Site Area Emergency
On March 20, 1990 at 9:20 a.m., a truck carrying fuel and lubricants in the plant's low voltage switchyard backed into a support column for the feeder line supplying power to the Unit 1-A reserve auxiliary transformer (RAT). This set off a complicated chain of events that was exacerbated both by planned maintenance, in which some back-up systems were off-line, and by equipment failures in some back-up systems. The resulting loss of electrical power in the plant's "vital circuits" shut down the residual heat removal (RHR) pump that was cooling Unit 1 (which was nearing the end of a refueling outage) and prevented the back-up RHR from activating. Even though Unit 1 was not operating at full-power, residual heat from the natural decay of the radioactive fuel needed to be removed to prevent a dangerous rise in core temperature.
At 9:40 a.m., the plant operators declared a site area emergency (SAE) per existing procedures which called for an SAE whenever "vital" power is lost for more than 15 minutes. At 9:56 a.m., plant operators performed a manual start of the A-train emergency diesel generator (EDG), which bypassed most of the EDG's protective trips which had prevented it from coming on-line. RHR-A was then started using power from EDG-A. With core cooling restored, the SAE was downgraded to an alert at 10:15 a.m.
The temperature of the Unit 1 core coolant increased from 90 °F to 136 °F during the 36 minutes required to re-energize the A-side bus. Throughout the event, non-vital power was continuously available to Unit 1 from off-site sources. However, the Vogtle electrical system was not designed to permit easy interconnection of the Unit 1 vital busses to non-vital power or to the Unit 2 electrical busses. Since this incident, Plant Vogtle has implemented changes to the plant that allow power to be transferred from one side to the other from an off-site source.
Read more about this topic: Vogtle Electric Generating Plant
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