Waterford — Millstone Unit 3 began producing power again early Sunday morning after a 35-day shutdown for refueling, but not until after a problem with instruments that monitor the equipment that controls the nuclear fission reaction was corrected.

Kenneth Holt, spokesman for Millstone owner Dominion, said the plant was undergoing a series of tests on Thursday leading up to the restart when operators found that a set of control rods was not in the correct position. The rods are used primarily during startup and shutdown of the plant to control the nuclear reaction, Holt said

After further testing, the operators determined that the the rods were moving properly, but the instruments that recorded their movement were not. The instruments were replaced. The incident was reported to the Nuclear Regulatory Commission and delayed the startup by “less than a day,” Holt said.

The complete series of tests required for restart was completed by Saturday, and the unit went back on line just after midnight Sunday, Holt said. The 27-year-old plant was shut down April 13 for routine refueling.

The larger and newer of the two working reactors at the site, Millstone 3 produces 1,230 megawatts of power.