05000440/LER-2007-003
| Docket Numbersequential Rev | |
| Event date: | 08-27-2007 |
|---|---|
| Report date: | 10-29-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4402007003R01 - NRC Website | |
PRINTED ON RECYCLED PAPER
Energy Industry Identification System Codes are identified in the text as [XX].
I. INTRODUCTION
At 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br /> on August 27, 2007, an engineering review determined that a section of grating in the reactor containment building [NH] suppression pool swell region was not properly restrained. When the grating condition was discovered, the plant was in Mode 1 at approximately 100% power. All emergency core cooling systems (ECCS) and emergency diesel generators [EK], except for residual heat removal (RHR)[BO] B, C and high pressure core spray (HPCS)[BG], were operable. RHR B, C and HPCS were in standby and available for operation. At 2044 hours0.0237 days <br />0.568 hours <br />0.00338 weeks <br />7.77742e-4 months <br /> on August 27, 2007, an eight-hour Non-Emergency notification (ENF Number 43603) was made to the NRC Operations Center as required by 10CFR50.72(b)(3)(ii)(B) and 10CFR50.72(b)(3)(v)(D).
Subsequently, a review of the Unit log identified that RHR A had been inoperable for the low pressure coolant injection mode (LPCI), on August 6, 2007, while in the suppression pool cooling mode, resulting in RHR A, B and C being inoperable for LPCI mode. Additionally, the Division 1 emergency diesel generator was identifed to have been inoperable for a portion of August 7-9, 2007, as a result of an overspeed trip during a surveillance . These conditions meet the reporting requirements for a Licensee Event Report in accordance with 10CFR50.73(a)(2)(ii)(B), "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety." This event was also determined to meet the the reporting criteria for 10CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of systems needed to mitigate the consequences of an accident. Since the inoperability of RHR B, C and HPCS was unrecognized, Limiting Condition for Operation (LCO) 3.5.1, ECCS Operating; 3.6.1.7, RHR Containment Spray; 3.6.2.3, RHR Suppression Pool Cooling and 3.6.3.3, Combustible Gas Mixing System Required Action Completion Times were exceeded and Mode changes were made with inoperable equipment from Mode 4 to Mode 2 and 1 which is prohibited by LCO 3.0.4. This condition is reportable per 10CFR50.73(a)(2)(i)(B), any operation or condition prohibited by Technical Specifications.
II. EVENT DESCRIPTION
On August 26, 2007, a plant roving operator discovered that a three by seven foot section of grating was not properly fastened down. The grating is located in the plant containment building on the 599 foot elevation near the lower containment air lock door. The fasteners required to hold the piece of grating in place were not installed as required. The grating had been removed in July 2007, to support the reactor recirculation system [AD] pump motor [MO] replacement. As a result of the engineering review, Technical Specification Limiting Conditions for Operation (LCO) 3.5.1, 3.6.1.7, 3.6.2.3 and 3.6.3.3 were entered and Required Actions taken within the specified completion time. The grating was subsequently restored to design configuration and the Technical Specification LCOs were exited at 1838 hours0.0213 days <br />0.511 hours <br />0.00304 weeks <br />6.99359e-4 months <br /> on August 27, 2007.
III.CCAUSE OF EVENT The cause of the error was determined to be inadequate detail in the work order, inadequate turnover of the work, and inadequate training for configuration control. The standards necessary to assure configuration control for the plant were not executed or enforced with the appropriate rigor. The work order should have provided detailed steps to remove and reinstall structural components; provide proper storage of removed components for accountability and reinstallation and provide for a formal turnover of partially completed work from one work organization to another. The lack of rigor resulted in:
- Not providing adequate detail in the work order to remove and reinstall the structural interferences for the Reactor Recirculation System motor replacement or to remove and reinstall the structural components and store the removed material properly for accountability and reinstallation.
- Not performing a formal turnover of the partially completed work from the contractor organization to the plant organization as required by plant procedures that provided for the disassembly and subsequent reassembly of the grating.
- Inadequate training received by the contractor personnel to ensure the recognition of the need to maintain proper configuration control or to understand the necessity for accurate segregation and labeling of the parts, components and hardware that were disassembled and would subsequently be needed for final installation and design configuration control.
IV.
CEVENT ANALYSIS
Following a postulated design basis Loss of Coolant Accident (LOCA) in the drywell, the atmosphere is rapidly compressed due to blow-down mass and energy addition to the drywell volume. This compression is transmitted to the water in the weir annulus in the form of a compressive wave which propagates through the horizontal vent system into the suppression pool. Following vent clearing, the air-steam-water mixture flows from the dry well through vents and is injected into the suppression pool. During vent flow the steam component of the flow mixture condenses in the suppression pool while the air, since it is non-condensable, is released to the suppression pool in the form of high pressure air bubbles. The continued addition and expansion of air within the suppression pool causes the pool volume to swell resulting in acceleration of the pool surface vertically upwards. The area in containment where this occurs is called the "pool swell region". The grating with the missing hold down fasteners is located in the pool swell region.
During this postulated event, the grating could become dislodged and subsequently impact the emergency core cooling system (ECCS) suction strainer [STR] located in the suppression pool below the grating. An engineering review determined that the force of the impact could be larger than the impingement forces that had been previously evaluated. The resulting damage could cause one or more of the following systems to be inoperable as a result of debris being allowed through the damaged strainer and cause fouling of the connected pumps: RHR B, C or HPCS. The improperly restrained grating resulted in an unanalyzed condition for part of the ECCS suction strainer and the connected ECCS systems. Since HPCS is considered a single train safety system, the improperly restrained grating is also considered to have caused a loss of safety function for this high pressure system.
A review of the Unit log identified that RHR A had been placed in the suppression pool cooling mode of operation from 0956 hours0.0111 days <br />0.266 hours <br />0.00158 weeks <br />3.63758e-4 months <br /> until 1702 hours0.0197 days <br />0.473 hours <br />0.00281 weeks <br />6.47611e-4 months <br /> on August 6, 2007, which made it inoperable for injection for 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> and 6 minutes due to valves being unable to reposition in time to meet injection response times. However, the RHR A loop was available for LPCI with managed restoration controls in place. Additionally, the division 1 emergency diesel generator was determined to have been inoperable for 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br /> and 53 minutes on August 7-9, 2007, as a result of an overspeed trip (unavailable for 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> and 26 minutes). Offsite power was available for division 1 equipment during this time interval. Since the inoperability of RHR B, C and HPCS was unrecognized, Limiting Condition for Operation (LCO) 3.5.1, ECCS Operating; 3.6.1.7, RHR Containment Spray; 3.6.2.3, RHR Suppression Pool Cooling and 3.6.3.3, Combustible Gas Mixing System Required Action Completion Times were exceeded and Mod changes were made with inoperable equipment from Mode 4 to Mode 2 and 1 which is prohibited by LCO 3.0.4. Additionally, since the grating condition was not recognized when the division 1 emergency diesel generator became inoperable, LCO 3.8.1, Action B.2 and F, were not met. This condition is reportable per 10CFR50.73(a)(2)(i)(B), 10CFR50.73(a)(2)(v)(D) and 10CFR50.73(a)(2)(ii)(B).
An evaluation was performed assuming the aforementioned conditions occurred using the plant's Probabilistic Risk Assessment model.
The duration that the grating was not properly fastened with the potential of a large or intermediate LOCA was from the time the plant entered Mode 2 at 1643 hours0.019 days <br />0.456 hours <br />0.00272 weeks <br />6.251615e-4 months <br /> on July 23, 2007, until the grating was restored to full qualification at 1838 hours0.0213 days <br />0.511 hours <br />0.00304 weeks <br />6.99359e-4 months <br /> on August 27, 2007. This was a period of 841.92 hours0.00106 days <br />0.0256 hours <br />1.521164e-4 weeks <br />3.5006e-5 months <br />, which is rounded up to 842 hours0.00975 days <br />0.234 hours <br />0.00139 weeks <br />3.20381e-4 months <br /> for this assessment. While. RHR A was inoperable in suppression pool cooling mode, it was available and did not impact the risk significance. The division 1 emergency diesel generator was also unavailable for 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> and 26 minutes but was evaluated to have had minimal impact on the risk significance. The calculated Conditional Core Damage Probability was determined to be 3.08 E-08. Therefore, this event was determined to be of a very low safety significance.
V.� CORRECTIVE ACTIONS The grating was restored to proper design configuration at 1838 hours0.0213 days <br />0.511 hours <br />0.00304 weeks <br />6.99359e-4 months <br /> on August 27, 2007.
The planning organization will review the requirements of the "Order Planning Process" procedure which requires a sign-off in the work order after each important attribute including verification that fasteners and locking devices are installed properly and verification of configuration control activities.
The "Post Maintenance Test Manual" will be changed to include verification of the restoration of structural steel components. Specifically, a caution will be added for verification of structural items such as removable grating, including hold down plates, handrails and other components in the pool swell region.
A lessons-learned presentation will be provided to the plant supervisory staff regarding work in the pool swell region, configuration control and formal turnover of work between work groups.
A lessons-learned presentation will be developed and presented to the Maintenance Organization, Construction Management and the Project Support Team to ensure that the process for configuration control in the plant is reviewed and understood. The lessons-learned presentation will include training on process barriers such as the walk-down process, order documentation, material accountability, housekeeping, turnovers and briefings, configuration control, post maintenance testing and sensitivity to un-secured material in the pool swell region.
A supervisor from each maintenance shop will perform at least one configuration control observation per month for six months.
VI. PREVIOUS SIMILAR EVENTS
in the containment pool swell region, the region immediately above the grating discussed in the containment would not have been expected to prevent the improper grating installation.
VII. COMMITMENTS
There are no regulatory commitments contained in this report. Actions described in this document represent intended or planned actions, are described for the NRC's information, and are not regulatory commitments.