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Event Report Forms (ERFs) are filed to the IAEA. Sometimes there are multiple event reports, updating the rating, the event description. Here you only see the latest event report

ERF send date: Thu, 21-03-1996
ERF description: Shortly before 0300 EST, on 30 January 1996, operators at the Wolf Creek Generating Station received indication of decreasing water levels in the Circulating (CW) and Service (SW) Water pump intake structure and started both Essential Service Water (ESW) pumps. The decrease in water level was caused by the formation of ice on the intake travelling screens. At 0437 operators manually tripped the reactor from 80% power in anticipation of the need to trip the CW pumps. Following reactor trip, five control rods failed to fully insert, (10 to 29 cm out). This failure to fully insert did not directly affect the event, but it did complicate the operator's response. Decay heat was removed through the steam generators using the atmospheric relief valves with feed from the Auxiliary Feedwater pumps. At 0614 the turbine-driven Auxiliary Feedwater pump was secured and declared inoperable due to a shaft gland leak. At 0847 the "A" ESW pump was secured due to low ESW pump intake level caused by the formation of "Frazil" ice below the surface of the water on the intake trash rack. Although the "B" ESW train remained in operation throughout the event, there were several occasions when the "B" ESW pump intake water level decreased to a level that threatened to render this pump inoperable as well.
Justification: the manual reactor trip and loss of the Main Condenser following the imminent loss of the Circulating Water pumps resulted in initiation of the safety related Auxiliary Feedwater System. Core cooling with the Motor-driven Auxiliary Feedwater pumps (after the turbine-driven auxiliary feedwater pump was declared inoperable) was dependent upon either the ESW system (provides emergency diesel generator cooling) or offsite power. Offsite power remained available through the event. The "B" ESW train and one SW pump remained in operation throughout the event. The SW pump could also have been used to supply ESW loads. In addition, the plant had sufficient make-up inventory and a diesel driven fire pump that could have been used to feed the steam generators if necessary. Based on Severity Classificaiton Criteria for Degradation of Defense in Depth with an initiator (Table II), this event would be rated a level 1/2. However, there are additional factors that were considered. Taken together, these additional factors justify rating the event as a level 2. The first addional factor was the common cause failure potential introduced by the "frazil" ice phenomena. The second factor was that a control room operator did not align the ESW system in accordance with the operating procedure which resulted in a decrease in flow through the ESW pump intake structure warming lines. The decrease in warming flow was a contributor to the loss of the "A" ESW train and the potential loss of the "B" ESW train due to "frazil" ice buildup on the trash racks early that morning.
On-site and off-site impact criteria were not relevant for this event.
ERF rating date: 15-02-1996
ERF contact person: R.J. BARRETT
ERF address: United States
ERF affiliation: USNRC, EMERGENCY RESPONSE BRANCH, OFFICE FOR ANALYSIS AND EVALU

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