05000263/LER-2001-003
| Docket Number | |
| Event date: | 02-20-2001 |
|---|---|
| Report date: | 04-13-2001 |
| 2632001003R00 - NRC Website | |
FACILITY NAME(1) DOCKET LER NUMBER (6) PAGE (3)
MONTICELLO NUCLEAR YEAR SEQUENTIAL NUMBER REVISION NUMBER
Description During past Standby Gas Treatment System' (SBGT) on-line maintenance activities, the following sequence of events has normally been followed when working on one train of the SBGT system:
1. A seven-day Technical Specification Limiting Condition for Operation (LCO) entered 2. The train was declared inoperable 3. The train was isolated and associated control panels were deenergized.
4. The system filter2 train doors were opened to access components in the filter train.
On February 20, 2001, during normal full power operation, engineering review of this SBGT on-line maintenance procedure determined that it was non-conservative in that it did not consider the potential adverse impact on the redundant train when the filter train doors were opened on the train undergoing maintenance at the same time that the associated control panels were deenergized.
When any filter unit door is opened, a flow pathway is created from the open door, through the downstream components in the filter unit, through the outlet isolation damper (which fails open when the train is electrically isolated), through the cross-connect duct and restricting orifice3, and into the operable SBGT exhaust fan. The flow rate through this pathway would be larger than the design idle train flow rate resulting in degraded performance of the operable SBGT train if it were required to function following an accident.
In addition to the flow path described above, there would also be a second pathway created into the open filter train door, backwards through the upstream components and the inlet isolation damper (which fails open when the train is electrically isolated), and into the operable SBGT train.
A third concern arises if any internal filter train components are removed during maintenance and the train is electrically isolated. Flow resistance is lower when filter components are removed and the train isolation dampers fail open on loss of power. This lower flow resistance would also allow more flow through the idle train into the operable train and provide an unfiltered bypass of the operable train, thereby degrading system performance.
The first two pathways can result in excessive air being drawn from the SBGT room into the operating train. This could allow unfiltered post-accident secondary containment air to enter the SBGT room and released from the plant stack without filtration.
1 EIIS System Code:
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VA
2 EIIS Component Code: FLT EIIS Component Code: OR FACILITY NAME(1) DOCKET LER NUMBER (6) PAGE (3)
MONTICELLO NUCLEAR YEAR SEQUENTIAL NUMBER REVISION NUMBER
Since the potential existed for the operable SBGT train to be degraded during on-line maintenance of the other train, a 36-hour LCO period should have been conservatively entered. A 36-hour LCO is appropriate for inoperability of both SBGT trains.
Event Analysis
Analysis of Reportability This event was determined to be reportable in accordance with 10 CFR Part 50, Section 50.73(a)(2)(vii) since it had the potential to cause the inoperability of two independent trains of a single system designed to mitigate the consequences of an accident. Reporting in accordance with 10 CFR Section 50.73(a)(2)(v)(C) is also applicable.
Safety Significance
Following engineering evaluation, the safety significance of this event was determined to be small for the following reasons:
1. A review of work orders for the last five years of plant operation found that there were 12 periods within the last five years when SBGT on-line maintenance was performed. The maximum duration of maintenance during these periods was 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and 35 minutes. It is estimated that a filter train door was open for 50% or less of each period during which both trains of SBGT could be affected. There were no occasions when maintenance exceeded 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
2. An open filter train door downstream of the second high efficiency particulate filter is expected to create the largest potential bypass flow into the redundant operating SBGT train. An extremely conservative analysis has been performed assuming this bypass flow consists of 540 cfm of secondary containment post-accident atmosphere. Under these conditions offsite doses would remain less than 10% of 10 CFR Part 100 guidelines and dose to control room operators would remain less than 60% of the 10 CFR 50, Appendix A, General Design Criterion 19 guideline.
3. The impact on an operating SBGT train caused by removing a filter train element during maintenance on the other train would not significantly degrade the capability of the operable train to maintain a negative pressure in secondary containment. The dose consequences of the removal of a charcoal adsorber element would be bounded by the condition described above of operation with an open filter train door.
FACILITY NAME(1) DOCKET LER NUMBER (6) PAGE (3)
MONTICELLO NUCLEAR YEAR SEQUENTIAL NUMBER REVISION NUMBER
Cause
The root cause of this event was determined to be inadequate procedures caused by the failure of plant engineering and operations personnel to recognize the potential for adverse interaction between the SBGT trains during maintenance.
Corrective Actions
SBGT hold and secure card isolations have been revised to require tagging the SBGT doors in the closed position when control power to an SBGT train is deenergized thereby causing the train isolation damper to fail open. Operation control documents associated with SBGT maintenance and testing have been placed on hold and will be revised prior to their next use to require tagging SBGT access doors closed when train control power is deenergized for maintenance.
Work control directives will be reviewed for possible enhancements to emphasize evaluating conditions during maintenance activities that could have adverse effects on redundant systems.
Engineering will evaluate the possibility of using mechanical closure stops on the SBGT train isolation dampers to hold them closed during train maintenance.
Engineering, operations, and technical staff personnel will receive training on this event with emphasis on prevention of similar maintenance interactions in other systems.
Failed Component Identification Not applicable.
Similar Events This failure of plant engineering and operations personnel to recognize the potential for adverse interaction between the SBGT trains during maintenance is similar to the discovery of SBGT train isolation activities which affected the operability of secondary containment isolation dampers. This event was previously reported as LER 2000-006.