An "Unusual Event" is the lowest of fours emergency classification levels. "The purpose of this classification is to assure that the first step in future response has been carried out, to bring the operations staff to a state of readiness, and to provide systematic handling of unusual event information and decision-making."
Pretty much declared anytime something abby-normal occurs. Not a big deal. A plant is designed to be totally divorced from outside services for 28 days.
Retired nuclearpowerfag here