A valve that unexpectedly opened on an 8,000-gallon coolant tank pushed back by at least 24 hours FirstEnergy Corp.'s critical seven-day test of the Davis-Besse nuclear power plant's coolant system.

The test should start sometime today, said spokesman Todd Schneider. Nothing was damaged or will need to be replaced, he said.

The tank is designed to flood the reactor core with coolant during an emergency. When the main valve opened unexpectedly, a check valve prevented the tank from draining. The utility said it changed procedures and trained operators to ward off future problems.