The following event description is based on information currently available. If through subsequent reviews of this event additional information is identified that is pertinent to this event or alters the information being provided at this time, a follow-up notification will be made via the
ENS or under the reporting requirements of
10 CFR 50.73.
This notification describes a previously undeclared event that is now reported pursuant to the requirements of 10 CFR 50.72(a)(3). No emergency situation exists at this time.
On November 6, 2013, while operating at 100% reactor power under steady state conditions, a U-1 control room alarm was received indicating low fluid pressure in the Train 'A' hydraulic accumulator of Main Steam system isolation valve MSIV-170. A plant operator was dispatched according to the alarm response procedure to perform a pre-charge check of the accumulator. A pre-charge check returns hydraulic fluid from the accumulator to a vented tank that serves as a fluid reservoir. During this operation the fluid reservoir tank and supporting piping were damaged by the rapid expansion of nitrogen gas that had entered the hydraulic system by way of a leak across an accumulator piston O-ring. As a result of the damage and loss of hydraulic fluid, MSIV-170 and its associated hydraulic accumulators were declared inoperable at 1307 Mountain Standard Time. It was determined at that time that the failure of the fluid reservoir due to rapid expansion did not constitute an explosion and a declaration of an Unusual Event was not made according to Emergency Action Level (EAL) HU2, 'Fire within the Protected Area not extinguished within 15 minutes of detection or explosion within the Protected Area.'
Subsequent review has determined that the reservoir failure should have been characterized as an explosion within the Protected Area and that a declaration of an Unusual Event should have been made per EAL HU2.
The NRC Resident Inspector has been notified.
The licensee has notified the State of
Arizona and Maricopa County.
