An unresolved item (
URI) was identified that was associated with a loss of cooling water flow to the reactor while was shutdown. On July 11, 2007, the plant was shutdown in Mode 4 with the B
residual heat removal (
RHR) loop in service providing
decay heat removal for the reactor vessel. A maintenance technician was performing a surveillance test associated with the
RCIC steam line flow high instrument channel. At about 11:13 p.m. the technician disconnected a wire for a relay associated with a
RCIC system isolation function. This caused the relay to change state. The licensee determined that when the affected relay de-energized, it created an electrical
transient that affected the relays power supply. The affected power supply wires were in close physical proximity to the power wires for an optical isolator associated with a relay for the B
RHR pump trip circuit. The licensee determined that the electrical
transient resulted in the actuation of the optical isolator and the trip of the B
RHR pump. When the B
RHR pump tripped, the reactor vessel bottom head drain temperature instrument indicated about 108 EF (degrees Fahrenheit). Operators monitored vessel temperature using the bottom head drain temperature indication and developed response plans based on an observed 11 Enclosure heat up rate of about 1 to 2 EF per hour. Operators took immediate action to restore cooling flow to the reactor vessel using the A
RHR loop and established a plan to begin containment closure when reactor vessel indicated temperature reached 140 EF. The inspectors noted that the licensee had calculated a timeto- boil (
TTB) value of 2.
5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, and that this
TTB value remained in effect through the entire loss of
decay heat removal event. The inspector was concerned with a statement from the shift manager regarding his belief that a significantly longer response time (potentially greater than a day) was available based on observed heat-up rate derived from reactor bottom head temperature indication. During the event, the inspectors questioned licensee personnel in the control room on the discrepancy between the calculated
TTB and observed heatup rate. In response to the inspectors questions, a licensee management representative in the control room remarked that the posted calculated
TTB was potentially in need of an update. The inspector was also concerned that the operators may have been misled by the use of bottom head drain temperature as an indication of reactor vessel bulk temperature during this event. Operators established
shutdown cooling flow using the A
RHR loop at about 12:42 a.m. on July 12, 2007. When the A
RHR pump was started, bottom head temperature indicated about 111 EF and then rose to about 140 EF as water flow was restored to the vessel. The Regional Senior Reactor Analyst commenced a risk evaluation of the loss of the B
RHR loop and subsequent licensee actions. The risk evaluation was ongoing at the end of the inspection period. This issue is considered an
URI pending completion of the risk assessment (
URI 05000440/2007004-01)
