 2. The control room is located at elevation 735' within the Auxiliary Building approximately equidistance between Unit 1 and Unit 2.
The chiller rooms are located directly above the control room at elevation 755'. The cable spreading room on the 715' elevation (directly below the control room) is not part of the control room envelope.
1 EIIS System Code SEAL 2 EllS System Code CDMP HVAC ducting between the cable spreading room and the control room has been isolated by closing the dampers and installing blanks. The control room ventilation system is entirely located within the two chiller rooms (one train of ventilation system in each room), with the exception of the outside air supply. The outside air supply ducting is routed through the Auxiliary Building. The outside air supply dampers are located at the envelope boundary. There are no other ventilation systems that penetrate the control room envelope.
The CRE is the area that in the event of an emergency can be isolated from other plant areas and the environment external to the CRE. For Prairie Island, the CRE includes the Control Room and the 121 and 122 Control Room Chiller Rooms. The CRE provides the boundary to protect the operators from radiological events, toxic hazards, fire, smoke, and high energy line breaks.
A more detailed review of the Waste Liquid (WL) design was performed. It was identified during this review that the loop seals on the WL system were considered Category I vent zone openings. CAT I vent zone provides the boundary control for the Auxiliary Building Special Vent Zone (ABSVZ). In this particular location they adjoin the Control room envelope boundary. This interface called operability into question regarding the Control Room habitability program and Control Room loop seal drain design and function. Pouring water down the floor drains prior to testing was called preconditioning because of the configuration differences between the 1998 testing (blank plates) and the bubble tight dampers in place during the 2010 testing. Therefore the CRE was considered inoperable with measures in place to keep the loop seals full of water until testing could be performed to substantiate the results.
During the weeks of 9/9/2013 through 9/18/2013 WO 00485074 tasks 1 and 3 performed in leakage testing of the CRE with both the floor drain loop seals full using SP 1449 and empty using ST 1449.
During the initial performance of SP 1449 Control Room in leakage testing Control Damper CD-34177 was found to be leaking by. The SP was stopped and T-Mod EC 22746 was put in place to isolate and blank off the damper on 9/15/13. SP 1449 was re-performed with the blank damper design installed for CD-34177. The Surveillance SP 1449 passed with acceptable results. A special test procedure (ST 1449) was written to empty the floor drain loop seals and perform the in leakage testing to validate past operability concerns with the floor drain loop seals dry. ST 1449 was performed using the same train lineups as the SP 1449 procedure. The ST 1449 also passed with acceptable results.
With the failure of CD-34177, the CRE was inoperable, this condition is reportable per 10 CFR 50.73(a)(2)(v)(C), event or condition that could have prevented fulfillment of a safety function, control the release of radioactive material, and 10 CFR 50.73(a)(2)(i)(B), operation or condition prohibited by Technical Specifications. The site operated outside T.S. 3.7.10 "Control Room Special Ventilation System." The CRE was declared operable with the retesting of SP 1449 and ST 1449, with the damper was blank flanged on September 18, 2013.
There was a potential nuclear, radiological, and industrial impact, in that this preconditioning could have masked degradation of the CRE, which would reduce the protection of the CRE provided to the operators in the event of a Design Basis Accident. However, special testing was conducted that indicated that the preconditioning did not impact the CRE function.
The control damper that was identified with the excessive leakage was blank flanged, and the CRE was retested with the floor drains loop seals dry and full of water. Both tests were performed and successfully met the acceptance criteria for in-leakage. The results from these tests indicate that the Control Room personnel are protected in the event of a Design Basis Accident. The CRE was determined to be inoperable from December 10, 2010. The Technical Specification Required Action Statement for 3.7.10 Condition B was exited on September 18, 2013.
CAUSE
The station current licensing bases do not describe the boundary function of the Control Room Chiller floor drain loop seals with regard to the CRE.
Based on engineering's past operability evaluation, Control Damper CD-34177 was determined to be inoperable from December 10, 2010. The Technical Specification Required Action Statement for 3.7.10 Condition B was exited on September 18, 2013.
CORRECTIVE ACTION
Work Order 00486988 — Inspect and repair CD-34177 for excessive leakage past blades.
TMOD (EC 22746) "Install Blank Flanges for CD-34177, Control Room Outside Air Roof Isolation Damper" was installed.
Equipment Evaluation is in progress to determine the cause of the CD-34177 failure and any other contributing causes. Corrective actions will be generated following the completion of the investigation.
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