05000313/LER-2011-001
| Arkansas Nuclear One - Unit 1 | |
| Event date: | 02-14-2011 |
|---|---|
| Report date: | 04-11-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3132011001R00 - NRC Website | |
A. Plant Status At the time this condition was identified, Arkansas Nuclear One, Unit 1 (ANO-1) was operating at -100% heat balance power.
B. Event Description
During the NRC 4th Quarter Resident Inspection, the inspectors identified a non-cited violation of Technical Specifications (TS) due to failure to enter the appropriate TS or complete the associated required action prior to the appropriate completion time when the associated emergency chillers were out of service. Specifically, ANO-1 did not enter the appropriate TS for an inoperable system, subsystem, train or component when the necessary attendant non-technical specification support equipment was not capable of performing their support function. The inspectors determined that not entering the appropriate TS when the emergency switchgear room chillers or applicable room cooling units were not available to provide the TS support function for TS emergency switchgear equipment was a performance deficiency.
Original design specifications for the ANO-1 emergency switchgear room cooling arrangement were based on hand calculations of switchgear room known heat loads. Since implementation of the 1977- 1979 design changes that installed this equipment, more robust software modeling of the switchgear rooms and associated heat load calculations have identified previously unrecognized heat loads in these areas. As these heat load calculations have evolved, certain additional procedurally controlled compensatory actions have been implemented to ensure continued switchgear room cooling functionality under accident conditions (LOCA with loss of offsite power) for as long as 30 days post accident, and to provide full capacity cooling to both Red and Green switchgear areas in the event that one emergency train of cooling is out of service for maintenance. In December, 2009, ANO-1 implemented a revision to the "Battery and Switchgear Emergency Cooling System" operating procedures that introduced the use of the non-safety related VUC-13A/B "Battery Room Normal Cooling Units" as contingency measures during emergency switchgear room cooling equipment maintenance. The use of VUC-13A/B was justified by an engineering change to a calculation that provided analysis for compensatory measures used to maintain certain ANO-1 switchgear rooms temperatures acceptable for the most limiting qualification temperatures for equipment located within those rooms. Additional compensatory measures implemented during the subject time period included manual action to open doors or dampers in specified switchgear areas and were validated using software modeling analysis.
During the subject time frame, upon failure of the switchgear room coolers, ANO-1 has entered the applicable TS for the switchgear room equipment and upon establishing compensatory measures to establish cooling, exited the TS. The compensatory measures were additionally utilized when taking a single emergency switchgear chiller train out of service for maintenance. Because the chillers were not originally designed to provide two trains of 100% redundant cooling, guidance used for justifying these alternate compensatory measures was inappropriately applied.
The VUC-13A/B unit coolers were originally considered by ANO-1 to be acceptable for compensatory emergency use, based on the fact that they were powered from on-site emergency power. A misapplication of industry guidance led to the belief that with proper Engineering analysis, the method of crediting the VUC-13A/B was an acceptable alternative compensatory measure.
Because the chillers were not originally designed to provide two trains of 100% redundant cooling, these non-qualified components should not have been used to replace functions performed by qualified components and the guidance used for justifying these alternate compensatory measures was inappropriately applied.
D. Corrective Actions
ANO-1 has ceased reliance on the non-safety related VUC-13 unit coolers and the additional subject compensatory measures previously outlined in plant operating procedures. Technical Specification LCO compliance is being maintained whenever the switchgear room cooling is removed from service.
Based on Engineering evaluation; the inverters and associated DC equipment remain operable even when associated emergency chilled water equipment is inoperable up to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> post accident.
Additionally, ANO-1 is in the process of developing appropriate actions to address time periods when a non-TS room cooling subsystem is out-of-service considering its impact on TS-required equipment.
E. Safety Significance
A Probabilistic Risk Assessment risk analysis has shown that the increase in risk of ANO-1 operation with one emergency switchgear room chiller out of service beyond the TS 8-hour completion time and within the window of any maintenance or failures of Emergency Switchgear Chillers, during the period of January 22, 2008 until January 4, 2011, is insignificant and considered acceptable. Additionally, considering the fact that compensatory measures were implemented during the time that the TS action statements were exceeded, the significance of this event is considered to be minimal.
10 CFR 50.73(a)(2)(i)(B) - "Any operation or condition which was prohibited by the plant's Technical Specifications except when:
(1) The Technical Specification is administrative in nature; (2) The event consisted solely of a case of a late surveillance test where the oversight was corrected, the test was performed, and the equipment was found to be capable of performing its specified safety functions; or (3) The Technical Specification was revised prior to discovery of the event such that the operation or condition was no longer prohibited at the time of discovery of the event.
G. Additional Information
There have been no previous similar events reported by ANO.
Energy Industry Identification System (EllS) codes are identified in the text as [XX].