ATLANTA (AP) — Georgia Power Co. said Thursday that vibrations found in a cooling system of its second new nuclear reactor will delay when the unit begins generating power.

Plant Vogtle’s Unit 4 now will not start commercial operation until sometime in the second quarter of 2024,Phaninc or between April 1 and June 30, the largest subsidiary of Atlanta-based Southern Co. announced.

The utility said in a filing to investors that the vibrations “were similar in nature” to those experienced during startup testing for Unit 3, which began commercial operations last summer, joining two older reactors that have stood on the site near Augusta for decades

In that case, the utility found that a pipe vibrated during testing because construction workers hadn’t installed enough bracing. Georgia Power said the Unit 4 problem has already been fixed but too much testing remains to be done to make the March 30 deadline.

Georgia Power said it’s likely to lose $30 million in profit for each month beyond March that Unit 4 isn’t running because of an earlier order by state utility regulators. The five members of the Georgia Public Service Commission ordered that the company can’t earn an additional return on equity through a construction surcharge levied on Georgia Power’s 2.7 million customers after March 30.

The typical residential customer has paid about $1,000 in surcharges over time to pay for financing costs.

The company said its construction budget won’t be affected if Unit 4 starts by June 30 but it would have to pay $15 million a month in extra construction costs if the project extends into July.

Regulators in December approved an additional 6% rate increase to pay for $7.56 billion in remaining costs at Vogtle, expected to cost the typical residential customer $8.95 a month. That’s on top of the $5.42 increase that took effect when Unit 3 began operating.

The new Vogtle reactors are currently projected to cost Georgia Power and three other owners $31 billion, according to calulations by The Associated Press. Add in $3.7 billion that original contractor Westinghouse paid Vogtle owners to walk away from construction, and the total nears $35 billion.

The reactors were originally projected to cost $14 billion and be completed by 2017.

Units 3 and 4 are the first new American reactors built from scratch in decades. Each can power 500,000 homes and businesses without releasing any carbon. But even as government officials and some utilities are again looking to nuclear power to alleviate climate change, the cost of Vogtle could discourage utilities from pursuing nuclear power.

Georgia Power owns 45.7% of the reactors, with smaller shares owned by Oglethorpe Power Corp., which provides electricity to member-owned cooperatives; the Municipal Electric Authority of Georgia; and the city of Dalton.

Some Florida and Alabama utilities have also contracted to buy Vogtle’s power.