Four reactors at France's Gravelines nuclear plant will return to operation throughout the week, slightly later than initially planned, EDF data showed on Tuesday, after they were forced offline by a swarm of jellyfish in the cooling systems.

A nuclear power plant in northern France hit by a jellyfish invasion was getting back online on Wednesday, operator EDF said, with service restored at the first of four closed reactors.

Late Sunday, a jellyfish swarm clogged the cooling-water filters at Gravelines, a six-reactor plant on France’s North Sea coast. Four units shut down automatically. It was a bad timing, as two other reactors were already offline for maintenance,