OL3 is now expected to restart at 1800 GMT on Thursday. The reactor, Europe's largest with a capacity of 1,600 megawatt (MW), suffered an unexpected "rapid shutdown" during testing on Wednesday, operator TVO has said.
OL3 had originally been expected to be offline for eight hours, but the restart was since delayed several times.
In Sweden, Vattenfall extended an outage at Ringhals 4 with a capacity of 1,130 MW by two full days to 2259 GMT on Saturday.
The reactor had tripped on Wednesday morning due to a valve failure, with troubleshooting ongoing, according to the market message on Nord Pool.
The Nordic region has been hit by unusually cold weather in late November, pushing up demand for heating and boosting power prices.
